A tuberculin skin test (THT, TST) is a method to determine whether someone has come into contact with the tuberculosis pathogen (Mycobacterium tuberculosis). The test is often used to detect latent tuberculosis infection (LTBI). However, it does not provide any information as to whether the person has active tuberculosis.
A small amount of tuberculin (killed components of tuberculosis bacteria) is injected into the inside of the forearm using a small needle. The result is read after 48 to 72 hours. Tuberculin RT23 is used for this purpose in Germany.
If a hardening or swelling forms, the test is considered positive. It is important to determine the size of the induration. A positive test result means that the person tested has had contact with tuberculosis pathogens. A latent tuberculous infection is probably present. A positive test result can also be observed in people who have received a BCG vaccination against tuberculosis in the past or with non-tuberculous mycobacteria (see NTM). An IGRA should be performed to differentiate between the two. Tuberculosis must be ruled out with an X-ray of the lungs and, if necessary, further examinations.
The tuberculin skin test should be applied and read by experienced personnel. In addition to the THT, there are blood tests to detect a latent infection (see IGRA). A positive result does not mean that tuberculosis is present; further tests are necessary for this. A negative test does not always rule out tuberculosis or infection. Once a diagnosis of latent tuberculosis infection (LTBI) has been made, preventive treatment should be given. This can significantly reduce the risk of contracting the disease.
After tuberculosis has taken place, the THT usually remains positive and can no longer be used in the diagnosis of LTBI.