Diagnosing Latent TB Infection

Latent TB Infection (LTBI) can convert to active disease, especially in people with a weakened immune system that may be unable to control the latent infection. It is therefore important to identify those people with LTBI and treat them before they convert to active TB disease and pass it onto others.

Risk groups with higher rates of conversion to active disease include children, the elderly, transplant patients, people who are HIV positive, and those being treated for rheumatoid arthritis.

The only two methods for detecting latent TB infection are:
Blood Test (e.g. the T-SPOT.TB test)
Tuberculin Skin Test (TST)

Blood Test (e.g. T-SPOT.TB)
Blood tests are a relatively new development in TB testing for both active disease and LTBI. A tube of blood is needed to run this test which is performed in the laboratory. The result is available to the doctor in one or two days. The T-SPOT.TB test holds several major advantages over the tuberculin skin test: it does not require a second visit, it is not affected by BCG vaccination and it is very reliable–even in patients with weakened immune systems*.

Tuberculin Skin Test (TST)
The TST has been in existence for over 100 years. A small amount of liquid containing TB proteins is injected into the lower part of the arm. The injection site is examined by a trained healthcare professional 48-72 hours later. If the person has LTBI, the body recognizes the proteins that were injected and responds by forming a lump where the TB proteins were injected.

The accuracy of the TST varies and can be affected by a previous BCG vaccination, a weakened immune system and by other illnesses or medical treatments.

Both tests identify LTBI by using the body’s immune response to TB proteins, whether the person is showing signs and symptoms of TB disease or not.

*Based on a study involving 963 subjects which was performed to investigate the effects of selected risk factors on the T-SPOT.TB assay. The study demonstrated no association between the T-SPOT.TB results and immunocompromised status or BCG vaccination. T-SPOT.TB was also not impacted by age. T-SPOT.TB US Pivotal Clinical Study, PI-TB-US-V1