How to Deal with Cost Differences at Baseline

Authors: van Asselt, Antoinette D.I.¹; van Mastrigt, Ghislaine A.P.G.¹; Dirksen, Carmen D.¹; Arntz, Arnoud²; Severens, Johan L.; Kessels, Alfons G.H.¹

Source: PharmacoEconomics, Volume 27, Number 6, 6 June 2009 , pp. 519-528(10)

Publisher: Adis International

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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 for SFT and €4238 for TFP, a mean difference of €899. Total unadjusted follow-up costs were €30 822 for SFT and €36 812 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.

Document Type: Research article

Affiliations: 1: 1 Department of Clinical Epidemiology and Medical Technology Assessment, Maastricht University Medical Center, Maastricht, the Netherlands 2: 3 Department of Clinical Psychological Science, Maastricht University, Maastricht, the Netherlands

Publication date: 2009-06-06

How to Deal with Cost Differences at Baseline

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