Endogenous substances such as serum creatinine and cystatin C are cheaper, provide more rapid results and are widely available.16 Although the National Institute for Health and Care Excellence (NICE) and the Kidney Disease Improving Global Outcomes (KDIGO) groups both base their eGFR assessment (equations) on cystatin C, its cost and lack of standardisation excludes its general use.17,18 Creatinine is produced at a constant rate, is present in all body fluids, and is filtered by the glomerulus. This evidence concerns the gene CST3 and kidney disorder.