Evaluating the cost-effectiveness of diagnostic tests in combination: is it important to allow for performance dependency?

OBJECTIVES: To investigate the importance of accounting for potential performance dependency when evaluating the cost-effectiveness of two diagnostic tests used in combination. METHODS: Two meta-analysis models were fitted to estimate the diagnostic accuracy of Wells score and Ddimer in combination. The first model assumes that the two tests perform independently of one another; thus, two separate meta-analyses were fitted to the Ddimer and Wells score data and then combined. The second model allows for any performance dependency of the two tests by incorporating published data on the accuracy of Ddimer stratified by Wells score, as well as studies of Ddimer alone and Wells score alone. The results from the two meta-analysis models were input into a decision model to assess the impact that assumptions regarding performance dependency have on the overall cost-effectiveness of the tests. RESULTS: The results highlight the importance of accounting for potential performance dependency when evaluating the cost-effectiveness of diagnostic tests used in combination. In our example, assuming the diagnostic performance of the two tests to be independent resulted in the strategy "Wells score moderate/high risk treated for DVT and Wells score low risk tested further with Ddimer" being identified as the most cost-effective at the £20,000 willingness-to-pay threshold (probability cost-effective 0.8). However, when performance dependency is modeled, the most cost-effective strategies were "Ddimer alone" and "Wells score low/moderate risk discharged and Wells score high risk further tested with Ddimer" (probability cost-effective 0.4). CONCLUSIONS: When evaluating the effectiveness and cost-effectiveness of diagnostic tests used in combination, failure to account for diagnostic performance dependency may lead to erroneous results and nonoptimal decision making.