GFR estimation using standardized cystatin C in kidney transplant recipients.

BACKGROUND: The utility of serum cystatin C (SCysC) as a filtration marker in kidney transplantation is uncertain. We took advantage of the recent validation of a reference calibrator for SCysC and of newly developed CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equations (2012) expressed for use with standardized SCysC level to reassess the performance of SCysC as a filtration marker in kidney transplant recipients.

STUDY DESIGN: Study of diagnostic test accuracy.

SETTING & PARTICIPANTS: 670 kidney transplant recipients from 3 centers undergoing glomerular filtration rate (GFR) measurements from December 2006 to November 2012.

INDEX TEST: Estimated GFR (eGFR) using the 2012 SCysC-based and serum creatinine (SCr)/SCysC-based CKD-EPI equations (eGFR(cys) and eGFR(cr-cys), respectively) and the 2009 SCr-based CKD-EPI equation (eGFR(cr)), with SCysC and SCr measured at a single laboratory between April 2011 and June 2011.

REFERENCE TEST: Measured GFR (mGFR) using urinary clearance of inulin.

RESULTS: Bias (the difference between mGFR and eGFR) was significantly smaller for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (-2.82 and -0.54 vs +4.4 mL/min/1.73 m(2), respectively; P < 0.001). Precision (standard deviation of the mean bias) also was better for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (12 and 11 vs 13 mL/min/1.73 m(2) [P < 0.001 for both comparisons]). Accuracy (percentage of GFR estimates within 30% of mGFR) was greater for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (81% and 86% vs 75%, respectively [P = 0.004 and P < 0.001]). Net reclassification index with respect to mGFR of 30 mL/min/1.73 m(2) for eGFR(cr-cys) and eGFR(cys) versus eGFR(cr) was 18.8% [95% CI, 8.6%-28.9%] and 22.5% [95% CI, 10.2%-34.9%].

LIMITATIONS: Patients were exclusively of European descent; association with transplant outcome was not evaluated.

CONCLUSIONS: Our data validate the use of both the newly developed SCysC-based and SCr/SCysC-based CKD-EPI equations (2012) in kidney transplant recipients. Both equations perform better than the SCr-based CKD-EPI equation (2009).