Testing for Immune Response to M. Tuberculosis

Prior to the development of interferon-gamma release assays (IGRA), the tuberculin skin test (TST) was the only test available for identifying an immune response to infection with tuberculosis. Both can be used to screen for latent TB infection. Although the IGRA is more specific, does not react to BCG vaccine, and may be more sensitive than the TST, "Limited data exist on (IGRA) use in groups such as children younger than 5 years of age, persons recently exposed to TB, immuno-compromised persons, and those who will be tested repeatedly (serial testing)." Read more about IGRA in the Targeted LTBI (PDF) guide on the CDC's website.

Reliable Testing Solutions

Only the Mantoux method for tuberculin skin testing should be used. The tine test is not a reliable test. TST should only be applied and read by health care providers trained and proficient in it. For details on administration and measurement of TST, see Appendix C (PDF) in the Guide for Primary Health Care Providers. TST results should always be recorded in specific millimeters of induration, with "0 mm" indicating no induration. They should not be recorded as a range (e.g. " less than 10 mm" or "10-15 mm").

Interpreting a TST Result

The interpretation of a TST result (positive or negative) depends on the exposure history and clinical status of the patient as well as the induration in response to intradermal tuberculin (PPD). See "Classification of tuberculin skin test reactions (PDF)” in Latent Tuberculosis Infection: A Guide for Primary Health Care Providers for more information.