QJM Advance Access originally published online on January 10, 2007
QJM 2007 100(2):142-143; doi:10.1093/qjmed/hcl142
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Calculating eGFR using the MDRD equation
Sir,In a recent paper, MacGregor et al. suggested that the recently developed IDMS-traceable Modification of Diet in Renal Disease (MDRD) equation should be used to calculate GFR from serum creatinine measurements.1 This equation was developed for use with creatinine results from measurement procedures with calibrations that are traceable to the isotope dilution gas chromatography mass spectroscopy (IDMS) creatinine reference method. Since most routine measurement procedures for serum creatinine have not been calibrated to be traceable to the IDMS reference method, the authors suggest that measurement-procedure-specific correction factors derived from EQAS data can be used by individual laboratories to adjust the reported values measured in their laboratory to match the IDMS reference method. While this approach has theoretical appeal, it has a significant limitation unless the EQAS materials used to derive the correction factors have been validated to demonstrate commutability with native clinical samples, for all the routine measurement procedures for which such a correction is intended.2–4
Commutability has been defined in metrological terms in several ISO documents and the following working definition was suggested in a recent editorial: ‘the equivalance of the mathematical relationships between the results of different measurement procedures for a reference material and for representative samples from healthy and diseased individuals’.4
Commutable EQAS materials with target values assigned by a reference method are required to obtain the trueness bias for a given routine method. When non-commutable materials are used, the observed bias may include contributions from trueness bias as well as from non-commutability. Because it is not possible to determine the sources of the total observed bias from EQAS results alone, an incorrect assessment of trueness bias can result when materials of unknown commutability status are used as EQAS samples. It has been reported in numerous investigations that a substantial and variable fraction of typical EQAS materials are not commutable with native clinical samples for many routine measurement procedures, including commonly used measurement procedures for serum creatinine.2,3,5
Clinical laboratories and EQAS providers are cautioned to avoid the use of correction factors based on EQAS results to adjust serum creatinine values from routine measurement procedures to agree with an IDMS reference method, unless the EQAS samples have been validated for commutability with native clinical samples for each routine measurement procedure of interest. If commutability is not validated for the EQAS samples, there is significant risk that a correction factor based on EQAS results may be incorrect, potentially leading to even greater, rather than reduced, bias in the estimation of GFR.
Virginia Commonwealth Univ
Richmond
Virginia
Centers for Disease Control and Prevention
Atlanta
Georgia
University of Minnesota
Minneapolis
Minnesota
Ortho Clinical Diagnostics
Rochester
New York State
USA
email: gmiller{at}vcu.edu
Disclaimer
The findings and conclusions presented in this letter are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention, or those of Ortho Clinical Diagnostics.
References
1. MacGregor MS, Boag DE, Innes A. (2006) Chronic kidney disease: evolving strategies for detection and management of impaired renal function. Q J Med 99 365–75.
2. Miller WG. (2003) Specimen materials, target values and commutability for external quality assessment (proficiency testing) schemes. Clin Chim Acta 327 25–37.[CrossRef][Web of Science][Medline]
3. Franzini C and Ceriotti F. (1998) Impact of reference materials on accuracy in clinical chemistry. Clin Biochem 31 449–57.[CrossRef][Web of Science][Medline]
4. Miller WG, Myers GL, Rej R. (2006) Why commutability matters. Clin Chem 52 553–4.
5. Miller WG, Myers GL, Ashwood ER, Killeen AA, Wang E, Thienpont LM, Siekmann L. (2005) Creatinine measurement: State of the Art in Accuracy and Inter-Laboratory Harmonization. Arch Pathol Lab Med 129 297–304.[Web of Science][Medline]
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