To explore whether the difference between formulae with and without factor for black Americans was confounded by the cystatin C equation calculated by van Deventer et al. in black South-Africans, we compared CKD-EPI with and without the factor with the CKD-EPI cystatin C equation by Stevens et al., which has been developed based on results of different pooled cohorts with GFR measured by iothalamate. This evidence concerns the gene CST3 and chronic kidney disease.