What is the value of collecting detailed costing data in clinical trials?

Objectives Cost data for trial-based economic evaluation can be obtained through micro-costing (collecting resource use and unit cost data for each centre or patient), grosscosting (average costs based on top-line budgets) or provider tariffs (e.g. healthcare resource groups, HRGs). Most studies use a combination of approaches due to data availability, although there is little guidance on which is best. We report a systematic comparison of the three costing approaches in IVAN: a non-inferiority randomised controlled factorial trial of treatment regimens for age-related macular degeneration (AMD), where policy makers are interested in the efficacy and cost-effectiveness of two dosing regimens of bevacizumab (Avastin) and ranibizumab (Lucentis). We aimed to assess the extent to which micro-costing, gross-costing and HRGs differ, and to investigate resource use variation between UK hospitals and explore possible reasons for this variability.