Cystatin C levels are unaltered in patients with diabetes mellitus and normal renal function.

To the Editor: In their recent article, Stevens et al.1 have reported that factors other than glomerular filtration rate (GFR) affect serum cystatin C (CysC) levels. Specifically, they mentioned the impact of diabetes mellitus, which was associated with 8.5% higher levels of CysC. However, proteinuria was also associated with CysC and, as admitted by the authors, diabetes and proteinuria were strongly associated in their patient cohorts. Thus, their data did not allow for determining the independent effects of diabetes mellitus or proteinuria. We have previously reported GFR data of a cohort of patients with normo-albuminuric diabetes type I and age- and sex-matched controls.2 From this cohort, we have selected 33 patients and 38 controls with matching GFR (inulin clearance) and evaluated the relationship between CysC and GFR.