Tuberculosis infection tests (TST and IGRA)

The tests for tuberculosis infection are the tuberculin skin test, also called the PPD, and the IGRA, which is done on a blood sample. Both answer the same question: has this person been infected with Mycobacterium tuberculosis, the bacterium that causes TB? And they get there in an interesting way. They measure the immune response, meaning how the body's defenses react to proteins from that bacterium. Think of showing the body's security guards a photo of a suspect: the ones who react are the ones who have run into him before.

For the tuberculin skin test, PPD, a preparation of proteins from the bacterium, is injected intradermally, meaning into the skin itself. In someone who has been infected, it usually sets off delayed-type hypersensitivity: a reaction that doesn't happen right away and is driven by T lymphocytes, or T cells, white blood cells that specialize in recognizing invaders. In the IGRA, short for interferon-gamma release assay, white blood cells from the blood sample are exposed to antigens, pieces that mimic those proteins, and some of them respond by releasing interferon gamma, a molecule the body's defenses use to signal one another.

That's why a positive result is read as infection, but on its own it doesn't say whether there's active TB. Ruling that out takes other tests, such as a chest X-ray, the picture that shows the lungs. If those tests don't find it, the term is latent infection: a small number of bacteria stay in the body, alive but inactive, sort of asleep, and the person can't pass them on to anyone.