Prerenal azotemia

The serum BUN and creatinine are both elevated (results repeated below), but the BUN is elevated to a much greater extent than the creatinine. Remember that BUN that is filtered at the glomerulus is reabsorbed in the renal tubules in a process that is dependent on the rate of flow of fluid through the tubules (lower flow rate = more reabsorption = higher serum BUN). Creatinine is not reabsorbed in this way. Thus an elevated BUN/creatinine ratio (>20) suggests that low flow of fluid through the nephrons is the basis for the elevation in BUN and, to a lesser extent, creatinine.

Low flow states may be produced by hypotension, hypovolemia, or blockage of blood flow to the kidneys. These conditions effect blood flow before the blood reaches the kidney; hence the term, prerenal azotemia.

Creatinine (S)          1.8 mg/dl                0.7 - 1.2
Urea nitrogen (S)        52 mg/dl                   7 - 18
BUN/creatinine ratio     29:1                  12:1 - 20:1

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