The urinary tract consists of the kidneys, ureters, bladder, and urethra (Figure 73-1). The function of the urinary tract is to make and process urine. Urine is an ultrafiltrate of blood that consists mostly of water but also contains nitrogenous wastes, sodium, potassium, chloride, and other analytes. Urine is normally a sterile fluid, Often, urinary tract infections (UTIs) are characterized as being either upper (U-UTI) or lower (L-UTI) based primarily on the anatomic location of the infection: the lower urinary tract encompasses the bladder and urethra, and the upper urinary tract encompasses the ureters and kidneys. Upper urinary tract infections affect the ureters (ureteritis) or the renal parenchyma (pyelonephritis). Lower urinary tract infections may affect the urethra (urethritis), the bladder (cystitis), or the prostate in males (prostatitis).

Figure 73-1 Overview of the anatomy of the urinary tract. (From Potter PH, Perry AG: *Fundamentals of nursing,* St. Louis, 1985, Mosby.)

The anatomy of the female urethra is of particular importance to the pathogenesis of UTIs. The female urethra is relatively short compared with the male urethra and also lies in close proximity to the warm, moist, perirectal region, which is teeming with microorganisms. Because of the shorter urethra, bacteria can reach the bladder more easily in the female host, thus urinary tract infections are primarily a disorder of women. For men, the incidence of urinary tract infections increases after the age of 60, when the enlargement of the prostate interferes with the removal of urine from the bladder.

Resident Microorganisms of the Urinary Tract

The urethra has resident microflora that colonize its epithelium in the distal portion; these organisms are lactobacilli, corynebacteria, and coagulase-negative staphylococci (Box 73-1). Potential pathogens, including gram-negative aerobic bacilli (primarily Enterobacteriaceae) and occasional yeasts, are also present as transient colonizers. All areas of the urinary tract above the urethra in a healthy human are sterile. Urine is typically sterile, but noninvasive methods for collecting urine must rely on a specimen that has passed through a contaminated milieu. Therefore, quantitative cultures for the diagnosis of UTIs have been used to discriminate among contamination, colonization, and infection.

Infections of the Urinary Tract

Epidemiology

UTIs are among the most common bacterial infections that lead patients to seek medical care. It has been estimated that more than 7 million outpatient visits, 1 million visits to the emergency department, and 100,000 hospital stays every year in the United States are due to UTIs. Approximately 10% of humans will have a UTI at some time during their lives. Of note, UTIs are also the most common hospital-acquired infection, accounting for as many as 35% of nosocomial infections.

The exact prevalence of UTIs is age and sex dependent. During the first year of life, UTIs are less than 2% in males and females. The incidence of UTIs among males remains relatively low after 1 year of age and until approximately 60 years of age when enlargement of the prostate interferes with emptying of the bladder. Extensive studies have shown that the incidence of bacteriuria (presence of bacteria in urine) among girls 5 through 17 years of age is 1% to 3%. The prevalence of bacteriuria in females increases gradually with time to as high as 10% to 20% in older women. In women between 20 and 40 years of age who have had UTIs, as many as 50% may become reinfected within 1 year. The association of UTIs with sexual intercourse may also contribute to this increased incidence because sexual activity increases the chances of bacterial contamination of the female urethra. Finally, as a result of anatomic and hormonal changes that favor development of UTIs, the incidence of bacteriuria increases during pregnancy. These infections can lead to serious infections in both mother and fetus.

UTIs are important complications of diabetes, renal disease, renal transplantation, and structural and neurologic abnormalities that interfere with urine flow. In 40% to 60% of renal transplant recipients, the urinary tract is the source of bacteremia, and in these patients, the recurrence rate is about 40%. In addition, UTIs are a leading cause of gram-negative sepsis in hospitalized patients and are the origin for about half of all nosocomial infections caused by urinary catheters.

Etiologic Agents

Pathogenesis

Routes of Infection

Bacteria can invade and cause a UTI via three major routes: ascending, hematogenous, and lymphatic pathways. Although the ascending route is the most common course of infection in females, ascent in association with instrumentation (e.g., urinary catheterization, cystoscopy) is the most common cause of hospital-acquired UTIs in both sexes. For UTIs to occur by the ascending pathway, enteric gram-negative bacteria and other microorganisms that originate in the gastrointestinal tract must be able to colonize the vaginal cavity or the periurethral area. Once these organisms gain access to the bladder, they may multiply and then pass up the ureters to the kidneys. UTIs occur more often in women than men, at least partially because of the short female urethra and its proximity to the anus. As previously mentioned, sexual activity can increase chances of bacterial contamination of the female urethra.

