CST3 and chronic kidney disease: The equations commonly used for eGFR are the serum creatinine (cr) based Modification of Diet in Renal Disease (MDRD) study equation (5) and the Chronic Kidney Disease Epidemiology Collaboration (2009 CKD-EPI eGFRcr) equation (6), the serum cystatin C (cys) based CKD-EPI (2012 CKD-EPI eGFRcys) and the combination of creatinine and cystatin C based CKD-EPI (2012 CKD-EPI eGFRcr-cys) (7).