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Letters to the Editor |
1 Nutrition-Diabétologie and
2 Biochimie, Hôpital Haut-Lévêque Pessac, France
3 Néphrologie and
4 Médecine Nucléaire Hôpital Pellegrin Bordeaux, France
aAddress correspondence to this author at: Nutrition-Diabétologie, Hôpital Haut-Lévêque, Avenue de Magellan, 33600 Pessac, France. Fax 33 5 57 65 60 79; e-mail vincent.rigalleau{at}wanadoo.fr.
To the Editor:
Pucci et al. (1) recently reported their study of the use of cystatin C (cysC) to detect decreases in renal function in patients with diabetes. CysC had an advantage over other methods only for detecting very early impairment of renal function [glomerular filtration rate (GFR) < 90 and 75 mL · min–1 · (1.73 m2)–1], whereas detection of the 70 patients with GFR <60 studied by Pucci et al.(1) was not improved by use of cysC compared with conventional estimated GFR estimations (e-GFR) predicted by Cockcroft and Gault (CG) and Modification of Diet in Renal Disease (MDRD) equations. Better GFR estimations are required for those patients considered to have chronic kidney disease (CKD) according to the new American Diabetes Association recommendations, particularly to assess the progression of their CKD, which may not be accurately estimated by the CG and MDRD equations(2). Rule et al.(3) recently proposed a composite GFR estimation based on both serum creatinine (sCr) and cysC, highly correlated with the results of 204 iothalamate clearance determinations in patients with diseases of the native (nontransplanted) kidney. We wondered whether this estimation could be useful in the assessment and monitoring of kidney function in patients with diabetes and CKD.
Seventy-six patients [43 males, 33 females, mean (SD) age 65 (12) years, body mass index 27.6 (4.9) kg/m2] gave informed consent to participate in the study, which was performed in accordance with the declaration of Helsinki. The inclusion criteria were diabetes [19 type 1 and 57 type 2, hemoglobin A1C 8.1 (1.4%)] and CKD [MDRD<60 mL · min–1 · (1.73 m2)–1, sCr 182 (84) µmol/L]. GFR was measured by 51Cr-EDTA clearance [i-GFR, 33.2 (14.8) mL · min–1 · (1.73 m2)–1], and compared to the recommended and new composite estimations by use of correlations, paired t tests, Bland-Altman procedures, and absolute percentage difference with i-GFR. sCr was quantified on an Olympus AU 640 analyzer by use of the Jaffé method with bichromatic measurements according to the manufacturer’s specifications. The MDRD estimation was calculated by the abbreviated 4-variable version of the equation. The composite estimation was based on both sCr and cysC (3):
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The relationships between i-GFR and 1/sCr, 1/cysC, and both were tested by linear regression. The diagnostic accuracy of each estimation for the diagnosis of severe renal failure (i-GFR<30) was tested by ROC curve analysis. Sixteen patients had a 2nd i-GFR [31.0 (18.0) mL · min–1 · (1.73 m2)–1 vs 36.1 (14.0) at baseline, not significant (NS)], and an e-GFR determination 2 years later to test whether e-GFR changes correlated to i-GFR changes [–5.1 (17.5) mL · min–1 · (1.73 m2)–1].
The initial mean (SD) cysC was 1.97 (0.83) mg/L. i-GFR correlated to 1/sCr (r = 0.65, P <0.001) and 1/cysC (r = 0.64, P <0.001). The statistical significance was maintained when both were included in a multiple linear regression model (r = 0.69, P <0.001; P = 0.002 for 1/sCr and P = 0.009 for 1/cysC). Areas under the ROC curves were 0.874 for cysC and 0.870 for sCr (NS). The results of the GFR estimations are presented in Table 1
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As already reported in patients with diabetes (4), the performances of the CG equation (correlation with i-GFR, accuracy, area under the ROC curve, Bland-Altman procedure) were poor. In accordance with the findings of Pucci et al.(1), the MDRD remains the best estimation of GFR in renally insufficient diabetic patients: the best estimates for correlation coefficient, absolute percentage difference, and ROC curve for the diagnosis of severe renal failure were all obtained with this equation. However, 1/cysC still predicted i-GFR after taking account of 1/sCr. The performances of the composite estimations of GFR, which included both markers, were close to the MDRD equation. These similar performances contrast with the better correlation coefficient for the composite estimation (r2 = 0.891) than the MDRD (r2 = 0.825) in the report by Rule et al.(3), but this outcome is not unexpected because the lower i-GFR for our patients [33.2 (14.8) mL · min–1 · (1.73 m2)–1, vs 57 (29) for the patients studied by Rule et al.] decreased the impact of the well-known underestimation of high GFR by the MDRD. Most interestingly, the correlation between the composite estimation change and the i-GFR change almost reached significance, whereas the MDRD did not. Even for advanced CKD patients, the inclusion of cysC in the composite estimation may have diagnostic value, for monitoring GFR decline, a finding that is in line with recent reports for the use of 100/cysC(5).
Acknowledgments
Grant/funding support: None declared. Financial disclosures: None declared.
References
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