Nierfunctie bij kinderen: Meten, schatten, nieuwe markers of toch creatinine?

It is widely accepted that renal function is best measured as glomerular filtration rate (GFR). It is the single most important renal function parameter in clinical practice. The ideal GFR marker has a stable plasma concentration and is exclusively eliminated by the glomerulus. Inulin shares these features of the ideal marker, its clearance is therefore regarded as the gold standard for measuring GFR. Because clearance studies are invasive and cumbersome, it has become preferred practice to use an endogenous renal function marker. Creatinine remains the most widely used marker, despite its well-known limitations of which muscle dependency and nonglomerular elimination are the most important. The inverse relationship between marker concentrations and GFR complicates rapid detection of impaired renal function. International guidelines therefore advise to estimate GFR using concentrations of an endogenous marker. The most widely used pediatric equation is by Schwartz et al. and corrects for the age-dependent increase in muscle mass by including height in the equation. In the last decades, cystatin C has emerged as an alternative GFR marker. It is a low-molecular weight protein that shares the properties of the ideal renal function marker. Despite its advantages over creatinine, of which the stable reference range above the age of 2 years and independency of body composition are the most important, widespread clinical use remains limited. Similar to the Schwartz equation for creatinine, several equations have been developed to estimate GFR from cystatin C. This review summarizes the different methods available to the pediatrician to determine the GFR.