title: A comparison of three methods to estimate baseline creatinine for RIFLE classification.
abstract: BACKGROUND
  A pre-morbid 'baseline' creatinine is required in order to diagnose and stage acute kidney injury (AKI) using the RIFLE classification. Estimation of baseline creatinine by solving the Modification of Diet in Renal Disease (MDRD) equation assuming a glomerular filtration rate of 75 ml/min/1.73 m(2) has been widely used but never validated.
  
  
  METHODS
  We analysed four cohorts of intensive care unit (ICU) patients from three centres (two from Pittsburgh and one from Mayo and Austin). Three cohorts consisted of preselected patients without AKI (Pittsburgh 1 n = 1048, Mayo n = 737, Austin n = 333), and measured creatinine values in these cohorts were taken to represent baseline creatinine values. The last cohort (Pittsburgh 2 n = 468) consisted of unselected ICU patients with baseline creatinine values recorded within 1 year before ICU admission. Using the Pittsburgh 1 cohort, we derived an equation using the same anthropometric variables as the MDRD equation: baseline creatinine = 0.74 - 0.2 (if female) + 0.08 (if black) + 0.003 × age (in years). We then compared measured creatinine in the Mayo and Austin cohorts and recorded creatinine in the Pittsburgh 2 cohort to the estimated creatinine from: (i) the MDRD equation; (ii) our new equation; (iii) a gender-fixed creatinine of 0.8 mg/dl for females and 1.0 mg/dl for males.
  
  
  RESULTS
  Using any of the three methods, the median absolute error of the estimates was of the order of 0.1-0.2 mg/dl, and overall accuracy was similar. When the definition of AKI was limited to the severity grades of Injury and Failure, all three methods were able to generate 78-90% reliable results for preselected normal range cohorts, and 63-70% for the unselected cohort of ICU patients.
  
  
  CONCLUSIONS
  Estimates of incidence of AKI in the critically ill using RIFLE classification can be affected by the bias and limited accuracy of methods to estimate baseline creatinine. Whenever possible, recorded creatinine values should be used as a reference of baseline. The use of the MDRD equation to estimate baseline creatinine when it is unknown may over- or underestimate some mild (Risk) AKI cases but is unlikely to misclassify patients in Injury and Failure.
authors.affiliation: The CRISMA (Clinical Research, Investigation, and Systems Modeling of Acute Illness) Laboratory, Department of Critical Care Medicine, University of Pittsburgh, School of Medicine, Pittsburgh, USA.
authors.forename: Jakub
authors.initials: J
authors.lastname: Závada
authors.forename: Eric
authors.initials: E
authors.lastname: Hoste
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Cartin-Ceba
authors.forename: Paolo
authors.initials: P
authors.lastname: Calzavacca
authors.forename: Ognjen
authors.initials: O
authors.lastname: Gajic
authors.forename: Gilles
authors.initials: G
authors.lastname: Clermont
authors.forename: Rinaldo
authors.initials: R
authors.lastname: Bellomo
authors.forename: John A
authors.initials: JA
authors.lastname: Kellum
mesh_terms: D058186:Acute Kidney Injury / Q000097:blood* / Q000145:classification* / Q000503:physiopathology; D000368:Aged; D015415:Biomarkers / Q000097:blood; D044383:Black People; D002965:Classification / Q000379:methods*; D003404:Creatinine / Q000097:blood*; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology; D006801:Humans; D007676:Kidney Failure, Chronic / Q000453:epidemiology; D007677:Kidney Function Tests; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D012307:Risk Factors; D012720:Severity of Illness Index; D044465:White People
chemical_list: D015415:Biomarkers; D003404:Creatinine