CDC | TB | Testing & Diagnosis

Tuberculosis

Testing & Diagnosis

Testing for TB Infection

There are two kinds of tests that can be used to help detect TB infection – the TB skin test (TST) and special TB blood tests. A positive TB skin test or TB blood test only tells that a person has been infected with TB bacteria. It does not tell whether or not the person has progressed to TB disease. Other tests, such as a chest x-ray and a sample of sputum, are needed to see whether the person has TB disease.

Mantoux tuberculin skin test

The TB skin test (Mantoux tuberculin skin test) is performed by injecting a small amount of fluid (called tuberculin) into the skin in the lower part of the arm. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm.
- Tuberculin Skin Testing (fact sheet)

Special TB blood tests

The special blood tests (interferon-gamma release assays [IGRAs]) measure how the immune system reacts to the bacteria that cause TB. Since they are relatively new, few health departments offer these blood tests. If your health department does offer the special blood tests, only one visit is required to draw blood for the test. The QuantiFERON^®-TB Gold test (QFT-G) and T-SPOT^®.TB test are two examples of special TB blood tests.
- QuantiFERON^®-TB Gold Test Fact Sheet

Who Should Get Tested for TB

Persons should get tested for TB by their doctor or local health department if they

have spent time with a person known or suspected to have active TB disease; or
have HIV infection or another condition that weakens the immune system and puts them at high risk for active TB disease; or
have symptoms of active TB disease; or
are from a country where active TB disease is very common (most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia); or
live somewhere in the United States where active TB disease is more common such as a homeless shelter, migrant farm camp, prison or jail, or some nursing homes); or
inject illegal drugs.

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.

The special blood tests (interferon-gamma release assays [IGRAs]), unlike the TST, are not affected by prior BCG vaccination and are less likely to give a false-positive result.

BCG Vaccine (Fact sheet)

Diagnosis of TB Disease

Persons suspected of having TB disease should be referred for a medical evaluation, which should include a

- Medical history,
- Physical examination,
- Test for TB infection (TB skin test or special blood test),
- Chest radiograph (X-ray), and
- Appropriate bacteriologic or histologic examinations (tests to see if TB bacteria are in the sputum).