title: Calibration and random variation of the serum creatinine assay as critical elements of using equations to estimate glomerular filtration rate.
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.
authors.affiliation: Department of Epidemiology, The Johns Hopkins University School of Hygiene and Public Health, Baltimore, MD, USA. coresh@jhu.edu
authors.forename: Josef
authors.initials: J
authors.lastname: Coresh
authors.forename: Brad C
authors.initials: BC
authors.lastname: Astor
authors.forename: Geraldine
authors.initials: G
authors.lastname: McQuillan
authors.forename: John
authors.initials: J
authors.lastname: Kusek
authors.forename: Tom
authors.initials: T
authors.lastname: Greene
authors.forename: Frederick
authors.initials: F
authors.lastname: Van Lente
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Levey
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002138:Calibration; D003404:Creatinine / Q000097:blood* / Q000592:standards*; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007674:Kidney Diseases / Q000503:physiopathology; D008297:Male; D008432:Mathematical Computing; D008875:Middle Aged; D015233:Models, Statistical; D015588:Observer Variation; D012737:Sex Factors
chemical_list: D003404:Creatinine