In most hospitalized patients, UTI is preceded by urinary catheterization or other manipulation of the urinary tract. The pathogenesis of catheter-associated UTI is not fully understood. It is certain that soon after hospitalization, patients become colonized with bacteria endemic to the institution, often gram-negative aerobic and facultative bacilli carrying resistance markers. These bacteria colonize the patient’s skin, gastrointestinal tract, and mucous membranes, including the anterior urethra. With insertion of a catheter, the bacteria may be pushed along the urethra into the bladder or, with an indwelling catheter, may migrate along the track between the catheter and the urethral mucosa, gaining access to the bladder. It is estimated that approximately 10% to 30% of catheterized patients will develop bacteriuria (presence of bacteria in urine).

UTIs may also occur by the hematogenous, or blood-borne, route. Hematogenous spread usually occurs as a result of bacteremia. Any systemic infection can lead to seeding of the kidney, but certain organisms, such as Staphylococcus aureus or Salmonella spp., are particularly invasive. Although most infections involving the kidneys are acquired through the ascending route, yeast (usually Candida albicans), Mycobacterium tuberculosis, Salmonella spp., Leptospira spp., or Staphylococcus aureus in the urine often indicates pyelonephritis acquired via hematogenous spread, or the descending route. Hematogenous spread accounts for less than 5% of UTIs.

Finally, increased pressure on the bladder can cause lymphatic flow into the kidneys, resulting in UTI. However, evidence for the significance of this potential route is insufficient, indicating that the ascending route remains the major mechanism for the development of UTI.

The Host-Parasite Relationship

Many individuals, women in particular, are colonized in the vaginal or periurethral area with organisms originating from the gastrointestinal tract, yet they do not develop urinary infections. Whether an organism is able to colonize and then cause a UTI is determined in large part by a complex interplay of host and microbial factors.

In most cases, the host defense mechanisms are able to eliminate the organisms. Urine itself is inhibitory to some of the urethral flora such as anaerobes. In addition, if urine has a low pH, high or low osmolality, high urea concentration, or high organic acid content, even organisms capable of growth in the urinary tract may be inhibited. If bacteria do gain access to the bladder, the constant flushing of contaminated urine from the body either eliminates bacteria or maintains their numbers at low levels. Clearly, any interference with the act of normal voiding, such as mechanical obstruction resulting from kidney stones or strictures, will promote the development of UTI. Also, the bladder mucosal surface has antibacterial properties. If the infection is not eradicated, the site of infection remains in the superficial mucosa; deep layers of the bladder are rarely involved.

In addition to the previously described host defenses, a valvelike mechanism at the junction of the ureter and bladder prevents the reflux (backward flow) of urine from the bladder to the upper urinary tract. Therefore, if the function of these valves is inhibited or compromised in any way, such as by obstruction or congenital abnormalities, urine reflux provides a direct route for organisms to reach the kidney. Hormonal changes associated with pregnancy and their effects on the urinary tract increase the chance for urine reflux to the upper urinary tract.

Activation of the host immune response by uropathogens also plays a key role in fending off infection. For example, bacterial contact with urothelial cells initiates an immune response via a variety of signaling pathways. Bacterial lipopolysaccharide (LPS; see Chapter 2) activates host cells to ultimately release cytokines such as tumor necrosis factor and interferon-gamma. In addition, bacteria can activate the complement cascade, leading to the production of biologically active components such as opsonins, as well as augment the host’s adaptive immune response. Host factors that lead to host susceptibility or resistance to uropathogens have been identified. For example, a glycoprotein synthesized exclusively by epithelial cells in a specific anatomic location in the kidney, referred to as Tamm-Horsfall protein or uromodulin, serves as an anti-adherence factor by binding to E. coli–expressing type 1 fimbriae (discussed later). Defensins, a group of small antimicrobial peptides, are produced by a variety of host cells such as macrophages, neutrophils, and cells in the urinary tract and attach to the bacterial cell, eventually causing its death.

Although many microorganisms can cause UTIs, most cases are a result of infection by a few organisms. To illustrate, only a limited number of serogroups of E. coli cause a significant proportion of UTIs. Numerous investigations indicate that UPEC possesses virulence factors that enhance their ability to colonize and invade the urinary tract. Some of these virulence factors include increased adherence to vaginal and uroepithelial cells by bacterial surface structures (adhesins, in particular, pili), alpha-hemolysin production, and resistance to serum-killing activity (Box 73-2). Also, genome sequences of some UPEC strains have been determined, indicating that several potential virulence factor genes associated with the acquisition and development of UTIs are encoded on pathogenicity islands (e.g., hemolysins and E. coli P. fimbriae). Uropathogenic E. coli (UPEC) possess pathogenicity islands containing a variety of virulence factors. By definition, pathogenicity islands (see Chapter 3) contain genes that are associated with virulence and are absent from avirulent or less virulent strains of the same species.

