CST3 and obesity due to melanocortin 4 receptor deficiency: Age, weight, and smoking are known to be associated with increased cystatin C levels even after controlling for creatinine clearance.[39] Consistent with this, these patient characteristics were all associated with increased reclassification in our cohort supporting the notion that a high prevalence of obesity and smoking would limit the ability of eGFRcys to accurately identify CKD in older cohorts.