Evaluation of blood urea nitrogen and serum creatinine concentrations as indicators of renal dysfunction: a study of 111 cases and a review of related literature.

Blood urea nitrogen concentration (BUN), serum creatinine concentration (SC), and BUN/SC ratios, as recorded for 111 dogs and cats with azotemia, were evaluated to determine their usefulness in evaluation of renal dysfunction. Cases were categorized into prerenal, renal, and post-renal causes of azotemia, on the basis of histologic and clinical criteria. The severity of azotemia varied within groups, but the mean value for degree of azotemia was lowest in the prerenal group. The BUN/SC ratios were highest when azotemia was mild, regardless of cause. The BUN/SC ratios in the 3 groups were not significantly different when the degree of azotemia was considered, indicating that differentiation of renal from extrarenal azotemia was not possible in the dog and cat, on the basis of relative values of BUN and SC. Comparison of BUN/SC ratios in acute and chronic azotemia of comparable magnitude revealed no significant differences between groups, indicating that this ratio cannot be used to differentiate acute and chronic azotemia. Following supportive therapy, BUN decreased significantly (P less than 0.05) more than SC, suggesting that extrarenal factors contributed to the increased BUN and that proportionately more urea than creatinine was excreted by the kidneys during therapy. The BUN/SC ratios varied widely and to a similar degree in all groups examined. Inasmuch as the BUN and SC were not correlated with more accurate measurements of renal function, conclusions could not be drawn concerning the superiority of either factor as a measure of renal function. Many nonrenal factors, previously identified, influence serum concentration of urea and creatinine. In recent studies involving azotemic human beings and rats, it was suggested that up to 25% of the urea and 65% of the creatinine produced in the body is degraded by enteric bacteria rather than excreted directly by the kidneys. These data indicate that neither BUN or SC can be used as precise tests of renal function, although SC is subject to alteration by fewer nonrenal factors than is BUN. In older medical and veterinary medical literature, the use of SC as a prognostic indicator had been advocated. Newer findings on the pathophysiology of creatininemia and retrospective case studies do not support this view. In the present study, severe creatininemia was documented in cases in which primary renal dysfunction did not exist. It was concluded that BUN and SC should continue to be regarded as crude indexes of renal function. Clinical value lies in the relative ease of their determination. Because of their lack of sensitivity, more specific evaluation of renal function (urine concentrating ability, phenolsulfonphthalein excretion) may be indicated when BUN and SC are normal or only slightly elevated. Because extrarenal factors may alter BUN and SC, it is necessary to correlate these values with clinical and other laboratory data to differentiate renal from extra-renal azotemia. Single determinations of BUN or SC provide no basis for prognosis.