GERIATRIC RENAL-FUNCTION - ESTIMATING GLOMERULAR-FILTRATION IN AN AMBULATORY ELDERLY POPULATION

In elderly individuals, serum creatinine may remain normal as glomerul ar filtration rate (gfr) declines. Therefore, the estimation of glomer ular filtration utilizing mathematical models incorporates age as an i mportant variable. In order to adjust drug dosages and diagnose renal disease earlier in the elderly, a variety of such simplified estimates of gfr have been applied. Unfortunately, no estimator is as accurate as the cumbersome gold standards (e.g. inulin or iothalamate clearance ) and the reliability of each may vary with the particular clinical se tting. The purpose of this study was to critically evaluate three comm only used estimators of gfr - i.e., creatinine clearance (CC), Cockrof t-Gault (CG), and 100 over serum creatinine (100/SC)- comparing them t o iothalamate clearance (IC) in a group of healthy ambulatory geriatri c subjects (n = 41; ages 65-85). IC declined 1 ml/min per year of age in our sample. CC demonstrated a similar decline, a correlation of 0.8 3 with IC, and moderate error relative to IC of 17% at the mean (stand ard error [SE] = 12.3), In contrast, 100/SC correlated only 0.56 with IC, demonstrated a large positive bias (41 ml/min). and showed no age- related decline. An age correction to 100/SC similar to that utilized in the CG formula was clearly necessary. Despite the age and weight co rrection used in the CG formula, we found the estimates from it to be inaccurate (correlation = 0.5; SE = 23.8). A simpler age-corrected for mula (Est. IC = 1/2 [100/SC] + 88 - age) was derived and proved signif icantly superior to CG in our ambulatory geriatric sample, but still e xhibited enough error (SE = 16.4) to question its clinical utility. It appears that serum creatinine based estimates of gfr in the elderly m ay not provide accurate results.