Box 73-2 Examples of Probable Virulence Factors of Uropathogenic E. coli

Type 1 fimbriae that bind to uroepithelial cells

Type P fimbriae that recognize kidney glycosphingolipids

Siderophores that help gather iron from the host

Alpha and beta hemolysins that lyse host erythrocytes

Capsules

Sat protein that acts as a proteolytic toxin

The importance of adherence in the pathogenesis of UTIs has also been demonstrated with other species of bacteria. Once introduced into the urinary tract, Proteus strains appear to be uniquely suited to cause significant disease in the urinary tract. Data indicate that these strains are able to facilitate their adherence to the mucosa of kidneys. Also, Proteus is able to hydrolyze urea via urease production. Hydrolysis of urea results in an increase in urine pH that is directly toxic to kidney cells and also stimulates the formation of kidney stones. Similar findings have been made with Klebsiella spp. Staphylococcus saprophyticus also adheres better to uroepithelial cells than does S. aureus or S. epidermidis.

Other bacterial characteristics may be important in the pathogenesis of UTIs. Motility may be important for organisms to ascend to the upper urinary tract against the flow of urine and cause pyelonephritis. Some organisms demonstrate greater production of K antigen(capsule or outer cell wall); this antigen protects bacteria from being phagocytosed.

Finally, despite numerous host defenses and even antibiotic treatments that can effectively sterilize the urine, a significant proportion of patients have recurrent UTIs. Studies show that uropathogens can invade superficial epithelial cells in the bladder and replicate, forming large foci of intracellular E. coli. This invasion of bladder epithelial cells triggers the host immune response, which in turn causes the superficial cells to exfoliate within hours following infection. Although this exfoliation is considered a host defense mechanism by eliminating infected cells, intracellular organisms are able to reemerge from the bladder epithelial cells and invade the underlying, new superficial layer of epithelial cells, consequently persisting within the urinary tract. Anderson and colleagues reported that intracellular bacteria mature into numerous, large protrusions on the bladder surface they referred to as “pods.” This bacterial organization—in which the intracellular bacteria are embedded in a fibrous, polysaccharide-rich matrix resembling that of a biofilm—may help further explain the persistence of bladder infections despite strong host defenses.

Types of Infection and Their Clinical Manifestations

UTI encompasses a broad range of clinical entities that differ in terms of clinical presentation, degree of tissue invasion, epidemiologic setting, and requirements for antibiotic therapy. There are several types of UTIs: urethritis, ureteritis, asymptomatic bacteriuria, cystitis, the urethral syndrome, and pyelonephritis. Sometimes UTIs are classified as uncomplicated or complicated. Uncomplicated infections occur primarily in otherwise healthy females and occasionally in male infants and adolescent and adult males. Most uncomplicated infections respond readily to antibiotic agents to which the etiologic agent is susceptible. Complicated infections occur in both sexes. In general, individuals who develop complicated infections often have certain risk factors. Some of these risk factors are listed in Box 73-3. In general, complicated infections are more difficult to treat and have greater morbidity (e.g., kidney damage, bacteremia) and mortality compared with uncomplicated infections.

Box 73-3 Risk Factors Associated with Complicated Urinary Tract Infections

Underlying diseases that predispose the kidney to infection (e.g., diabetes, sickle cell anemia)

Kidney stones

Structural or functional abnormalities of the urinary tract (e.g., a tipped bladder)

Indwelling urinary catheters

The clinical presentation of UTIs may vary, ranging from asymptomatic infection to full-blown pyelonephritis (infection of the kidney and its pelvis). Some UTI symptoms may be nonspecific, and frequently symptoms overlap considerably in patients with lower UTIs and in those with upper UTIs.

Urethritis

Symptoms associated with urethritis (infection of the urethra), dysuria (painful or difficult urination), and frequency are similar to those associated with lower UTIs. Urethritis is a common infection. Because Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis are common causes of urethritis and considered to be sexually transmitted, urethritis is discussed as a sexually transmitted disease in Chapter 74.

Ureteritis

Inflammation or infection within the ureters is considered in combination with kidney infections. UTI within the ureters indicates that organisms have begun or are in the process of ascending into the kidneys and should be treated similarly to prevent further infection.

Asymptomatic Bacteriuria

Asymptomatic bacteriuria or asymptomatic UTI is the isolation of a specified quantitative count of bacteria in an appropriately collected urine specimen obtained from a person without symptoms or signs of urinary infection. Asymptomatic bacteriuria is common, but its prevalence varies widely with age, gender, and the presence of genitourinary abnormalities or underlying diseases. For example, the prevalence of bacteriuria increases with age in healthy women from as low as about 1% among school-age females to greater than or equal to 20% among women 80 years of age or older living in the community, whereas bacteriuria is rare in healthy young men. Because its clinical significance was controversial (asymptomatic bacteriuria precedes UTI but does not always lead to asymptomatic infection), guidelines have been published for the diagnosis and treatment of asymptomatic bacteriuria in adults older than 18 years of age. The foundation of these guidelines rests on the premise that screening of asymptomatic subjects for bacteriuria is appropriate if bacteriuria has adverse outcomes that can be prevented by antimicrobial therapy. Thus, screening and treatment for asymptomatic bacteriuria is recommended for pregnant women (because the risk of progression to severe symptomatic UTI and possible harm to the fetus), males undergoing transurethral resection of the prostate, and individuals undergoing urologic procedures for which mucosal bleeding is anticipated. In contrast, screening for or treatment of asymptomatic bacteriuria is not recommended for premenopausal, nonpregnant women, diabetic women, older persons living in the community, older institutionalized subjects, persons with spinal cord injury, or catheterized patients while the catheter is in place.

