The two tests available to test for latent TB infection are:
Tuberculin Skin Test (TST)
The tuberculin skin test (also known as PPD or Mantoux test) has been in existence for over 100 years. The test works by injecting purified protein derivative (PPD) intradermally (between the layers of the skin). PPD is an extract of inactive cultures of M. tuberculosis, the bacterium that causes TB disease. The basis of the test is to determine if the body has been exposed to TB infection. If previously infected, the body will recognize the proteins of the PPD and will generate an immune response. This response will take place in the form of an induration (lump, swelling or blister) at the site of injection. An induration may mean the person is infected. The injection site must be evaluated by a trained healthcare professional 48-72 hours after the PPD was administered. The sensitivity of the TST can vary and it can be affected by a previous Bacille Calmette-Guerin (BCG) vaccination, a depressed immune system (immunocompromised) and by other illnesses or medical treatment;
Blood Tests (e.g., The T-SPOT.TB Test)
Blood tests are relatively new developments for detecting latent TB infection. These tests are also known as Interferon Gamma Release Assays (IGRAs). The principle of the T-SPOT.TB test is the detection of effector T cells that respond to stimulation by M. tuberculosis antigens (ESAT-6 and CFP 10) by capturing interferon gamma (IFN-γ) in the vicinity of T cells from which it was secreted.
To view an overview of the TST and the T-SPOT.TB test click here.
