Evaluation of plasma cystatin C as a marker for glomerular filtration ratein patients with type 2 diabetes

Aim: To evaluate plasma cystatin C as a marker of the glomerular filtration rate in patients with type 2 diabetes and their age and sex-matched contro ls. Materials and methods: Forty-seven patients with one decade of type 2 d iabetes and 51 non-diabetic control subjects were studied. Plasma cystatin C was measured by particle-enhanced turbidimetric immunoassay in a new appl ication for the Hitachi 704 analyzer. For comparison, plasma creatinine and creatinine clearance were measured. The plasma clearance of Cr-51-EDTA by the single injection method was utilized as reference. Results: In patients with type 2 diabetes the correlation coefficient between plasma cystatin C and the plasma clearance of Cr-51-EDTA was 0.774 (Spearman's coefficient) and that between plasma creatinine and the plasma clearance of Cr-51-EDTA w as 0.556 (p = 0.001 for the difference). The correlation between creatinine clearance and the plasma clearance of Cr-51-EDTA was 0.411. In receiver op erating characteristic (ROC) curve analysis the diagnostic accuracy of plas ma cystatin C was significantly better than that of plasma creatinine (p = 0.047) or creatinine clearance (p = 0.001). The best diagnostic efficiency (98%) for cystatin C was obtained when the cut-off limit was set at 1.32 mg /l. In the control group the correlation coefficients were: between cystati n C and the plasma clearance of Cr-51-EDTA 0.627, between creatinine and th e plasma clearance of Cr-51-EDTA 0.466 and between creatinine clearance and the plasma clearance of Cr-51-EDTA 0.416. The area under the ROC plot curv e of cystatin C was also greatest in the control group, but the diagnostic accuracy of cystatin C was marginally better than that of either plasma cre atinine (p = 0.05) or creatinine clearance (p = 0.08). Among the control su bjects Various non-renal causes may have interfered with cystatin C concent rations reducing the correlations. Conclusions: Cystatin C measurement is a more sensitive and specific test for GFR in patients with type 2 diabetes than plasma creatinine or its clearance, when GFR is normal or only slightl y reduced. If an elevated cystatin C concentration is found, non-renal fact ors have to be excluded. The turbidimetric application described here can e asily be applied for most clinical chemistry analyzers and is therefore use ful in daily clinical practice.