Cystitis

Typically, patients with cystitis (infection of the bladder) complain of dysuria, frequency, and urgency (compelling need to urinate). These symptoms are due not only to inflammation of the bladder but also to multiplication of bacteria in the urine and urethra. Often, there is tenderness and pain over the area of the bladder. In some individuals, the urine is grossly bloody. The patient may note urine cloudiness and a bad odor. Because cystitis is a localized infection, fever and other signs of a systemic (affecting the body as a whole) illness are usually not present.

Acute Urethral Syndrome

Another UTI is acute urethral syndrome. Patients with this syndrome are primarily young, sexually active women, who experience dysuria, frequency, and urgency but yield fewer organisms than 10⁵ colony-forming units of bacteria per milliliter (CFU/mL) urine on culture. (The criterion of greater than 10⁵ CFU/mL of urine is highly indicative of infection in most patients with UTIs.) Almost 50% of all women who seek medical attention for complaints of symptoms of acute cystitis fall into this group. Although Chlamydia trachomatis and N. gonorrhoeae urethritis, anaerobic infection, genital herpes, and vaginitis account for some cases of acute urethral syndrome, most of these women are infected with organisms identical to those that cause cystitis but in numbers less than 10⁵ CFU/mL of urine. One must use a cutoff of 10² CFU/mL, rather than 10⁵ CFU/mL, for this group of patients but must insist on concomitant pyuria (presence of eight or more leukocytes per cubic millimeter on microscopic examination of uncentrifuged urine). Approximately 90% of these women have pyuria, an important discriminatory feature of infection.

Pyelonephritis

Pyelonephritis refers to inflammation of the kidney parenchyma, calices (cup-shaped division of the renal pelvis), and pelvis (upper end of the ureter that is located inside the kidney) and is usually caused by bacterial infection. The typical clinical presentation of an upper urinary tract infection includes fever and flank (lower back) pain and, frequently, lower tract symptoms (frequency, urgency, and dysuria). Patients can also exhibit systemic signs of infection such as vomiting, diarrhea, chills, increased heart rate, and lower abdominal pain. Of significance, 40% of patients with acute pyelonephritis are bacteremic.

Urosepsis

Approximately 25% of sepsis cases (severe blood infection) are a result of urosepsis, a systemic infection that may develop from community- or hospital-acquired urinary tract infections. Early diagnosis and treatment of urinary tract infections are essential in preventing urosepsis.

Laboratory Diagnosis of Urinary Tract Infections

As previously mentioned, because noninvasive methods for collecting urine must rely on a specimen that has passed through a contaminated milieu, quantitative cultures for the diagnosis of UTI are used to discriminate between contamination, colonization, and infection. Refer to Table 5-1 for a quick reference for collecting, transporting, and processing urinary tract specimens.

Specimen Collection

Prevention of contamination by normal vaginal, perineal, and anterior urethral flora is the most important consideration for collection of a clinically relevant urine specimen.

Clean-Catch Midstream Urine

The least invasive procedure, the clean-catch midstream urine specimen collection, must be performed carefully for optimal results, especially in females. Good patient education is essential. Guidelines for proper specimen collection should be prepared on a printed card (bilingual, if necessary), with the procedure clearly described and preferably illustrated to help ensure patient compliance. The patient should be instructed to clean the periurethral area well with a mild detergent to avoid contamination. Of importance, the patient should also be instructed to rinse well because the detergent may be bacteriostatic. Once cleansing is completed, the patient should retract the labial folds or glans penis, begin to void, and then collect a midstream urine sample. Studies showed that uncleansed, first-void specimens from males were as sensitive as (but less specific than) midstream urine specimens.

Straight Catheterized Urine

Although slightly more invasive, urinary catheterization provides a method for the collection of uncontaminated urine from the bladder. Either a physician or another trained health professional performs this procedure. Risk exists, however, that urethral organisms will be introduced into the bladder with the catheter. An example of a collection device to obtain “straight,” or “in and out,” catheterized urine is shown in Figure 73-2.

Figure 73-2 Collection device to obtain urine by “in and out” or “straight” catheterization. (Courtesy Tristate Hospital Supply Corp., Howell, Mich.)

Suprapubic Bladder Aspiration

With suprapubic bladder aspiration, urine is withdrawn directly into a syringe through a percutaneously inserted needle, thereby ensuring a contamination-free specimen. The bladder must be full before performing the procedure. This collection technique may be indicated in certain clinical situations, such as pediatric practice, when urine is difficult to obtain. In brief, the full bladder is punctured using a needle and syringe and sampled following proper skin preparation (antisepsis). If good aseptic techniques are used, this procedure can be performed with little risk in premature infants, infants, small children, and pregnant women and other adults with full bladders.

