title: How to deal with cost differences at baseline.
abstract: BACKGROUND
  To our knowledge, adjustment for baseline imbalances in costs has never been performed in trial-based cost-effectiveness analyses.
  
  
  METHODS
  We used data from a clinical trial performed in the Netherlands comparing two outpatient psychotherapies: schema-focused therapy (SFT) versus transference-focused psychotherapy (TFP). Costs were assessed with a cost interview. Outcome was the proportion of recovered patients measured with the Borderline Personality Disorder Severity Index (BPDSI-IV). We used three methods to adjust the costs for baseline differences: (i) mean difference adjustment, calculating total costs after baseline by adjusting the difference between groups with the difference of the mean baseline costs; (ii) delta adjustment, calculating the individual differences between patient baseline and the subsequent measurements (concerning incremental costs, this is the same as mean difference adjustment); and (iii) regression-based adjustment, adjusting total costs with a regression model, with total costs as the dependent variable and baseline costs as the independent variable.
  
  
  RESULTS
  Mean baseline costs were 3339 euros for SFT and 4238 euros for TFP, a mean difference of 899 euros. Total unadjusted follow-up costs were 30822 euros for SFT and 36812 euros for TFP. The fraction of recovered patients was 45% for SFT and 24% for TFP. Cost-effectiveness acceptability curves show that mean difference and delta adjustments are different from the regression-based methods.
  
  
  CONCLUSIONS
  Although the routine starting point of an analysis should always be an unadjusted analysis of the cost effectiveness, a baseline difference between treatment groups should be adjusted for. This should be done by reported patient characteristics or, when these are not sufficiently present, by baseline costs as a substitute. This adjustment should be carried out most preferably with a regression-based method.
authors.affiliation: Department of Clinical Epidemiology and Medical Technology Assessment, Maastricht University Medical Center, Maastricht, the Netherlands. thea.van.asselt@mumc.nl
authors.forename: Antoinette D I
authors.initials: AD
authors.lastname: van Asselt
authors.forename: Ghislaine A P G
authors.initials: GA
authors.lastname: van Mastrigt
authors.forename: Carmen D
authors.initials: CD
authors.lastname: Dirksen
authors.forename: Arnoud
authors.initials: A
authors.lastname: Arntz
authors.forename: Johan L
authors.initials: JL
authors.lastname: Severens
authors.forename: Alfons G H
authors.initials: AG
authors.lastname: Kessels
mesh_terms: D001883:Borderline Personality Disorder / Q000191:economics / Q000628:therapy; D017281:Cost of Illness; D003362:Cost-Benefit Analysis / Q000379:methods*; D006801:Humans; D018803:Models, Economic; D011613:Psychotherapy / Q000191:economics; D016032:Randomized Controlled Trials as Topic