Commercial TB Tests Turn Up High False-Positive Rates

Mycobacterium tuberculosis bacteria (CDC)

Tests of three commercially available tests to diagnose the presence of latent tuberculosis turned up false positive results in a majority of positive cases on at least one of the tests. James Mancuso of the Walter Reed Army Institute of Research, with colleagues from the Uniformed Services University of the Health Sciences and Otsuka Pharmaceuticals, discuss their findings in the American Journal of Respiratory and Critical Care Medicine.

The study sampled 2,017 recruits stationed at Fort Jackson, South Carolina, who completed a risk factor questionnaire and underwent testing for tuberculosis with:
1) Tuberculin skin test (TST)
2) Interferon gamma release assays (IGRAs) QuantiFERON-TB Gold In-Tube test (QFT-GIT)
3) TSPOT TB test (T-Spot)

The subjects also received the Battey Skin Test to assess the impact of non-tuberculosis mycobacteria reactivity on test discordance. Nontuberculous mycobacterium infections are caused by mycobacteria that are found in water and soil. They can become chronic and require ongoing treatment.

The findings show a wide range of positive results from the three tests. Of 88 subjects returning a positive test, 68 (77%) were positive to one test, 10 (11.4%) were positive to two tests, and the same number of cases — 10 (11.4%) — were positive to all three tests. “Our results suggest that in low-prevalence populations,” says Mancuso, “most positive results obtained with these tests are false positives.”

The researchers found a number of factors associated with the postive tuberculin skin test results, specifically the Bacille Calmette Guerin vaccination for tuberculosis, prevalence of tuberculosis in the subject’s country of birth, and Battey skin test reaction size.

Read more: Small Business Grant Awarded for Tuberculosis Test

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