Indwelling Catheter

The number of patients in hospitals and nursing homes with long-term, indwelling urinary catheters continues to increase. These patients ultimately develop bacteriuria, which predisposes them to more severe infections.⁵ Specimen collection from patients with indwelling catheters requires scrupulous aseptic technique. Health care workers who manipulate a urinary catheter in any way should wear gloves. The catheter tubing should be clamped off above the port to allow the collection of freshly voided urine. The catheter port or wall of the tubing should then be cleaned vigorously with 70% ethanol, and urine aspirated via a needle and syringe; the integrity of the closed drainage system must be maintained to prevent the introduction of organisms into the bladder. Specimens obtained from the collection bag are inappropriate, because organisms can multiply there, obscuring the true relative numbers. Cultures should be obtained when patients are ill; routine monitoring does not yield clinically relevant data.

Specimen Transport

Because it is an excellent supportive medium for growth of most bacteria, urine must be immediately refrigerated or preserved. Bacterial counts in refrigerated (4° C) urine remain constant for as long as 24 hours. Urine transport tubes (BD Urine Culture Kit [Becton Dickinson Vacutainer Kits, Rutherford, New Jersey]) containing boric acid, glycerol, and sodium formate have been shown to preserve bacteria without refrigeration for as long as 24 hours when greater than 10⁵ CFU/mL (100,000 organisms per milliliter) were present in the initial urine specimen. The system may inhibit the growth of certain organisms, and it must be used with a minimum of 3 mL of urine. Another preservative system (Starplex Scientific, Inc., Etobicoke, Cleveland, TN) is also available. Both boric acid products preserve bacterial viability in urine for 24 hours in the absence of antibiotics. For patients from whom colony counts of organisms of less than 100,000/mL might be clinically significant, plating within 2 hours of collection is recommended. The kits provide a convenient method for preserving and transporting urine from remote areas where refrigeration is not practical.

Screening Procedures

As many as 60% to 80% of all urine specimens received for culture by the acute care medical center laboratory may contain no etiologic agents of infection or contain only contaminants. Procedures developed to identify quickly those urine specimens that will be negative on culture and circumvent excessive use of media, technologist time, and the overnight incubation period are discussed in this section. A reliable screening test for the presence or absence of bacteriuria provides physicians important same-day information that a conventional urine culture may take a day or longer to provide. Many screening methods have been advocated for use in detecting bacteriuria and/or pyuria. These include microscopic methods, colorimetric filtration, bioluminescence, electrical impedance, enzymatic methods, photometric detection of growth, and enzyme immunoassay. Because a discussion of all available urine-screening methods is beyond the scope of this chapter, only the more commonly used methods are highlighted.

Gram Stain

A Gram stain of urine is an easy, inexpensive means to provide immediate information as to the nature of the infecting organism (bacteria or yeast) to guide empiric therapy. After a drop of well-mixed urine is allowed to air-dry, the smear is fixed, stained, and examined under oil immersion (1000×) for the presence of 1 or 5 bacteria per oil immersion field (OIF). The performance characteristics of the urine Gram stain are not well defined in that different criteria have been used to define a positive result (1 or 5 bacteria per OIF). Using either 1 or 5 bacteria/OIF has a sensitivity of 96% and 95%, respectively, and a specificity of 91% when correlated with significant bacteriuria (>10⁵ CFU/mL). The Gram stain should not be relied on for detecting polymorphonuclear leukocytes in urine because leukocytes deteriorate quickly in urine that is not fresh or not adequately preserved. Many microbiologists have not adopted Gram stain examination of urine specimens because of its unreliability in detecting lower yet clinically significant numbers of organisms and because of its labor intensity. If employed, urine Gram stain should be limited to patients with acute pyelonephritis, patients with invasive UTIs, or other patients for whom immediate information is necessary for appropriate clinical management.

Pyuria

Pyuria is the hallmark of inflammation, and the presence of polymorphonuclear neutrophils (PMNs) can be detected and enumerated in uncentrifuged specimens. This method of screening urine correlates fairly well with the number of PMNs (neutrophils) excreted per hour, the best indicator of the host’s state. Patients with more than 400,000 PMNs excreted into the urine per hour are likely to be infected, and the presence of more than 8 PMNs/mm³ correlates well with this excretion rate and with infection. This test can be performed using a hemocytometer, but it is not easily incorporated into the workflow of most microbiology laboratories. The standard urinalysis (usually done in hematology or chemistry sections) includes an examination of the centrifuged sediment of urine for enumeration of PMNs, results of which do not correlate well with either the PMN excretion rate or the presence of infection. Pyuria also can be associated with other clinical diseases, such as vaginitis, and therefore is not specific for UTIs.

