Nice to meet you, Mr. GFR!

Teleologically speaking kidney’s function is designed to maintain life parameters as close as possible to the normal level. Clearance is a tool to compare renal function among different individuals independently on urine flow, body size and solute concentration in blood. Since the tubulus manipulates the glomerular filtrate composition, for the computation of clearance as a surrogate of GFR, we need a molecule with ideal features: fully filtered by the glomerular membrane, absent reabsorption or secretion in the tubular part of the nephron and easily measurable. In a steady state condition the serum level of an endogenous marker is correlated to the reciprocal of the level of GFR making possible GFR estimation without urine collection. GFR can be estimated using different equations that include race, gender, age and body size. The MDRD equation, derived from the study carried out in 1999 resulted reasonably accurate and probably more precise than the previous Cockcroft-Gault equation developed in 1973 for patients with chronic kidney disease. Both equations however have been reported to be less accurate in patients without chronic kidney disease. In several conditions, estimated GFR (from MDRD formula) can result significantly lower than direct measurements of renal clearance potentially leading to a false positive diagnosis of chronic renal disease (GFR < 60 ml/min/1.73m2) with important consequences. This phenomenon is particularly evident in Europe compared to United States especially for a different calibration of serum creatinine assays among laboratories. A potential GFR underestimation from inaccurate serum creatinine measurements (or better, calibrations) could cause a “false epidemic” of mild chronic kidney. So, from one side we really encourage GFR monitoring in the general population and especially in the population at risk for kidney and cardiovascular disease; on the other side we must remind general practitionners that patients cannot be frightened with potentially false positive diagnoses of kidney dysfunction.