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A midarm muscle circumference (AMC) < 10th percentile indicates severe protein-energy malnutrition. We identified 20 consecutive patients receiving specialized nutritional therapy from the Nutrition Support Service of the New England Deaconess Hospital during a 3-mo period who had AMC < 10th percentile and 24-h urine and serum creatinine determinations performed. Mean weight was 96 +/- 25% of ideal body weight (IBW), with only 7 patients < 85% IBW, reaffirming that weight is often insensitive for the identification of lean tissue loss. Twenty-four-hour urine creatinine excretion confirmed most to be severely lean body mass depleted (creatinine-height index < 60% in 17 subjects) as predicted by AMC. Sixteen patients had normal kidney function as assessed by serum creatinine levels and hospital norms (0.3-1.2 mg/dl). However, abnormal creatinine clearances were found in 15 of 20 subjects (normal 80-100 ml/min), with a mean value (53 +/- 23 ml/min) also below normal. These results suggest that the usual normal range for serum creatinine, based on well-nourished patients, should be adjusted in severely malnourished patients. Considering the upper limit of normal serum creatinine in this population to be 0.7 mg/dl, to reflect the approximately 60% reduction in lean tissue indicated by an AMC < or = 10th percentile, would identify all but 1 patient with an abnormal creatinine clearance (sensitivity 93%, specificity 67%). Use of a predictive formula (the Cockcroft-Gault equation) also identified those with kidney dysfunction but overestimated actual function to a level not usually requiring drug dose adjustment and did not obviate the need for further testing.(ABSTRACT TRUNCATED AT 250 WORDS)