Reciprocal creatinine slopes often give erroneous estimates of progression of chronic renal failure

In 10 of 22 observation periods (lasting an average of 15 months) in 17 patients with moderate to severe chronic renal failure (GFR 4 to 23 ml/min), rates of progression as estimated from the linear regression on time of reciprocal plasma creatinine concentration (multiplied by average 24 hr creatinine excretion) (b2) differed significantly from rates of progression as estimated from the regression on time of urinary clearance of 99mTc-DTPA (b1), during all or part of the period of observation, b2 exceeded b1 in six cases and was less than b1 in the other four. Owing to these changes, measurements of reciprocal creatinine concentration gave erroneous impressions of the rate or existence of progression, during all or a portion of the period of observation, in nearly half of these patients. However, in the 22 studies as a group, using the entire periods of observation, b2 indicated nearly the same mean rate of progression as b1, and had the same variance. We conclude that sequential plasma or serum creatinine measurements in individual patients are often misleading as measures of progression and should, when feasible, be replaced by urinary clearances of isotopes in following patients with chronic renal failure.