Diagnosing Active TB Disease

Because active TB disease can be difficult to diagnose, especially in children and those who have weakened immune systems, additional tests beyond medical examinations are required. The following tests may be used to determine if a patient has active TB disease:

Tuberculin Skin Test (TST)
Chest Radiograph (X-ray)
Sputum Smear Microscopy (SSM)
Culture
Polymerase Chain Reaction (PCR)
Blood Test (e.g. the T-SPOT.TB test)

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 2 – 3 days 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 TB vaccination (BCG), a weakened immune system and by other illnesses or medical treatments.

Chest X-rays
Chest X-rays are used to check for lung abnormalities in people who have signs and symptoms of TB disease in the lungs. Although chest x-rays may suggest that TB disease is present, a chest X-ray alone cannot definitely diagnose a tuberculosis infection in the lungs or anywhere else in the body.

Sputum Smear Microscopy (SSM)
This is a simple laboratory test that examines sputum for bacteria using a microscope. Since some other non-TB bacteria appear similar Mycobacterium tuberculosis, it cannot always distinguish between TB and other infections. It is commonly used to diagnose active TB disease because it can quickly determine if a person is infected. However, it sometimes gives a negative result even in people with TB disease so a negative result cannot be relied upon.

Culture
Culture techniques are used to grow live TB bacteria in a laboratory. This is a reliable method for detecting active TB disease as long as a suitable sample containing the TB bacteria can be obtained. TB can be cultured from a variety of specimens. This test can also provide information on which antibiotics would be effective in treating the infection. A major drawback of this test is the length of time it takes to obtain the results (2-6 weeks).

Polymerase Chain Reaction (PCR)
These tests detect the presence of genetic material in bacteria. PCR can detect small amounts of genetic material. However, to be effective, the samples still have to contain a certain number of TB bacteria. It is somethimes difficult to obtain a good sample so people with TB disease may give a negative PCR result. The test is also quite complicated and can be expensive.

Blood Test (e.g. the T-SPOT.TB test)
Blood tests are a relatively new development in TB testing for both active disease and Latent TB Infections. A tube of blood is needed to run the 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*.

*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