Endogenous markers for kidney function in children: a review.

Although glomerular filtration rate (GFR) in children can be measured using a gold-standard technique following injection of an exogenous marker, this invasive and cumbersome technique is not widely available and GFR is commonly estimated using serum levels of endogenous markers. Creatinine, urea, cystatin C, beta-trace protein, and beta-2 microglobulin are well-established endogenous markers of kidney function. These markers differ in site of production and effects of diet and medication, as well as renal-tubular handling and extra-renal elimination. For each marker, different methods are available for measurement. Importantly, the measurements of creatinine and cystatin C have recently been standardized with the introduction of international reference standards. In order to allow estimation of GFR from serum marker concentrations, different equations for estimated GFR (eGFR) have been developed in children, using simple or more complex regression strategies with gold standard GFR measurements as a dependent variable. As a rule, estimation strategies relying on more than one marker - either by calculating the average of single parameter equations or by using more complex equations incorporating several parameters - outperform eGFR estimations using only a single marker. This in-depth review will discuss the physiology, measurement and clinical use of creatinine, urea, cystatin C, beta-trace protein, and beta-2 microglobulin in children. It will also address the generation of eGFR equations in children and provide an overview of currently available eGFR equations for the pediatric age group.