Indirect Indices

Frequently, screening tests detect bacteriuria or pyuria by examining for the presence of bacterial enzymes or PMN enzymes rather than the organisms or PMNs themselves.

Nitrate Reductase (Greiss) Test.

This screening procedure looks for the presence of urinary nitrite, an indicator of UTI. Nitrate-reducing enzymes that are produced by the most common urinary tract pathogens reduce nitrate to nitrite. This test has been incorporated onto a paper strip that also tests for leukocyte esterase, an enzyme produced by PMNs (discussed next).

Leukocyte Esterase Test.

As previously mentioned, evidence of a host response to infection is the presence of PMNs in the urine. Because inflammatory cells produce leukocyte esterase, a simple, inexpensive, and rapid method that measures this enzyme has been developed. Studies have shown that leukocyte esterase activity correlates with hemocytometer chamber counts. The nitrate reductase and leukocyte esterase tests have been incorporated into a paper strip. Numerous manufacturers sell these strips commercially, and the strips are one of the most widely used enzymatic tests. Although the sensitivity of the combination strip is higher than either test alone, the sensitivity of this combination screening is not great enough to recommend its use as a stand-alone test in most circumstances. Of note, the leukocyte esterase test is not sensitive enough for determining pyuria in patients with acute urethral syndrome.

Catalase.

The Accutest Uriscreen (JANT Pharmacal Crop., Encino, Calif.) is another rapid urine-screening system based on the detection of catalase present in somatic (pertaining to the body) cells and in most bacterial species commonly causing UTIs except for streptococci and enterococci. Approximately 1.5 to 2 mL of urine is added to a tube containing dehydrated substrate. Hydrogen peroxide is added to the urine, and the solution is mixed gently. The formation of bubbles above the liquid surface is interpreted as a positive test. Some studies have reported that this system does not offer significant advantages over the leukocyte esterase-nitrite strip.

Automated and Semiautomated Systems

Automated screening systems offer the promise of a large throughput with minimal labor and a rapid turnaround time compared with conventional cultures. However, these advantages may be offset by a substantial cost for the instrumentation. Often these costs can be justified only in laboratories that receive many specimens.

Various automated or semi-automated urine-screening systems are commercially available, such as the iRIcell Systems (IRIS International, Inc., Chatsworth, Calif.) and are capable of analyzing a urine or body fluid sample in one instrument. The instrument analyzes both the microscopic components and the urine chemistries by combining technology of both types of analyzers into one automated system. The Sysmex UF-100 (TOA Medical Electronics; Kobe, Japan) are able to recognize many cellular structures, including leukocytes and bacteria.

General Comments Regarding Screening Procedures

In general, screening methods are insensitive at levels below 10⁵ CFU/mL. Therefore, they are not acceptable for urine specimens collected by suprapubic aspiration, catheterization, or cystoscopy. Screening methods may also fail to detect a significant number of infections in symptomatic patients with low colony counts (10² to 10³ CFU/mL) such as young, sexually active females with acute urethral syndrome. Further complicating the laboratory’s decision as to whether to adopt a screening method is whether screening results will be used to rule out infection in asymptomatic patients. Under these circumstances, testing for pyuria is essential.

Therefore, given the importance of the 10² CFU/mL count and the PMN count, no screening test should be used indiscriminantly. Selecting a screening method largely depends on the laboratory and the patient population being served by the laboratory. For example, there will be a cost advantage in screening urine in laboratories that receive many culture-negative specimens. On the other hand, urine from patients with symptoms of UTI plus a selected group expected to have asymptomatic bacteriuria should be cultured. For example, patients in their first trimester of pregnancy should be cultured because these women might appear asymptomatic but have a covert infection and become symptomatic later; UTIs in pregnant women may lead to pyelonephritis and the likelihood of a premature birth. Other situations in which patients with no symptoms of UTI might be cultured include the following:

• Bacteremia of unknown source

• Urinary tract obstruction

• Follow-up after removal of an indwelling catheter

• Follow-up of previous therapy

Other factors that must be considered when selecting a rapid urine screen include accuracy, ease of test performance, reproducibility, turnaround time, and whether bacteriuria or pyuria is detected.

Urine Culture

Inoculation and Incubation of Urine Cultures

Once it has been determined that a urine specimen should be cultured for isolation of the common agents of UTI, a measured amount of urine is inoculated to each of the appropriate media. The urine should be mixed thoroughly before plating. The plates can be inoculated using disposable sterile plastic tips with a displacement pipetting device calibrated to deliver a constant amount, but this method is somewhat cumbersome. Most often, microbiologists use a calibrated loop designed to deliver a known volume, either 0.01 or 0.001 mL of urine. These loops, made of platinum, plastic, or other material, can be obtained from laboratory supply companies.

