Diagnosing Active TB

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. To determine if a patient has active TB disease, the following tests may be used:

• Blood Test (e.g., The T-SPOT®.TB Test)
• Chest Radiograph (X-ray)
• Sputum Smear Microscopy
• Culture
• Nucleic Acid Amplification Tests (NAATs)
  
  

Blood Test (e.g., The T-SPOT.TB Test)
Blood tests are a relatively new development in TB disease testing for both active disease and latent TB infections. A blood sample is needed to run this test which is performed in the laboratory. The results are available to the doctor the next day. The T-SPOT.TB test holds several major advantages over the tuberculin skin test in that 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 tuberculin skin test has been in existence for over 100 years. The test works by injecting a small amount of liquid containing dead TB cells into the lower part of the arm. The injection site must then be evaluated by a trained healthcare professional 48-72 hours later. If a person is infected, the body will recognize the cells and respond by forming a lump at the injection site. The accuracy of the TST varies and can be affected a number of issues including a previous BCG vaccination,a weakened immune system and by other illnesses or medical treatment.

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

Sputum Smear Microscopy
This is a simple laboratory test that examines sputum for bacteria. This test also identifies other types of non-TB bacteria, so 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 cannot determine which antibiotics would best treat the infection.

Culture
Culture techniques are used to grow live TB bacteria in a laboratory. This test is routinely ordered for detecting Mycobacterium tuberculosis from a variety of specimens (i.e., sputum). 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 get the results back (2-6 weeks).

Nucleic Acid Amplification Tests (NAATs)
Nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR), are a relatively new development in active TB testing. These tests detect the presence of genetic material in bacteria.  Samples for these tests  have to contain a certain number of TB bacteria for the detection to occur.  This is not always possible, particularly with non-pulmonary TB where sensitivity can be low.  These tests are relatively complicated to run in the laboratory and can be expensive. 

*T-SPOT.TB Package Insert