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Reporter: Aviva Lev-Ari, PhD, RN

The combined creatinine–cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.)

Estimating Glomerular Filtration Rate from Serum Creatinine and Cystatin C

Lesley A. Inker, M.D., Christopher H. Schmid, Ph.D., Hocine Tighiouart, M.S., John H. Eckfeldt, M.D., Ph.D., Harold I. Feldman, M.D., Tom Greene, Ph.D., John W. Kusek, Ph.D., Jane Manzi, Ph.D., Frederick Van Lente, Ph.D., Yaping Lucy Zhang, M.S., Josef Coresh, M.D., Ph.D., and Andrew S. Levey, M.D. for the CKD-EPI Investigators

N Engl J Med 2012; 367:20-29  July 5, 2012

BACKGROUND

Estimates of glomerular filtration rate (GFR) that are based on serum creatinine are routinely used; however, they are imprecise, potentially leading to the overdiagnosis of chronic kidney disease. Cystatin C is an alternative filtration marker for estimating GFR.

METHODS

Using cross-sectional analyses, we developed estimating equations based on cystatin C alone and in combination with creatinine in diverse populations totaling 5352 participants from 13 studies. These equations were then validated in 1119 participants from 5 different studies in which GFR had been measured. Cystatin and creatinine assays were traceable to primary reference materials.

RESULTS

Mean measured GFRs were 68 and 70 ml per minute per 1.73 m2 of body-surface area in the development and validation data sets, respectively. In the validation data set, the creatinine–cystatin C equation performed better than equations that used creatinine or cystatin C alone. Bias was similar among the three equations, with a median difference between measured and estimated GFR of 3.9 ml per minute per 1.73 m2 with the combined equation, as compared with 3.7 and 3.4 ml per minute per 1.73 m2 with the creatinine equation and the cystatin C equation (P=0.07 and P=0.05), respectively. Precision was improved with the combined equation (interquartile range of the difference, 13.4 vs. 15.4 and 16.4 ml per minute per 1.73 m2, respectively [P=0.001 and P<0.001]), and the results were more accurate (percentage of estimates that were >30% of measured GFR, 8.5 vs. 12.8 and 14.1, respectively [P<0.001 for both comparisons]). In participants whose estimated GFR based on creatinine was 45 to 74 ml per minute per 1.73 m2, the combined equation improved the classification of measured GFR as either less than 60 ml per minute per 1.73 m2 or greater than or equal to 60 ml per minute per 1.73 m2 (net reclassification index, 19.4% [P<0.001]) and correctly reclassified 16.9% of those with an estimated GFR of 45 to 59 ml per minute per 1.73 m2 as having a GFR of 60 ml or higher per minute per 1.73 m2.

CONCLUSIONS

The combined creatinine–cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.)

Supported by grants (UO1 DK 053869, UO1 DK 067651, and UO1 DK 35073) from the National Institute of Diabetes and Digestive and Kidney Diseases.

Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.

This article was updated on July 5, 2012, at NEJM.org.

We thank Dr. Aghogho Okparavero for providing assistance with communications and manuscript preparation. (Additional acknowledgments are provided in the Supplementary Appendix.)

SOURCE INFORMATION

From Tufts Medical Center, Boston (L.A.I., C.H.S., H.T., Y.L.Z., A.S.L.); the University of Minnesota, Minneapolis (J.H.E.); the University of Pennsylvania School of Medicine, Philadelphia (H.I.F.); the University of Utah, Salt Lake City (T.G.); National Institutes of Health, Bethesda, MD (J.W.K.); Johns Hopkins University, Baltimore (J.M., J.C.); and Cleveland Clinic Foundation, Cleveland (F.V.L.).

Address reprint requests to Dr. Inker at the Division of Nephrology, Tufts Medical Center, 800 Washington St., Box 391, Boston, MA 02111, or at linker@tuftsmedicalcenter.org.

Additional investigators in the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) are listed in the Supplementary Appendix, available at NEJM.org.

N Engl J Med 2012; 367:20-29

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