The calibrated loop that delivers the larger volume of urine (0.01 mL) is recommended to detect lower numbers of organisms in certain specimens. For example, urine collected from catheterization, nephrostomies, ileal conduits, and suprapubic aspirates should be plated with the larger calibrated loop. The communication of pertinent clinical history to the laboratory is essential so that appropriate processing can be performed.

The choice of media to inoculate depends on the patient population served and the microbiologist’s preference. The use of a 5% sheep blood agar plate and a MacConkey agar plate allows detection of most gram-negative bacilli, staphylococci, streptococci, and enterococci. To save cost and somewhat streamline culture processing, many laboratories use an agar plate split in half (biplate); one side contains 5% sheep blood agar and the other half contains MacConkey agar.

In some circumstances, enterococci and other streptococci may be obscured by heavy growth of Enterobacteriaceae. Because of this possibility, some laboratories add a selective plate for gram-positive organisms, such as Columbia colistin-nalidixic acid agar (CNA) or phenylethyl alcohol agar. Although some discriminatory capability may be added, cost is also added to the procedure. In addition to increased cost, inclusion of plated media selective for gram-positive organisms generally provides no or limited additional information. Many European laboratories use cystine-lactose electrolyte-deficient (CLED) agar. In recent years, chromogenic media have been introduced and become commercially available from a number of manufacturers, allowing for more specific direct detection and differentiation of urinary tract pathogens on primary plates, such as BD CHROMagar (Becton Dickison, Heidelberg, Germany). This medium uses enzymatic reactions to identify E. coli and Enterococcus without additional confirmatory testing from urine specimens as well as providing presumptive identification of S. saprophyticus, Streptococcus agalactiae, Klebsiella-Enterobacter-Serratia and the Proteus-Morganella-Providencia groups.

Before inoculation, urine is mixed thoroughly and the top of the container is then removed. The calibrated loop is inserted vertically into the urine in a cup. Otherwise, more than the desired volume of urine will be taken up, potentially affecting the quantitative culture result (Figure 73-3). A widely used method is described in Procedure 73-1, which can be found on the Evolve site. If the urine is in a small-diameter tube, the surface tension will alter the amount of specimen picked up by the loop. A quantitative pipette should be considered if the urine cannot be transferred to a larger container. Once inoculated, the plates are streaked to obtain isolated colonies (Figure 73-4).

Figure 73-3 Method for inserting a calibrated loop into urine to ensure that the proper amount of specimen adheres to the loop.

Figure 73-4 Method for streaking with calibrated urine loop to produce isolated colonies and countable colony-forming units.

Procedure 73-1 Inoculating Urine with a Calibrated Loop

Purpose

Quantitate the number of organisms and amount of each present in a urine specimen to determine the clinical significance of growth and organisms present.

Principle

The number of microorganisms per milliliter recovered on urine culture can aid in the differential diagnosis of UTI. Plastic or wire loops, available commercially, have been calibrated to deliver a known volume of liquid when handled correctly, thus enabling the microbiologist to estimate numbers of organisms in the original specimen based on CFU of growth on cultures.

Method

1. Flame a calibrated wire-inoculating loop and allow it to cool without touching any surface. Alternatively, aseptically remove a plastic calibrated loop from its package.

2. Mix the urine thoroughly and remove the top of the container. If the urine is in a small-diameter tube, the surface tension will alter the amount of specimen picked up by the loop. A quantitative pipette should be considered if the urine cannot be transferred to a larger container.

3. Insert the loop vertically into the urine (see Figure 73-3) to allow urine to adhere to the loop.

4. Spread the loopful of urine over the surface of the agar plate, as shown in Figure 73-4. A standard quadrant streaking technique is also acceptable.

5. Without reflaming, insert the loop vertically into the urine again for transfer of a loopful to a second plate. Repeat for each plate.

6. Incubate plates for at least 24 hours at 35° to 37° C in air. Colonies are counted on each plate. The number of CFUs is multiplied by 1000 (if a 0.001-mL loop was used) or by 100 (if a 0.01-mL loop was used) to determine the number of microorganisms per milliliter in the original specimen.

7. Because antimicrobial treatment or other factors may inhibit initial growth, reincubate plates with no growth or tiny colonies for an additional 24 hours before discarding plates.

8. To store the inoculating loop, place (handle down) in a test tube taped to the wall, rather than flat on the bench, to prevent bending, which would destroy the calibration.

Interpretation and Results

	Midstream or Catheter Specimen	Sterile Specimen (Suprapubic or Cytoscopy)
One organism isolated*	<10⁷ CFUs Growth insignificant >10⁷ CFUs Identification and sensitivity testing	Identification and sensitivity testing of any growth
Two organisms isolated >10⁷ CFUs	Identification and sensitivity testing of both organisms	Identification and sensitivity testing of any growth
Two organisms with one predominant isolate	Identification and sensitivity testing of predominant organism	Identification and sensitivity testing of any growth
Three or more organisms present	Mixed growth—report as probable contamination	Identification and sensitivity testing of any growth

Note: Some exceptions requiring the identification and sensitivity on small quantities of a single isolate may include symptomatic patients on antimicrobial therapy, patients with chronic infections, or young children.

