Impact of creatinine calibration on performance of GFR estimating equations in a pooled individual patient database.
ABSTRACT Variation in performance of glomerular filtration rate (GFR) estimating equations is related to variation in calibration of the creatinine assay across clinical laboratories.
6 research studies and 4 clinical populations including 5,504 participants who had GFR measured using urinary clearance of iothalamate.
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.
Noncalibrated serum creatinine assayed in research and clinical laboratories compared with standardized serum creatinine.
Difference between measured GFR versus GFR estimated from the Modification of Diet in Renal Disease (MDRD) Study and Cockcroft-Gault equations.
For a noncalibrated serum creatinine value of 1 mg/dL (88.4 micromol/L), standardized serum creatinine value was 0.07 mg/dL (6.2 micromol/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%.
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.
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).
- SourceAvailable from: psu.eduClinical Chemistry 04/1993; 39(3):424-32. · 7.15 Impact Factor
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ABSTRACT: A formula has been developed to predict creatinine clearance (Ccr) from serum creatinine (Scr) in adult males: (see article)(15% less in females). Derivation included the relationship found between age and 24-hour creatinine excretion/kg in 249 patients aged 18-92. Values for Ccr were predicted by this formula and four other methods and the results compared with the means of two 24-hour Ccr's measured in 236 patients. The above formula gave a correlation coefficient between predicted and mean measured Ccr's of 0.83; on average, the difference predicted and mean measured values was no greater than that between paired clearances. Factors for age and body weight must be included for reasonable prediction.Nephron 02/1976; 16(1):31-41. · 13.26 Impact Factor
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ABSTRACT: Equations using serum creatinine level, age, sex, and other patient characteristics often are used to estimate glomerular filtration rate (GFR) in both clinical practice and research studies. However, the critical dependence of these equations on serum creatinine assay calibration often is overlooked, and the reproducibility of estimated GFR is rarely discussed. We address these issues in frozen samples from 212 Modification of Diet in Renal Disease (MDRD) study participants and 342 Third National Health and Nutrition Examination Survey (NHANES III) participants assayed for serum creatinine level a second time during November 2000. Variation in serum creatinine level was assessed in 1,919 NHANES III participants who had serum creatinine measured on two visits a median of 17 days apart. Linear regression was used to compare estimates. Calibration of serum creatinine varied substantially across laboratories and time. Data indicate that serum creatinine assays on the same samples were 0.23 mg/dL higher in the NHANES III than MDRD study. Data from the College of American Pathologists suggest that a difference of this magnitude across laboratories is not unusual. Conversely, serum creatinine assays an average of 2 weeks apart have better precision (SD of percentage of difference in estimated GFR, 15%; 90% of estimates within 21%). Errors in calibration make little difference in estimating severely decreased GFR (<30 mL/min/1.73 m2), but result in progressively larger differences at higher GFRs. Both clinical and research use of serum creatinine or equations to estimate GFR require knowledge of the calibration of the serum creatinine assay.American Journal of Kidney Diseases 05/2002; 39(5):920-9. · 5.29 Impact Factor