Analytical approach:
This economic evaluation was based on a published decision model (a standard decision tree plus a Markov model) which was adapted to the Spanish setting using country-specific data. The time horizon of the study was 40 years. The authors stated that the perspectives of society and the health care system were adopted.
Effectiveness data:
The clinical data were derived from published studies which were selected by the authors. The effectiveness data were obtained from two sources. In the first analysis, the data were derived from an international double-blind, placebo-controlled, 12-week, clinical trial with open (double-blind removed) extension in 70 patients with confirmed AS and active disease who were followed up for 102 weeks. In the second analysis, the data were retrieved from an open, multicentre, clinical trial in Spain with 42 patients who were followed up for 60 weeks. The data on disease progression were obtained from a sample of 1,110 patients who responded to postal surveys at the University of Bath in the UK. Some key assumptions on treatment discontinuation and the long-term efficacy of treatment were necessary, given the lack of long-term data.
Monetary benefit and utility valuations:
The utility valuations were derived from a cross-sectional retrospective survey of 601 randomly selected AS patients in Spain, using the EQ-5D questionnaire. When data from these Spanish patients were not available, or when there were insufficient data to populate some health states, data from a larger cohort of 1,413 UK patients were used.
Measure of benefit:
Quality-adjusted life-years (QALYs) were the summary benefit measure used and a 3% annual discount rate was applied.
Cost data:
The three main cost categories considered were direct health care costs, non-medical direct costs associated with informal care, and indirect costs related to productivity losses. The cost of infliximab also included the management of adverse events. Resource consumption was derived from the previously mentioned cross-sectional retrospective survey using a validated questionnaire. The costs for health care services were valued using public sources. For example, the cost of infliximab was based on the official list price. Informal care was valued using the replacement approach, while productivity losses were calculated using the human capital method. All costs were in Euros (EUR) and the price year was 2005. Given the long-term horizon of the analysis, costs accrued after the first year were discounted at an annual rate of 3%.
Analysis of uncertainty:
A deterministic sensitivity analysis was carried out by varying specific model inputs such as discount rate, discontinuation rate, time horizon, and disease-specific mortality. Given the lack of definitive data on treatment progression in the long-term, the results were presented for three scenarios which were no worsening of Bath AS functional index (BASFI) while on treatment, no effect of treatment on BASFI, and half the natural rate of progression.