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The underlying cause of impaired kidney function may be glomerular dysfunction, tubular dysfunction, or urinary tract obstruction. Accurate clinical assessment of kidney function is often difficult and relies heavily on clinical laboratory determinations of glomerular filtration rate (GFR), including creatinine clearance, and other evaluations. The traditional diagnosis of AKI, based upon serum creatinine and urine output, has been refined into an increase of serum creatinine of 0.3 mg/dL or more within 48 h or a 1.5-fold or greater increase in baseline within 7 days. Since AKI is a systemic disorder, it is important to recall that the kidney excretory function assessed via serum creatinine and urine output ignores endocrine, metabolic, and immunological kidney functions.

1. Blood Urea Nitrogen

Blood urea nitrogen (BUN) is directly related to protein catabolism and inversely related to glomerular filtration. As a result, BUN is not a reliable indicator of the GFR unless protein catabolism is normal and constant.

The normal BUN concentration is 10–20 mg/dL. Lower values may be seen with starvation or liver disease; elevations usually result from decreases in GFR or increases in protein catabolism. The latter may be due to a high catabolic state (trauma or sepsis), degradation of blood either in the gastrointestinal tract or in a large hematoma, or a high-protein diet. BUN concentrations greater than 50 mg/dL are generally associated with impaired kidney function.

2. Serum Creatinine

Creatine is a product of muscle metabolism that is nonenzymatically converted to creatinine. Daily creatinine production in most people is relatively constant and related to muscle mass, averaging 20–25 mg/kg in men and 15–20 mg/kg in women. Creatinine is then filtered (and to a minor extent secreted) but not reabsorbed in the kidneys. Serum creatinine concentration is therefore directly related to body muscle mass and inversely related to glomerular filtration. Because body muscle mass is usually relatively constant, serum creatinine measurements are generally reliable indices of GFR in the ambulatory patient. However, the utility of a single serum creatinine measurement as an indicator of GFR is limited in critical illness: The rate of creatinine production and its volume of distribution is frequently abnormal in the critically ill patient, and a single serum creatinine measurement often will not accurately reflect GFR in the physiological disequilibrium of AKI.

The normal serum creatinine concentration is 0.8–1.3 mg/dL in men and 0.6–1 mg/dL in women. Each doubling of the serum creatinine represents a 50% reduction in GFR. As previously noted, many factors may affect serum creatinine measurement.

GFR declines with increasing age in most individuals (5% per decade after age 20), but because muscle mass also declines, the serum creatinine remains relatively normal; creatinine production may decrease to 10 mg/kg. Thus, in older adult patients, small increases in serum ...

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