Once plated, urine cultures are incubated overnight at 35° C. For the most part, incubation for a minimum of 24 hours is necessary to detect uropathogens. Thus, some specimens inoculated late in the day cannot be read accurately the next morning. These cultures should either be reincubated until the next day or interpreted later in the day when a full 24-hour incubation has been completed.

Interpretation of Urine Cultures

As previously mentioned, UTIs may be completely asymptomatic, produce mild symptoms, or cause life-threatening infections. Of importance, the criteria most useful for microbiologic assessment of urine specimens is dependent not only on the type of urine submitted (e.g., voided, straight catheterization) but the clinical history of the patient (e.g., age, sex, symptoms, antibiotic therapy).

One major problem in interpreting urine cultures arises because urine cultures collected by the voided technique may be contaminated with normal flora, including Enterobacteriaceae. Determining what colony count represents true infection from contamination is of utmost importance and is related to the patient’s clinical presentation. A number of studies have proposed the use of different cutoffs in colony counts based on clinical presentation; an example of one such set of guidelines is given in Table 73-1.

TABLE 73-1

Criteria for Classification of Urinary Tract Infections by Clinical Syndrome

Category	Clinical	Laboratory
Acute, uncomplicated UTI in women	Dysuria, urgency, frequency, suprapubic pain No urinary symptoms in last 4 weeks before current episode No fever or flank pain	≥10 WBC/mm³ ≥10³ CFU/mL uropathogens* in CCMS urine
Acute, uncomplicated pyelonephritis	Fever, chills Flank pain on examination Other diagnoses excluded No history or clinical evidence of urologic abnormalities	≥10 WBC/mm³ ≥10⁴ CFU/mL uropathogens in CCMS urine
Complicated UTI and UTI in men	Any combination of symptoms listed above One or more factors associated with complicated UTI^†	≥10 WBC/mm³ ≥10⁵ CFU/mL uropathogens in CCMS urine
Asymptomatic bacteriuria	No urinary symptoms	± >10 WBC/mm³ ≥10⁵ CFU/mL in two CCMS cultures >24 hours apart

UTI, Urinary tract infection; WBC, white blood cells; CFU, colony-forming unit; CCMS, clean-catch midstream urine.

^*Uropathogens: Organisms that commonly cause UTIs.

^†Factors associated with complicated UTI include any UTI in a male, indwelling or intermittent urinary catheter, >100 mL of postvoid residual urine, obstructive uropathy, urologic abnormalities, azotemia (excess urea in the blood, even without structural abnormalities), and renal transplantation.

From Stamm WE: Criteria for the diagnosis of urinary tract infection and for the assessment of therapeutic effectiveness, Infection 20 (suppl 3):S151, 1992.

Ideally, the clinician caring for the patient should provide the laboratory with enough clinical information to allow specimens from different patient populations to be identified. These specimens could then be selectively processed using the guidelines in Table 73-1. However, because microbiology laboratories frequently receive little or no clinical information about patients, questions have been raised as to whether these cutoffs are practical and realistic for routine laboratory use. Further complicating urine culture interpretation is the increasing difficulty in distinguishing between infection and contamination as the criterion for a positive culture is lowered from 10⁵ CFU/mL to 10² CFU/mL. Because of these issues, many laboratories establish their own interpretative criteria for urine cultures based on the type of urine submitted (e.g., clean-catch midstream, catheterized, and surgically obtained specimens such as suprapubic aspirates). Variations in interpretative guidelines occur from one laboratory to another but some generalities can be made; these are listed in Table 73-2. Some examples of urine culture results are shown in Figure 73-5 to illustrate some of these interpretations. See the Evolve site for a semi-quantitative procedure for inoculation of urine cultures.

TABLE 73-2

General Interpretative Guidelines for Urine Cultures

Result	Specific Specimen Type/Associated Clinical Condition, if Known	Workup
≥10⁴ CFU/mL of a single potential pathogen or for each of two potential pathogens	CCMS urine/pyelonephritis, acute cystitis, asymptomatic bacteriuria, or catheterized urines	Complete*
≥10³ CFU/mL of a single potential pathogen	CCMS urine/symptomatic males or catheterized urines or acute urethral syndrome	Complete
≥Three organism types with no predominating organism	CCMS urine or catheterized urines	None; because of possible contamination, ask for another specimen
Either two or three organism types with predominant growth of one organism type and <10⁴ CFU/mL of the other organism type(s)	CCMS urine	Complete workup for the predominating^† organism(s); description of the organism(s)
≥10² CFU/mL of any number of organism types (set up with a 0.001- and 0.01-mL calibrated loop)	Suprapubic aspirates, any other surgically obtained urines (including ileal conduits, cystoscopy specimens)	Complete