Measuring and estimating the GFR in children: state of the art in 2025.

Glomerular filtration rate (GFR) can be measured directly, or estimated from biomarkers like serum creatinine and cystatin C, or both. Measuring GFR in children is cumbersome, as it requires the intravenous injection of an exogenous filtration marker like iohexol, and several blood samples to determine the concentration-time decay curve. Serum creatinine (SCr) measurement is inexpensive and is part of the routine biochemical blood tests that are commonly requested in daily clinical practice. SCr-based estimated GFR is therefore still the most widely used test to obtain information on kidney function, although SCr varies with age and sex during childhood and GFR remains nearly constant over the 2-18-year age range. These issues are partially resolved by factoring SCr by height, or rescaling SCr by the median of healthy subjects, making interpretation of SCr and eGFR more straightforward. Cystatin C has become an interesting alternative kidney biomarker, and estimating GFR from cystatin C has therefore become more important. The aim of this review is to show recent advances in measuring and estimating GFR in children.