Serum creatinine is a poor marker of GFR in nephrotic syndrome

Background. In daily clinical practice creatinine clearance is used as marker of glomerular filtration rate (GFR). As a result of the tubular secretion process endogenous creatinine clearance (ECC) overestimates glomerular filtration rate, particularly in patients with impaired renal function. It has been suggested that the tubular handling of creatinine is altered in patients with a nephrotic syndrome. Methods. Inulin clearance (GFR) and creatinine clearance (ECC) have been simultaneously measured in a cohort of 42 patients with proteinuria and 45 healthy controls. The clearance of creatinine by tubular secretion (TScreat) can be estimated by ECC-GFR. TScreat was calculated in both groups. Regression analysis was performed to identify factors that independently influence tubular creatinine secretion. Results. The mean age (+/- SD) of the patients was 41 +/- 13 years, serum albumin 26 +/- 9 g/l, median (IQR) proteinuria 4.5 (3.6-8.2)g/10 mmol creatinine, serum creatinine 103 (84-143) mu mol/l, ECC 85 (69-118) ml/min/1.73 m(2), and GFR 54 (36-83) ml/min/1.73 m(2). Median TScreat amounted to 29 (21-36) ml/min/1.73 m(2). In the healthy controls serum creatinine was 75 (70-81) mu mol/l, ECC 118 (109-125) ml/min/1.73 m(2), GFR 106 (102-115) ml/min/1.73 m(2), and TScreat 11 (3.5-19) ml/min/1.73 m(2). By regression analysis serum albumin was identified as an independent predictor of tubular creatinine secretion. We divided the patients in two subgroups based on serum albumin levels. TScreat was 24 (14-29) ml/min/1.73 m(2) in patients with serum albumin levels > 25.8 g/l, and 36 (28-54)ml/min/1.73 m(2) in patients with serum albumin levels < 25.8 g/l (P < 0.01). Conclusion. Serum albumin levels influence tubular creatinine secretion. As a result, the endogenous creatinine clearance as well as estimated GFR using a modified MDRD equation more pronouncedly overestimate glomerular filtration rate in nephrotic syndrome.