A positive tuberculosis (TB) test means that the individual being tested has been exposed to the bacteria that causes tuberculosis, known as Mycobacterium tuberculosis. TB is a serious infectious disease that primarily affects the lungs, but can also affect other parts of the body. It is spread through the air when an infected person coughs or sneezes.
There are two primary tests used to diagnose TB: the tuberculin skin test (TST) and the interferon-gamma release assay (IGRA). Both tests measure the body’s immune response to the bacteria, but they do so in different ways. The TST involves injecting a small amount of tuberculin protein into the skin, usually on the forearm, and then observing the site for a reaction. The IGRA is a blood test that detects the release of interferon-gamma in response to specific TB antigens.
A positive TB test indicates that the person has been infected with M. tuberculosis, but it does not necessarily mean that they have active TB disease. Most people who become infected with TB are able to fight off the bacteria and never develop symptoms of the disease. This is known as latent TB infection (LTBI), and it can be treated with antibiotics to prevent the bacteria from becoming active in the future.
The TST is read by measuring the size of the induration, or raised area, at the injection site. A positive TST is defined as an induration of 5 mm or greater in people who are considered to be at high risk for TB, such as those with HIV infection, recent close contact with a person with TB disease, or abnormal chest x-ray findings consistent with TB. In people who are not considered to be at high risk, a positive TST is defined as an induration of 10 mm or greater.
The IGRA is interpreted based on the level of interferon-gamma released in response to TB antigens. There are two types of IGRA tests available: the QuantiFERON-TB Gold (QFT-G) and the T-SPOT.TB. Both tests require a blood sample to be taken and analyzed in a laboratory. A positive result on either test indicates that the person has been infected with M. tuberculosis.
If a person has a positive TB test, further testing is needed to determine whether they have active TB disease or LTBI. This may involve a chest x-ray, sputum culture, or other tests to look for evidence of active TB disease. If active TB disease is diagnosed, treatment with multiple antibiotics is necessary to cure the infection and prevent the spread of the bacteria to others.
It is important to note that a positive TB test does not necessarily mean that the person is contagious or that they will develop active TB disease. However, it is important for anyone with a positive TB test to follow up with a healthcare provider to determine the best course of action.
In summary, a positive TB test indicates that a person has been infected with M. tuberculosis, but it does not necessarily mean that they have active TB disease. Further testing is needed to determine the presence of active TB disease or LTBI. Treatment with antibiotics is necessary to cure active TB disease and prevent the spread of the bacteria to others. It is important for anyone with a positive TB test to follow up with a healthcare provider to determine the best course of action.