Table 5 Key issues that limit modelling cost‐utility in ankylosing spondylitis and recommendations for future research.

Model issue		Present study		Limitation		Proposal
Definition of disease states that are clinically and economically relevant		BASDAI		• Limited in relating clinically and economically relevant disease states		• Combined (BADAI‐BASFI) outcome measures. Application in models would require data from large RCTs and cohorts
				• Patient perspective without external (objective) criterion		• New measure including external criterion
Number of disease states that are distinguished		BASDAI <4 opposed to ⩾4		Limited number of disease states hampers identification of groups for which treatment is more or less cost‐effective		Determination of more disease states that are clinically (and economically) relevant. Application in models would require data from large RCTs
Natural course of the disease		• 5 year time horizon		Reduced ability to show additional long term beneficial effects of TNFα inhibition.		Gain insight into progression of the disease and measures to capture the progression
		• After initial placebo response no change in BASDAI over 5 years
Utilities		• Utilities measured in cohort		• Patients in the cohort did not experience the beneficial influence of the TNFα inhibitors		• Utilities derived directly from observational studies
		• EQ‐5D as only utility measure		• Different utilities can give different results		• Consensus of the recommended utility
Long term toxicity		From RCTs and open studies with low sample sizes and short observations		• Initial RCTs showed more toxicity than later RCTs		Long term observational studies
				• Limited long term observational data

BASDAI, Bath ankylosing spondylitis disease activity index; BASFI, Bath ankylosing spondylitis functional index; EQ‐5D, EuroQol 5 dimensions utility index; RCT, randomised controlled trial.