Unlike creatinine, the absence of urinary excretion of cystatin C makes it impossible to interpret GFR estimates based upon cystatin C. Therefore, our data suggest that cystatin C levels should be interpreted together with the knowledge of several non-renal factors, such as obesity, inflammation, diabetes, proteinuria, or drugs that may impact plasma cystatin C levels. Here, CST3 is linked to obesity due to melanocortin 4 receptor deficiency.