Estimating GFR: what it is, what it is not, and why prediction cannot settle the question.

Schultheiss et al. show that replacing Chronic Kidney Disease Epidemiology Collaboration with European Kidney Function Consortium cystatin C-based equations reclassifies women, but not men, into milder chronic kidney disease stages, while outcome prediction is unchanged. We argue that prediction of kidney failure cannot validate a glomerular filtration rate equation: dialysis initiation is a clinical decision, not a filtration measurement. The analysis is cross-sectional, leaving longitudinal performance untested. Equations are population tools; at the bedside, non-glomerular filtration rate determinants of both creatinine and cystatin C can mislead.