Commercially available tests for latent tuberculosis infection (LTBI) in a diverse population with a low LTBI prevalence in low-prevalence populace may be incompatible.

Scientist at the Walter Reed Army Institute (Silver Springs, MD, USA) conducted a cross-sectional study involving 2,017 military recruits at Fort Jackson in South Carolina. The participants were asked to complete a risk factor questionnaire and were checked for TB with the three commercially available tests, consisting of the tuberculin skin test (TST); the interferon gamma release assays (IGRAs), QuantiFERON-TB Gold In-Tube test (QFT-GIT) and the TSPOT TB test (T-Spot). The scientists also conducted the Battey Skin Test (BST) on all participants to assess the impact of nontuberculosis mycobacteria (NTM) reactivity on test discordance.

The results demonstrated that there was no substantial difference between the TST, QFT-GIT (Cellestis Inc.; Valencia, CA, USA) and T-Spot tests (Oxford Immunotec; Abingdon, UK). From 88 positive test results, 68 participants (77%) showed positive to one test, 10 participants (11.4%) showed positive in two tests, and only 10 people (11.4%) were positive to all three tests. The scientists linked Bacille Calmette Guerin vaccination, the tuberculosis prevalence in the country of birth, and Battey skin test reaction size to being TST positive with IGRA negative test discordance.

These results support evidence that false positive TST results can be caused by nontuberculous mycobacteria (NTM) sensitization. Greater quantitative test results and higher TB risk levels were linked to a higher agreement between tests. James Mancuso, MD, Dr. PH, senior author from the Walter Reed Army Institute of Research, explained that: "Our data support a high proportion of false positives with any of these three tests in a low- prevalence population as 77% of our subjects had positive results with only one test. Lower quantitative results were associated with a smaller risk for TB exposure and single positive tests, and lower risk for TB exposure was associated with decreasing test agreement." The study was published on December 8 2011, in the American Journal of Respiratory and Critical Care Medicine.

Related Links:
Walter Reed Army Institute
Cellestis
Oxford Immunotec