The most commonly used test for TB is a skin test called Mantoux test. In this test, a hypodermic needle is used to inject a small amount of purified TB protein called PPD tuberculin into the skin of the mid-forearm. Patients are asked to return to the doctor’s 48 to 72 hours later to have the result read. If the skin becomes a hard, raised bump at the injection site, this is a positive response and it indicates past exposure to TB. If there is no reaction, the result is negative, indicating that you have not been exposed to TB.

A positive test does not indicate an active TB infection, only that your immune system has been exposed to TB before. Thus, someone with a latent TB infection will test positive on a skin test. Further tests are needed to help determine whether you have active TB disease, which may include a chest X-ray, which can show nodules or cavities distributed through the lungs that indicate active TB. It can also sometimes show the walled-off area where TB is sequestered in a latent TB infection.

A culture test takes a sample of sputum (the mucus that comes up from your lungs when you cough) and allows it to grow on a special culture medium. If there is active TB, the bacteria will be visible under the microscope. It also allows a pathologist to add a common antibiotic to the medium and see if the bacteria respond to it. If not, other antibiotics can be tried and in this way the doctor can determine how best to treat the infection.