Extending the Lund-Malmö creatinine-based GFR equation to cystatin C - validation results from the European Kidney Function Consortium (EKFC) cohort of children and adults.

The aim of the present study was to extend the creatinine-based Lund-Malmö GFR equation for use with rescaled cystatin C (r-LMR_Cys) and validate it against measured GFR (mGFR) in the EKFC cystatin C cohort of children (n = 2,293) and adults (n = 7,727). Rescaling was obtained by dividing each biomarker by a Q-value, representing the population-specific median biomarker level among healthy individuals. Validation included median bias/precision/accuracy (percent estimates within ±30% of mGFR, P₃₀). Performance was compared with the EKFC-equation (EKFC_Cys), the CAPA cystatin C equation, the corresponding equations based on rescaled creatinine (r-LMR_Cr and EKFC_Cr) and the arithmetic mean of r-LMR_Cr and CAPA (r-LMR_Cr+CAPA), r-LMR_Cr and r-LMR_Cys (r-LMR_Mean), and EKFC_Cr and EKFC_Cys (EKFC_Mean). The overall P₃₀ of r-LMR_Cys in adults was 86.2% (95% CI 85.4%-86.9%), which was 6.6 percentage points (pp; 95% CI 5.8-7.4 pp) higher than for CAPA and similar to r-LMR_Cr (P₃₀ 87.4%, 95% CI 86.6%-88.1%). r-LMR_Cys and EKFC_Cys exhibited similar performance both overall and across subgroups of age, sex, GFR and BMI and in children. All three arithmetic mean equations had similar P₃₀-accuracy and generally performed better than the corresponding single-marker equations. Our results show that the Lund-Malmö GFR equation can be adapted for use with rescaled cystatin C with performance that is similar to the best-performing equations based on rescaled creatinine. The generality of the applied biomarker rescaling principle implies that the future demand for population- and biomarker-specific GFR estimating equations can be expected to decrease substantially.