Dual skin tests with Mycobacterium avium sensitin and PPD to detect misdiagnosis of latent tuberculosis infection

BACKGROUND:A positive tuberculin skin test (TST) may indicate cross-reacting immunity to non-tuberculous mycobacteria (NTM) and not latent tuberculosis infec- tion (LTBI). OBJECTIVES: To assess misclassification of LTBI, as assessed by skin testing with Mycobacterium avium sensitin (MaS), and to determine how this misclassifica- tion affects the analysis of risk factors for LTBI. METHODS: In a population-based survey, participants underwent skin testing with M. tuberculosis purified pro- tein derivative (PPD) and MaS. A PPD-dominant skin test was a reaction that was ≥ 3 mm larger than the MaS reaction; a MaS-dominant skin test was a reaction that was ≥ 3 mm larger than the PPD reaction. RESULTS: Of 447 randomly selected persons, 135 (30%) had a positive PPD test. Of these, 21 (16%) were MaS- dominant, and were therefore attributable to NTM and misclassified as LTBI. PPD reactions of 5–14 mm were more likely to be misclassified than those ≥ 15 mm (OR = 5.0, 95%CI 1.9–13.2). Adjusting for misclassification had only a small impact on the analysis of risk factors for LTBI. CONCLUSIONS: A substantial number of individuals who are diagnosed with LTBI are actually sensitized to NTM. Using dual skin testing would reduce misdiagnosis and prevent unnecessary treatment.