While non-GFR factors associated with cystatin C also exist, such as obesity, steroid use, and possibly inflammation, their associations with serum cystatin C levels are smaller in magnitude than those of non-GFR factors associated with serum creatinine.16,33,34,35 We adjusted for waist circumference and steroid use in our baseline, time-updated, and slope analyses. Here, CST3 is linked to obesity due to melanocortin 4 receptor deficiency.