title: Longitudinal formulas to estimate GFR in children with CKD.
abstract: OBJECTIVE
  Whereas current GFR estimating equations approximate direct GFR measurement at a single time point, formulas that capitalize on changes in easily measured biologic parameters could improve the accuracy and precision of GFR estimation.
  
  
  METHODS
  In the Chronic Kidney Disease in Children Cohort (aged 1 to 16 yr), we measured GFR by plasma disappearance of iohexol (iGFR) and biomarkers in the first two annual visits. Models took the form GFR(2) = a[GFR(1)/40](b)[X(2)/X(1)](c), where GFR(2) and GFR(1) represented the current and previous years' iGFR, 40 ml/min per 1.73 m(2) was the cohort mean, and X(2)/X(1) was the change in predictors over time. Using data from 360 participants with a median age of 12.1 yr, we evaluated the predictive performance of a past GFR measurement and 20 other variables using a two-thirds random sample of the data. A one-third sample was reserved for validation.
  
  
  RESULTS
  Previous iGFR measurements were strongly predictive of subsequent iGFR and adding change in height/serum creatinine significantly improved the explanatory power to 78%. In the validation set, the correlation between estimated and measured GFR was 0.88, and 48 and 88% of estimated GFRs were within 10 and 30% of observed iGFRs. When the past GFR measurement was not used, addition of change in markers to a cross-sectional model did not improve prediction.
  
  
  CONCLUSIONS
  Longitudinal formulas to estimate iGFR capitalize on the high predictive power of previous iGFR measurements and in this study yielded a parsimonious prediction model with the potential for assessing progression in the clinical setting.
authors.affiliation: Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe Street, Baltimore, MD 21205, USA. agump@jhsph.edu
authors.forename: Alison G
authors.initials: AG
authors.lastname: Abraham
authors.forename: George J
authors.initials: GJ
authors.lastname: Schwartz
authors.forename: Susan
authors.initials: S
authors.lastname: Furth
authors.forename: Bradley A
authors.initials: BA
authors.lastname: Warady
authors.forename: Alvaro
authors.initials: A
authors.lastname: Muñoz
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D003287:Contrast Media / Q000493:pharmacokinetics; D005260:Female; D005500:Follow-Up Studies; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007223:Infant; D007472:Iohexol / Q000493:pharmacokinetics; D007677:Kidney Function Tests / Q000379:methods* / Q000592:standards*; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D008954:Models, Biological; D011237:Predictive Value of Tests; D012015:Reference Standards; D051436:Renal Insufficiency, Chronic / Q000503:physiopathology*
chemical_list: D003287:Contrast Media; D007472:Iohexol