Analytical approach:
A Markov model was used to combine published evidence for the diagnostic strategies. Treatment for ulcerative colitis, Crohn's disease, and IBS was based on published models. The outcomes for true and false negative results for IBD were modelled; those for false positives were not. The time horizon was 10 years. The authors stated that the perspective of the patient benefits and NHS and Personal Social Services costs was adopted.
Effectiveness data:
Test accuracy data were from a systematic review of the literature. The estimates for the ELISA were from a meta-analysis. Those for CalDetect were from one study. Optimal cutoff points were chosen. Colonoscopy was assumed to have 100% specificity. The effectiveness of treatment for ulcerative colitis, Crohn's disease and IBS was from the published models, using the most cost-effective treatments in these models. The delay in correctly identifying patients with false negative results was estimated by clinical experts.
Monetary benefit and utility valuations:
Utilities were applied for the Crohn's disease, ulcerative colitis and IBS health states. The estimates were from publications identified by a review of the literature. The authors selected those estimates that had the most relevance to the population and to the UK.
Measure of benefit:
The measure of benefit was the quality-adjusted life-year gained. Benefits were discounted at an annual rate of 3.5%.
Cost data:
The costs included the provision of the tests, consultations, and treatment including medication and surgery. They were from NHS Reference Costs, the British National Formulary, and the Personal Social Services Research Unit. Staff time for the tests was estimated by experts. All costs were reported in UK £. The price year was 2011. Costs were inflated to 2011, where necessary, using the Hospital and Community Services Costs price index. They were discounted at an annual rate of 3.5%.
Analysis of uncertainty:
Probabilistic sensitivity analysis was conducted by varying all model parameters simultaneously. The results were presented in cost-effectiveness acceptability frontiers. One-way sensitivity analysis was conducted on the key parameters, including the whether calprotectin testing increased the number of patients being tested in general practice.