Different screening strategies (single or dual) for the diagnosis of suspected latent tuberculosis: a cost effectiveness analysis

Abstract

Background: Previous health economic studies recommend either a dual screening strategy [tuberculin skin test (TST) followed by interferon-gamma-release assay (IGRA)] or a single one [IGRA only] for latent tuberculosis infection (LTBI), the former largely based on claims that it is more cost-effective. We sought to examine that conclusion through the use of a model that accounts for the additional costs of adverse drug reactions and directly compares two commercially available versions of the IGRA: the Quantiferon-TB-Gold-In-Tube (QFT-GIT) and T-SPOT.TB.

Methods: A LTBI screening model directed at screening contacts was used to perform a cost-effectiveness analysis, from a UK healthcare perspective, taking into account the risk of isoniazid-related hepatotoxicity and post-exposure TB (2 years post contact) using the TST, QFT-GIT and T-SPOT.TB IGRAs.

Results: Examining costs alone, the TST/IGRA dual screening strategies (TST/T-SPOT.TB and TST/QFT-GIT; 162,387 pounds and 157,048 pounds per 1000 contacts, respectively) cost less than their single strategy counterparts (T-SPOT.TB and QFT-GIT; 203,983 pounds and 202,921 pounds per 1000 contacts) which have higher IGRA test costs and greater numbers of persons undergoing LTBI treatment. However, IGRA alone strategies direct healthcare interventions and costs more accurately to those that are truly infected.Subsequently, less contacts need to be treated to prevent an active case of TB (T-SPOT.TB and QFT-GIT; 61.7 and 69.7 contacts) in IGRA alone strategies. IGRA single strategies also prevent more cases of post-exposure TB. However, this greater effectiveness does not outweigh the lower incremental costs associated with the dual strategies. Consequently, when these costs are combined with effectiveness, the IGRA dual strategies are more cost-effective than their single strategy counterparts. Comparing between the IGRAs, T-SPOT.TB-based strategies (single and dual; 39,712 pounds and 37,206 pounds per active TB case prevented, respectively) were more cost-effective than the QFT-GIT-based strategies (single and dual; 42,051 pounds and 37,699 pounds per active TB case prevented, respectively). Using the TST alone was the least cost-effective (47,840 pounds per active TB case prevented). Cost effectiveness values were sensitive to changes in LTBI prevalence, IGRA test sensitivities/specificities and IGRA test costs.

Conclusion: A dual strategy is more cost effective than a single strategy but this conclusion is sensitive to screening test assumptions and LTBI prevalence.

Figure 1

TST screening strategy for diagnosis of presumed latent TB infection (LTBI). A decision tree for the diagnosis of LTBI using the TST alone in a single test strategy. Square nodes represent decision branches, circular nodes represent chance branches and triangular nodes represent terminal branches.

Figure 2

IGRA (T-SPOT.TB or QFT-GIT) screening strategy for diagnosis of presumed latent TB infection (LTBI). A decision tree for the diagnosis of LTBI using the IGRA (T-SPOT.TB or QFT-GIT) alone in a single test strategy. Square nodes represent decision branches, circular nodes represent chance branches and triangular nodes represent terminal branches. The same decision tree was used for both versions of IGRA single strategies as they both have identical screening steps in each scenario.

Figure 3

Dual screening strategy (IGRA on all TST positive) for diagnosis of presumed latent TB infection (LTBI). Decision tree for the diagnosis of LTBI using a dual diagnostic strategy (TST in all cases followed by the T-SPOT.TB or QFT-GIT for a positive TST result). Square nodes represent decision branches, circular nodes represent chance branches and triangular nodes represent terminal branches. The same decision tree was used for both versions of the IGRA dual strategy as they both have identical screening steps in each scenario.