Localization of azotemia: Test your skill

Feb 01, 2005

Azotemia is defined as an abnormal concentration of urea, creatinine, and other nonprotein nitrogenous substances in blood, plasma or serum.

Azotemia is a laboratory finding with several fundamentally different causes. Since nonprotein nitrogenous compounds (including urea and creatinine) are endogenous substances, abnormal concentrations in serum may be caused by

  • an altered rate of production (by the liver for urea; by muscles for creatinine), or by
  • an altered rate of loss (primarily by the kidneys). Assuming that the rates of production are constant, determination of serum urea nitrogen (SUN) and serum creatinine (SC) concentrations is commonly used to assess kidney function (specifically glomerular filtration rate).

However, azotemia may be caused by factors that are not directly related to the urinary system and by abnormalities of the lower urinary tract not directly related to the kidney. Therefore, azotemia should not be used as a synonym for primary renal failure or uremia.

Although the concentrations of serum urea nitrogen and creatinine commonly are used as crude indices of glomerular filtration rate, meaningful interpretation of these endogenous markers depends on recognition and evaluation of prerenal, primary renal, and post-renal factors that influence their rate of production and their rate of loss. In addition, knowledge of urine specific gravity values is of great significance in localizing different types of azotemia. If sufficient clinical evidence is present to warrant examination of a patient's renal function by determining the serum concentration of creatinine or urea nitrogen, then the pretreatment specific gravity of urine should be routinely evaluated at the same time.

Table 1: Can you localize the azotemia? (Interpretations at end of discussion)
Test your skill Table 1 provides a summary of findings observed in azotemic dogs and cats with a variety of different diseases. Based on the information provided, would you localize the type of azotemia in each patient as prerenal, primary renal and/or post renal?

My interpretations are summarized at the end of this Diagnote. The following discussion summarizes basic mechanisms associated with different types and combinations of azotemia associated with impaired excretion of urea and creatinine. It does not include non-urinary factors that may result in mild increases in the rate of production of these metabolites (consumption of high-protein diets, gastrointestinal hemorrhage).

Table 2: Species variation in maximum functionally adequate and inappropriate urine concentration of adult animals as evaluated by specific gravity
Normal values See Table 2 for urine specific gravity reference ranges. The normal reference range for serum urea nitrogen concentration in cats is approximately 14 to 34 mg/dl; it is approximately 10 to 30 mg/dl in dogs. The normal reference range for serum creatinine concentration in dogs and cats is approximately 0.6 to 1.5 mg/dl. (Note: In practice, it is usually best to use normal reference ranges that have been derived for the laboratory providing your hospital with test results.)