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.

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.

Interferon-Gamma Release Assay (IGRA)

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.

BCG Vaccination

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.

Testing and Treatment for TB in BCG-Vaccinated Persons

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.

Class B Tuberculosis Follow-up

Information regarding Tuberculosis Class B Follow-up Worksheets can be found on the IDPH Refugee Health web page. Please click here to access the Class B Follow-up Worksheet (scroll to Chapter 5), instructions for the worksheet and follow-up recommendations.

TST and Vaccine Administration

From the CDC's 12th Edition of the "Pink Book" Epidemiology and Prevention of Vaccine-Preventable Diseases: Infants and children who need a tuberculin skin test (TST) can and should be immunized. All vaccines, including MMR, can be given on the same day as a TST, or any time after a TST is applied. For most vaccines, there are no TST timing restrictions. MMR vaccine may decrease the response to a TST, poten­tially causing a false-negative response in someone who actually has an infection with tuberculosis. MMR can be given the same day as a TST, but if MMR has been given and 1 or more days have elapsed, in most situations a wait of at least 4 weeks is recommended before giving a routine TST. No information on the effect of varicella-containing vaccine or LAIV (live attenuated influenza vaccine) on a TST is available. Until such information is available, it is prudent to apply rules for spacing measles vaccine and TST to varicella vaccine and LAIV. There is a new type of tuberculosis test known as an interferon-gamma release assay (IGRA). Even though this test improves upon the TST because it is less affected by previous doses of BCG vaccine and less affected by previous doses of tuberculosis diagnostic testing, it still may be affected by previous doses of other live vaccines so it is prudent to apply the same spacing rules as for TST.

From the CDC's Core Curriculum for TB: Vaccination with live viruses may interfere with tuberculin skin test (TST) reactivity and cause false-negative reactions; this includes measles, mumps, rubella, oral polio, varicella, yellow fever, BCG, and oral typhoid. For persons scheduled to receive TST and live virus vaccines, the testing should be done either on the same day as vaccination or at least 1 month after vaccination to minimize the potential for a false-negative TST reaction.