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Estimated Glomerular Filtration Rate
- Andrew S. Levey, MD;
- Josef Coresh, MD, PhD; and
- Frederick Van Lente, PhD
IN RESPONSE:
Dr. Delanaye and colleagues question whether the precision of the creatinine assay within the reference range is sufficient for accurate GFR estimation. The Beckman Synchron CX3 assay used during the MDRD Study had analytic CVs of 4.3% and 1.5% at creatinine values of 88.4 µmol/L (1.0 mg/dL) and 477.3 µmol/L (5.4 mg/dL), respectively (1). The enzymatic assay used to calibrate the MDRD Study laboratory to standardized creatinine had analytic CVs of 2.0% and 1.8% at creatinine values of 78.7 µmol/L (0.89 mg/dL) and 518.0 µmol/L (5.86 mg/dL), respectively, in 2004 (n = 194) and 1.6% and 1.1% at creatinine values of 88.4 µmol/L (1.00 mg/dL) and 339.5 µmol/L (3.84 mg/dL), respectively, in 2005 (n = 409). Thus, the analytic CV for the enzymatic assay is as low as or is lower than that stated by Dr. Delanaye and colleagues.
We agree that the effect of imprecision in the serum creatinine assay and biological variation in GFR estimates is greater at lower values for serum creatinine (equal to higher values for estimated GFR) and that is one of several important reasons for lesser accuracy of higher GFR estimates (2). For these reasons, current recommendations are to report estimated GFR as a numerical value only when it is less than 60 mL/min per 1.73 m2 and to report “greater than 60 mL/min per 1.73 m2” for higher values. We believe that this is sufficient for most clinical circumstances requiring the clinical assessment of kidney function. New filtration markers, such as cystatin C, and improvement in estimation equations may be required for more accurate GFR estimation at higher values. Until then, if more accurate assessment of kidney function is required in patients with estimated GFRs greater than 60 mL/min per 1.73 m2, measurement of the clearance of an exogenous filtration marker or creatinine is necessary.
Andrew S. Levey, MD
Josef Coresh, MD, PhD
Frederick Van Lente, PhD
Tufts-New England Medical Center
Boston, MA 02111
Article and Author Information
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Potential Financial Conflicts of Interest: None disclosed.
