CST3 and nutritional deficiency disease: However, in situations where the accuracy of creatinine-based estimates may be compromised—such as in individuals with extremes of muscle mass, malnutrition or other con-founding factors—it is advisable to perform a confirmatory assessment using serum cystatin C. When cystatin C is measured, it should be incorporated into a validated estimating equation to provide a cystatin C–based eGFR (eGFRcr-cys) [7].