Urinalysis

Published on 01/03/2015 by admin

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Last modified 22/04/2025

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Urinalysis

Urinalysis is so important in screening for disease that it is regarded as an integral part of the complete physical examination of every patient, and not just in the investigation of renal disease. Urinalysis comprises a range of analyses that are usually performed at the point of care rather than in a central laboratory. Examination of a patient’s urine should not be restricted to biochemical tests. Figure 16.1 summarizes the different ways urine may be examined.

Fig 16.1 The place of biochemical testing in urinalysis.

Procedure

Biochemical testing of urine involves the use of commercially available disposable strips (Fig 16.2). Each strip is impregnated with a number of coloured reagent ‘blocks’ separated from each other by narrow bands. When the strip is manually immersed in the urine specimen, the reagents in each block react with a specific component of urine in such a way that (a) the block changes colour if the component is present, and (b) the colour change produced is proportional to the concentration of the component being tested for.

Fig 16.2 Multistix testing of a urine sample: (a) Immersion of test strip in urine specimen. (b) Excess urine removed. (c) Test strip is compared with colour chart on bottle label.

To test a urine sample:

fresh urine is collected into a clean dry container

the sample is not centrifuged

the disposable strip is briefly immersed in the urine specimen; care must be taken to ensure that all reagent blocks are covered

the edge of the strip is held against the rim of the urine container to remove any excess urine

the strip is then held in a horizontal position for a fixed length of time that varies from 30 seconds to 2 minutes

the colour of the test areas are compared with those provided on a colour chart (Fig 16.2). The strip is held close to the colour blocks on the chart and matched carefully, and then discarded.

The range of components routinely tested for in commonly available commercial urinalysis strips is extensive and includes glucose, bilirubin, ketones, specific gravity, blood, pH (hydrogen ion concentration), protein, urobilinogen, nitrite and leucocytes (white blood cells).

Urinalysis is one of the commonest biochemical tests performed outside the laboratory. It is most commonly performed by non-laboratory staff. Although the test is simple, failure to follow the correct procedure may lead to inaccurate results. A frequent example of this is where test strips are read too quickly or left too long. Other potential errors may arise because test strips have been stored wrongly or are out of date.

Glucose

The presence of glucose in urine (glycosuria) indicates that the filtered load of glucose exceeds the ability of the renal tubules to reabsorb all of it. This usually reflects hyperglycaemia and should, therefore, prompt consideration of whether more formal testing for diabetes mellitus is appropriate, e.g. by measuring fasting blood glucose. However, glycosuria is not always due to diabetes. The renal threshold for glucose may be lowered, for example in pregnancy, and glucose may enter the filtrate even at normal plasma concentrations (renal glycosuria).

Blood glucose rises rapidly after a meal, overcoming the normal renal threshold temporarily (alimentary glycosuria). Both renal and alimentary glycosuria are unrelated to diabetes.

Bilirubin

Bilirubin exists in the blood in two forms, conjugated and unconjugated. Only the conjugated form is water-soluble, so bilirubinuria signifies the presence in urine of conjugated bilirubin. This is always pathological. Conjugated bilirubin is normally excreted through the biliary tree into the gut where it is broken down; a small amount is reabsorbed into the portal circulation, taken up by the liver and re-excreted in bile. Interruption of this so-called enterohepatic circulation usually stems from mechanical obstruction, and results in high levels of conjugated bilirubin in the systemic circulation, some of which spills over into the urine.

Urobilinogen

In the gut, conjugated bilirubin is broken down by bacteria to products known collectively as faecal urobilinogen, or stercobilinogen. This too undergoes an enterohepatic circulation. However, unlike bilirubin, urobilinogen is found in the systemic circulation and is often detectable in the urine of normal subjects. Thus the finding of urobilinogen in urine is of less diagnostic significance than bilirubin. High levels are found in any condition where bilirubin turnover is increased, e.g. haemolysis, or where its enterohepatic circulation is interrupted, e.g. by liver damage.

Ketones

Ketones are the products of fatty acid breakdown. Their presence usually indicates that the body is using fat to provide energy rather than storing it for later use. This can occur in uncontrolled diabetes, where glucose is unable to enter cells (diabetic ketoacidosis), in alcoholism (alcoholic ketoacidosis), or in association with prolonged fasting or vomiting.

Specific gravity

This is a semi-quantitative measure of urinary density, which in turn reflects concentration. A higher specific gravity indicates a more concentrated urine. Assessment of urinary specific gravity usually just confirms the impression gained by visually inspecting the colour of the urine. When urine concentration needs to be quantitated, most people will request urine osmolality, which has a much wider working range.

pH (hydrogen ion concentration)

Urine is usually acidic (urine pH substantially less than 7.4 indicating a high concentration of hydrogen ions). Measurement of urine pH is useful either in cases of suspected adulteration, e.g. drug abuse screens, or where there is an unexplained metabolic acidosis (low serum bicarbonate). The renal tubules normally excrete hydrogen ions by mechanisms that ensure tight regulation of the blood hydrogen ion concentration. Where one or more of these mechanisms fail, an acidosis results (so-called renal tubular acidosis or RTA; see p. 30). Measurement of urine pH may, therefore, be used to screen for RTA in unexplained metabolic acidosis; a pH less than 5.3 indicates that the renal tubules are able to acidify urine and are, therefore, unlikely to be responsible.

Clinical note

Microbiological testing of a urine specimen (usually a mid-stream specimen or MSSU) is routinely performed to confirm the diagnosis of a urinary tract infection. These samples should be collected into sterile containers and sent to the laboratory without delay for culture and antibiotic sensitivity tests.

Protein

Proteinuria may signify abnormal excretion of protein by the kidneys (due either to abnormally ‘leaky’ glomeruli or to the inability of the tubules to reabsorb protein normally), or it may simply reflect the presence in the urine of cells or blood. For this reason it is important to check that the dipstick test is not also positive for blood or leucocytes (white cells); it may also be appropriate to screen for a urinary tract infection by sending urine for culture. Proteinuria and its causes are discussed in detail on pages 34–35.

Blood

The presence of blood in the urine (haematuria) is consistent with various possibilities ranging from malignancy through urinary tract infection to contamination from menstruation. Dipstick tests for blood are able to detect haemoglobin and myoglobin in addition to red blood cells – the presence in the urine sediment of large numbers of red cells establishes the diagnosis of haematuria. The absence of red cells, despite a strongly positive dipstick test for blood, points towards myoglobinuria or haemoglobinuria.

Nitrite

This dipstick test depends on the conversion of nitrate (from the diet) to nitrite by the action in the urine of bacteria that contain the necessary reductase. A positive result points towards a urinary tract infection.

Leucocytes

The presence of leucocytes in the urine suggests acute inflammation and the presence of a urinary tract infection.

Case history 11

A patient attending an obesity clinic is found to have ketonuria on urinalysis. There is no glycosuria and point-of-care glucose measurement using a strip test is 5.9 mol/L.

• What might explain these findings?

Comment on page 165.

Urinalysis

Urinalysis should be part of the clinical examination of every patient.

Chemical analysis of a urine specimen is carried out using commercially available disposable strips.

The range of components routinely tested for includes glucose, bilirubin, ketones, specific gravity, blood, pH, protein, urobilinogen, nitrite and leucocytes.

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