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Azotemia is the retention of nitrogenous waste products excreted by the kidney. Increased levels of blood urea nitrogen (BUN) (>10.7 mmol/L [>30 mg/dL]) and creatinine (>133 µmol/L [>1.5 mg/dL]) are ordinarily indicative of impaired renal function. Renal function can be estimated by determining the clearance of creatinine (CLcr) (normal >100 mL/min); this can be directly measured from a 24-h urine collection using the following equation:

Creatinine clearance (mL/min) = (uCr × uV)/(sCr × 1440)

  1. Where uCr is urine creatinine in mg/dL

  2. Where sCr is serum creatinine in mg/dL

  3. Where uV is 24-h urine volume in mL

  4. Where 1440 represents number of minutes in 24 h

The “adequacy” or “completeness” of the collection is estimated by the urinary volume and creatinine content; creatinine is produced from muscle and excreted at a relatively constant rate. For a 20- to 50-year-old man, creatinine excretion should be 18.5–25.0 mg/kg body weight; for a woman of the same age, it should be 16.5–22.4 mg/kg body weight. For example, an 80-kg man should excrete between ∼1500 and 2000 mg of creatinine in an “adequate” collection. Creatinine excretion is also influenced by age and muscle mass. Notably, creatinine is an imperfect measure of glomerular filtration rate (GFR), because it is both filtered by glomeruli and secreted by proximal tubular cells; the relative contribution of tubular secretion increases with advancing renal dysfunction, such that creatinine clearance will provide an overestimate of the “true” GFR in pts with chronic kidney disease. Isotopic markers that are filtered and not secreted (e.g., iothalamate) provide more accurate estimates of GFR.

A formula that allows for an estimate of creatinine clearance in men that accounts for age-related decreases in GFR, body weight, and sex has been derived by Cockcroft-Gault:

Creatinine clearance (mL/min) = (140-age) × lean body weight (kg)/plasma creatinine (mg/dL) × 72

This value should be multiplied by 0.85 for women.

GFR may also be estimated using serum creatinine–based equations derived from the Modification of Diet in Renal Disease Study. This “eGFR” (estimated glomerular filtration rate) is now reported with serum creatinine by most clinical laboratories in the United States and is the basis for the National Kidney Foundation classification of chronic kidney disease (Table 48-1).

TABLE 48-1The Classification of Chronic Kidney Disease

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