title: Estimating glomerular filtration rate from serum creatinine and cystatin C.
abstract: BACKGROUND
  Estimates of glomerular filtration rate (GFR) that are based on serum creatinine are routinely used; however, they are imprecise, potentially leading to the overdiagnosis of chronic kidney disease. Cystatin C is an alternative filtration marker for estimating GFR.
  
  
  METHODS
  Using cross-sectional analyses, we developed estimating equations based on cystatin C alone and in combination with creatinine in diverse populations totaling 5352 participants from 13 studies. These equations were then validated in 1119 participants from 5 different studies in which GFR had been measured. Cystatin and creatinine assays were traceable to primary reference materials.
  
  
  RESULTS
  Mean measured GFRs were 68 and 70 ml per minute per 1.73 m(2) of body-surface area in the development and validation data sets, respectively. In the validation data set, the creatinine-cystatin C equation performed better than equations that used creatinine or cystatin C alone. Bias was similar among the three equations, with a median difference between measured and estimated GFR of 3.9 ml per minute per 1.73 m(2) with the combined equation, as compared with 3.7 and 3.4 ml per minute per 1.73 m(2) with the creatinine equation and the cystatin C equation (P=0.07 and P=0.05), respectively. Precision was improved with the combined equation (interquartile range of the difference, 13.4 vs. 15.4 and 16.4 ml per minute per 1.73 m(2), respectively [P=0.001 and P<0.001]), and the results were more accurate (percentage of estimates that were >30% of measured GFR, 8.5 vs. 12.8 and 14.1, respectively [P<0.001 for both comparisons]). In participants whose estimated GFR based on creatinine was 45 to 74 ml per minute per 1.73 m(2), the combined equation improved the classification of measured GFR as either less than 60 ml per minute per 1.73 m(2) or greater than or equal to 60 ml per minute per 1.73 m(2) (net reclassification index, 19.4% [P<0.001]) and correctly reclassified 16.9% of those with an estimated GFR of 45 to 59 ml per minute per 1.73 m(2) as having a GFR of 60 ml or higher per minute per 1.73 m(2).
  
  
  CONCLUSIONS
  The combined creatinine-cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.).
authors: Division of Nephrology, Tufts Medical Center, 800 Washington St., Box 391, Boston, MA 02111, USA. linker@tuftsmedicalcenter.org
authors: Lesley A
authors: LA
authors: Inker
authors: Christopher H
authors: CH
authors: Schmid
authors: Hocine
authors: H
authors: Tighiouart
authors: John H
authors: JH
authors: Eckfeldt
authors: Harold I
authors: HI
authors: Feldman
authors: Tom
authors: T
authors: Greene
authors: John W
authors: JW
authors: Kusek
authors: Jane
authors: J
authors: Manzi
authors: Frederick
authors: F
authors: Van Lente
authors: Yaping Lucy
authors: YL
authors: Zhang
authors: Josef
authors: J
authors: Coresh
authors: Andrew S
authors: AS
authors: Levey
mesh_terms: D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000097:blood; D003404:Creatinine / Q000097:blood*; D003430:Cross-Sectional Studies; D055316:Cystatin C / Q000097:blood*; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D008297:Male; D055641:Mathematical Concepts; D008875:Middle Aged; D051436:Renal Insufficiency, Chronic / Q000097:blood / Q000145:classification / Q000175:diagnosis* / Q000503:physiopathology
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D003404:Creatinine