In this secondary analysis of a pre-specified secondary outcome of people enrolled in the STOP-ACEi trial [1, 2], we explored the impact of discontinuing (STOP) or continuing renin–angiotensin system (RAS) inhibitor therapy in people with advanced chronic kidney disease (CKD) on serum cystatin C levels and associated estimated glomerular filtration rate (eGFRcys), and compared this with both the American CKD Epidemiology Collaboration (CKD-EPI) and European Kidney Function Consortium (EKFC) equations for measuring eGFR. This evidence concerns the gene CST3 and chronic kidney disease.