TB Tests Yield Mostly False-Positives in Low-Risk Populations

Steven Fox

December 12, 2011

December 12, 2011 — In populations with low prevalence of tuberculosis (TB), the majority of positive results obtained with the 3 commercially available diagnostic tests are likely to be false-positives, according to findings from a new study published online December 8 in the American Journal of Respiratory and Critical Care Medicine.

"There is substantial discordance between the tuberculin skin test (TST) and the interferon gamma release assays (IGRAs) in populations with low prevalence of tuberculosis, and most positive results from the three tests identify different people," write James Mancuso, MD, DrPH, from the Division of Preventive Medicine, Walter Reed Army Institute of Research in Silver Spring, Maryland, and colleagues.

The researchers designed a cross-sectional comparison study of 2017 military enlistees at Fort Jackson, South Carolina. Data on the participants were collected between April and June 2009.

All the recruits completed risk factor questionnaires and then were administered the 3 commercially available latent TB diagnostic tests:

  • TST,

  • IGRAs QuantiFERON-TB Gold In-Tube test, and

  • TSPOT TB test.

The patients also were given the Battey Skin Test (BST) to see whether exposure to nontuberculosis mycobacteria (NTM) had affected their reactivity to the 3 TB tests.

The researchers report that none of the diagnostic tests demonstrated any significant differences with regard to specificity.

Of the 88 recruits who had positive test results, 68 (77%) were positive on 1 test, 20 (22.7%) were positive on 2 tests, and 10 (11.4%) were positive on all 3 tests, the researchers say.

Several factors were associated with TST-positive/IGRA-negative test discordance: history of Bacille Calmette-Guérin vaccination, prevalence of TB in the participants' native country, and the size of their reaction to the BST.

The authors say that finding supports previous evidence suggesting that NTM sensitization can trigger false-positive results on the TST test.

They also note that increased agreement between test results was associated with epidemiologic factors indicating risk for infection, as well as with quantitative test results.

"[L]ower quantitative results were associated with a smaller risk for TB exposure...[and] single positive tests," the researchers write, "and lower risk for TB exposure was associated with decreasing test agreement."

They conclude, "To support the current recommendations to treat TB diagnostics interchangeably, targeted testing using risk stratified interpretation should be used for the IGRAs as with the TST."

Support for this work was provided by the Infectious Disease Clinical Research Program, a Department of Defense program executed through the Uniformed Services University of the Health Sciences. This project was funded in whole, or in part, with federal funds from the National Institute of Allergy and Infectious Diseases, National Institutes of Health. The authors have disclosed no relevant financial interests.

Am J Respir Crit Care Med. Published online December 8, 2011. Abstract

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