Tuberculosis Testing Information

Testing

Persons at higher risk for Tuberculosis (TB) exposure and infection should be offered targeted testing by their healthcare provider.

There are two kinds of tests that can be used to help detect TB infection – the TB skin test (TST) and TB blood tests.  A positive TB skin test or TB blood test indicates that a person has been infected with TB bacteria.  It does not differentiate between latent TB infection (LTBI) or active TB disease in the individual.  Other tests, such as a chest x-ray and a sample of sputum, are needed to diagnose active TB disease.

Testing for TB in BCG-Vaccinated Persons

BCG, or bacille Calmette-Guérin, is a vaccine for TB disease. Many persons born outside of the United States have been BCG-vaccinated. BCG vaccination may cause a positive reaction to the TB skin test, which may complicate decisions about prescribing treatment. Despite this potential for BCG to interfere with test results, the TB skin test is not contraindicated for persons who have been vaccinated with BCG. The presence or size of a TB skin test reaction in these persons does not predict whether BCG will provide any protection against TB disease.  Furthermore, the size of a TB skin test reaction in a BCG-vaccinated person is not a factor in determining whether the reaction is caused by latent TB infection (LTBI) or the prior BCG vaccination.

TB blood tests (interferon-gamma release assays or IGRAs), unlike the TB skin tests, are not affected by prior BCG vaccination and are not expected to give a false-positive result in persons who have received prior BCG vaccination.