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)/SCysCbased
CKD-EPI equations (eGFRcys and eGFRcr-cys, respectively) and the 2009 SCr-based CKD-EPI equation
(eGFRcr), 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 eGFRcys and
eGFRcr-cys versus eGFRcr ( 2.82 and 0.54 vs 4.4 mL/min/1.73 m2, respectively; P 0.001). Precision
(standard deviation of the mean bias) also was better for eGFRcys and eGFRcr-cys versus eGFRcr (12 and 11 vs
13 mL/min/1.73 m2 [P 0.001 for both comparisons]). Accuracy (percentage of GFR estimates within 30% of
mGFR) was greater for eGFRcys and eGFRcr-cys versus eGFRcr (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 m2 for eGFRcr-cys and
eGFRcys versus eGFRcr 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).