Pseudoepidemics of tuberculin skin test conversions in the US Army after recent deployments

Rationale: The tuberculin skin test (TST) has many sources of error. These can lead to predominantly false-positive reactions when used in low-risk populations. The U.S. Army deploys to areas considered at high risk for tuberculosis (TB) infection, but often has limited contact with the local population. Objectives: We describe the investigation of eight pseudoepidemics of TST conversions in U.S. Army populations, five of which were associated with overseas deployments. Methods: Outbreak investigations of these pseudoepidemics consisted of several components: evaluation of active and latent TB surveillance data, review of medical records, investigation and interviews of active TB cases and their contacts, evaluation of materials and personnel screening procedures, and placement and reading of repeat skin testing. Measurements and Main Results: Initially reported risk of conversion in the outbreaks ranged from 1.3 to 15%. Repeat testing of converters (positives) found that 30 to 100% were negative on retesting. Several sources of false-positive results were identified in these pseudoepidemics, including variability in reading and administration, product variability, and cross-reactions to nontuberculous mycobacteria. Conclusions: Pseudoepidemics of TST conversions are a common occurrence after U.S. Army deployments and in U.S. Army populations. U.S. Army forces generally have a low risk of TB infection resulting from deployments due to limited exposure to local nationals with active TB, and universal testing in this population has a low positive-predictive value.