Impact of Creatinine Calibration on Performance of GFR Estimating Equations in a Pooled Individual Patient Database
Presented in abstract form at the Annual Meeting of the American Society of Nephrology, San Diego, CA, November 16, 2006.
Received 8 December 2006; accepted 2 April 2007. published online 30 May 2007.
Background
Variation in performance of glomerular filtration rate (GFR) estimating equations is related to variation in calibration of the creatinine assay across clinical laboratories.
Study Design
Cross-sectional analysis.
Setting & Participants
6 research studies and 4 clinical populations including 5,504 participants who had GFR measured using urinary clearance of iothalamate.
Measurements
Standardized serum creatinine values obtained by means of calibration to the Cleveland Clinic Research Laboratory using frozen specimens, a calibration panel, and/or survey results from the College of American Pathologists.
Predictor
Noncalibrated serum creatinine assayed in research and clinical laboratories compared with standardized serum creatinine.
Outcome
Difference between measured GFR versus GFR estimated from the Modification of Diet in Renal Disease (MDRD) Study and Cockcroft-Gault equations.
Results
For a noncalibrated serum creatinine value of 1 mg/dL (88.4 μmol/L), standardized serum creatinine value was 0.07 mg/dL (6.2 μmol/L) less than noncalibrated values. In the pooled data set, for the MDRD Study equation, calibration improved median percentage of difference between measured and estimated GFR from 9.0% (interquartile range [IQR], 28%) to 5.8% (IQR, 28%) and improved the percentage of estimates within 30% of measured GFR (P30) from 80% to 83%. The effect of calibration was greater at higher levels of GFR and varied across studies. For the Cockcroft-Gault equation, calibration worsened the median percentage of difference from −2.0% (IQR, 38%) to −11.4% (IQR, 39%), and the P30, from 74% to 69%.
Limitations
College of American Pathologist samples were used for calibration of clinical populations; calibration factors do not account for drift over time in the serum creatinine assay; calibration cannot account for variation in assay performance among individuals.
Conclusion
Calibration improves the performance of the MDRD Study equation. After calibration, larger errors remain for GFR estimates greater than 60 mL/min/1.73 m2 (>1 mL/s/1.73 m2).
Address correspondence to Lesley A. Stevens, MD, MS, Division of Nephrology, Tufts-New England Medical Center, 750 Washington St, Box #391, Boston, MA 02111.
Because an author of this manuscript is an editor for AJKD, the peer-review and decision-making processes were handled entirely by an outside editor, Marcello Tonelli, MD, SM, University of Alberta, who served as Acting Editor-in-Chief. Details of the journal’s procedures for potential editor conflicts are given in the Editorial Policies section of the AJKD Website.
Originally published online as doi: 10.1053/j.ajkd.2007.04.004 on June 1, 2007.
ABSTRACT