Two tests exist to test for the presence of a TB infection:
- the TB skin test (TST) or Mantoux test
- a blood test to detect for the existence of TB bacteria in the body
Testing usually occurs eight weeks after one’s last contact with an individual with pulmonary TB.
The TB skin test requires two visits to a healthcare provider.
During the first visit, a small amount of a substance called PPD tuberculin is injected into the skin of your forearm. If you have a latent TB infection, your skin will be sensitive to PPD tuberculin and a small, hard red bump will develop at the site of the injection, usually within 48 to 72 hours of having the test. If you have a very strong skin reaction, you may need a chest X-ray to confirm whether you have active TB disease.
If you do not have a latent infection, your skin will not react to the Mantoux test. However, as TB can take a long time to develop, you may need to be screened again at a later stage. If you’ve had the BCG vaccination, you may have a mild skin reaction to the Mantoux test. This does not necessarily mean you have latent TB.
The TB blood test consists of blood being drawn and sent to the lab for testing.
A positive skin and/or blood test indicates an INFECTION with TB bacteria. A TB infection does NOT indicate a latent TB infection or the progression to TB disease.
An TB active disease diagnosis must then be made through the medical evaluations.
Most individuals will experience inactive TB bacteria for the entirety of their lifetime.
Diagnosis of Latent TB Infection
A diagnosis of latent TB infection is made if a person has a positive TB test result and a medical evaluation does not indicate TB disease. The decision about treatment for latent TB infection will be based on a person’s chances of developing TB disease by considering their risk factors.
Diagnosis of TB Disease
TB disease is diagnosed by medical history, physical examination, chest x-ray, and other laboratory tests.
Several tests can be used to confirm suspected extrapulmonary TB, such as: a CT scan, MRI scan or ultrasound scan, endoscopy, urine and blood tests, or biopsy.