Although cystatin C testing is expected to identify a proportion of older adults with CKD who were not identified using eGFRcr alone (therefore qualifying some additional older adults for further monitoring, treatments or referral to specialty care) cystatin C testing as eGFRcys or eGFRcr‐cys may also de‐classify a proportion of older adults from the CKD definition, achieving one of the primary aims of age‐adapted CKD thresholds.2 This evidence concerns the gene CST3 and chronic kidney disease.