TB Testing

There are two kinds of tests that can be used to help detect TB infection - the TB skin test (TST) and TB blood tests or Interferon-Gamma Release Assays (IGRAS) (i.e., QuantiFERON®-TB Gold test (QFT-G), QuantiFERON®-TB Gold In-Tube test (GFT-GIT) and T-SPOT®.TB test).

IGRAS and TSTs should be used as an aid in diagnosing infection with M. tuberculosis. A positive test result suggests that M. tuberculosis infection is likely; a negative result suggests that infection is unlikely. For IGRA test results either an indeterminate or borderline test result (T-SPOT® only) indicates an uncertain likelihood of M. tuberculosis infection.

A diagnosis of Latent Tuberculosis Infection (LTBI) requires that TB disease be excluded by medical evaluation. This should include checking for signs and symptoms suggestive of TB disease, a chest x-ray, and, when indicated, examination of sputum or other clinical samples for the presence of M. tuberculosis. Decisions about a diagnosis of M. tuberculosis infection should also include epidemiological and the patient’s medical history.

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
• Tuberculosis Testing and Pregnancy

Interferon-gamma release assays (IGRAs) are blood tests that measure how the immune system reacts to the bacteria that cause TB. If your health care provider or local health department offers TB blood tests, only one visit is required to draw blood for the test. The QuantiFERON®-TB Gold test (QFT-G), QuantiFERON®-TB Gold In-Tube test (GFT-GIT) and T-SPOT®.TB test are approved TB blood tests by the Food and Drug Administration.

• General Information on IGRAs
• Interferon-Gamma Release Assays

BCG, or Bacille Calmette-Guérin, is a vaccine for TB disease. Many persons born outside of the United States have been BCG-vaccinated. BCG is used in many countries with a high prevalence of TB to prevent childhood tuberculous meningitis and miliary disease.

• BCG Fact Sheet

The question of the effect of BCG vaccine on TST results often causes confusion. TST reactivity caused by BCG vaccine generally wanes with the passage of time, but periodic skin testing may prolong (boost) reactivity in vaccinated persons. A history of BCG vaccine is not a contraindication for tuberculin skin testing or treatment for LTBI in persons with positive TST results. TST reactions should be interpreted regardless of BCG vaccination history.

IGRAs use M. tuberculosis specific antigens and, 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.