SEQUENTIAL CHANGES IN RENAL FUNCTION TESTS IN THE OLD‐OLD: RESULTS FROM THE BRONX LONGITUDINAL AGING STUDY

Abstract

To the Editor: We read with interest the article by Feinfeld et al.1 with respect to sequential changes in renal function tests in the old-old. We find somewhat misleading using serial serum creatinine measurements without measuring creatinine clearance in order to estimate changes in glomerular function with age. Normal aging is associated with a decline in renal creatinine excretion.2, 3 For example, creatinine excretion in a 70-kg man would decline, on average, from 1600 to 1000 mg/day with advancing age from 30 to 80 years. Possible explanations for the decline in creatinine excretion with aging include decreased creatinine generation attributable to reduced muscle mass or to reduced meat intake. In Feinfeld's study,1 the author's contention that the subjects’ muscle mass did not change is unjustified, based on a stable BMI. BMI is an index of total body weight. However, the ratio of muscle mass to total body weight decreases with age as a result of a relative increase in body fat.4 Therefore, the decline in serum creatinine in those subjects may well have resulted from a decrease in muscle mass and not from stable or improved renal function. Editors note: The letters of Dr. Drinka and of Drs. Marcus and Slotki were referred to the authors of the original article, and their reply follows.