Use of Serum Creatinine Concentration for Estimation of Glomerular Filtration Rate

Sir.—The data presented by Springate et al¹ provide convincing evidence that creatinine clearance (C_cr) calculated from serum creatinine level and height, and using the proportionality constant (k) derived by Schwartz et al,² is a better estimate of renal function as determined by diethylenetriaminepenta-acetic acid clearance than is the serum creatinine level alone. It is important to recall that creatinine is not only filtered by the glomerular capillaries but is also secreted by renal tubular cells, so both C_cr and diethylenetriaminepenta-acetic acid clearance, as measured by Springate et al, must be greater than the true glomerular filtration rate (GFR) as estimated by the accepted reference standard, inulin clearance (C_in).^3-7 This would not create a problem were C_cr to bear a constant relationship to C_in.

Petri et al⁸ did serial simultaneous determinations of C_in and C_cr over a period of