The glomerular filtration rate

The GFR is the amount of plasma filtered through glomeruli per unit of time and is the single best index of functioning renal mass. Inulin is a sugar that is completely filtered by the glomerulus but is neither secreted nor reabsorbed by the tubule. Thus, the volume of intravenously administered inulin cleared from the plasma can be used to calculate the GFR. However, inulin clearance is seldom used clinically because the assay is cumbersome and time consuming to perform. Creatinine (Cr) is a metabolic end product of creatine phosphate in skeletal muscle that is cleared by the kidney in a manner similar to that of inulin. Creatinine clearance (CrCl), the most clinically useful measure of GFR, is estimated using the following formula:

< ?xml:namespace prefix = "mml" />CrCl = (140–age)(body weight in kg)(serum creatinine)(72)

More precise measurement requires timed collections (over a period of 24 h) of urine and plasma samples and requires use of the following formula:

CrCl=(UCr×V)PCr

where U is the urinary Cr concentration in mg/dL, V is the volume of urine in mL/min, and P is the plasma Cr concentration.

Normal GFR is 120 ± 25 mL/min in men and 95 ± 20 mL/min in women. Mild, moderate, and severe impairment have corresponding values of approximately 40 to 60 mL/min, 20 to 40 mL/min, and less than 20 mL/min, respectively. Serial GFR measurements are important in determining the severity of renal dysfunction, as well as in monitoring disease progression.

Measuring the urinary sodium concentration (U_Na+) is useful in assessing volume status. A U_Na+ concentration of less than 20 mEq/L suggests intravascular volume depletion, whereas a U_Na+ concentration of more than 40 mEq/L suggests a decreased ability of the renal tubules to reabsorb sodium (e.g., acute tubular necrosis). Fractional excretion of sodium (FE_Na+) reflects renal tubular sodium reabsorption. FE_Na+ describes sodium clearance as a percentage of CrCl:

FENa+=(sodium clearance/CrCl)100%

An FE_Na+ less than 1% is seen in patients with a normal volume status or who are hypovolemic; an FE_Na+ greater than 1% indicates tubular damage (e.g., acute tubular necrosis).

Urinalysis is a useful noninvasive diagnostic tool available to assess renal function. Testing includes visual inspection (Table 46-1); dipstick determination of pH (normal 4.5-8.0), blood, glucose, and protein; specific gravity (normal 1.003-1.030); and examination of urinary sediment. In patients with porphyria, urine is of normal color when fresh, but discolors over time when exposed to light (which is a pathognomonic observation). The pH is rarely diagnostic, but in conjunction with serum pH and bicarbonate values, it is useful in evaluating renal tubule acidification function. Dipstick determinations register glucose, but not other reducing sugars; elevations in urine glucose concentrations suggest a diagnosis of hyperglycemia or a tubule defect (e.g., Fanconi syndrome or isolated glycosuria). When the plasma glucose concentration is 180 mg/dL or less, all of the glucose filtered by glomeruli is reabsorbed in the proximal tubules. Dipstick determination of glucose crudely indicates a blood glucose concentration of at least 230 mg/dL.

The presence of hemoglobin in the urine can be detected by a dipstick because peroxidase catalyzes the reaction of peroxide and chromogen to produce a colorimetric change. False-positive results may occur if the patient has myoglobin in the urine. Hemoglobin and myoglobin can be distinguished by dissolving 2.8 g of ammonium sulfate in 5 mL of urine, causing hemoglobin to precipitate. The two pigments can also be distinguished by spectrophotometry, electrophoresis, immunochemical methods, or looking for the presence or absence of red blood cells on microscopic examination. Protein testing is not sensitive for albumin on dipstick analysis.