To identify those who may have been exposed to M. tuberculosis, health care providers typically inject a substance called tuberculin under the skin of the forearm. If a red welt forms around the injection site within 72 hours, the person may have been infected. This doesn't necessarily mean he or she has active disease. People who may test positive on the tuberculin test include

·         Most people with previous exposure to M. tuberculosis

·         Some people exposed to bacteria related to M. tuberculosis

·         Some people born outside the United States who were vaccinated with the TB vaccine (see TB vaccine below) used in other countries

If people have an obvious reaction to the skin test, other tests can help to show if they have active TB. In making a diagnosis, doctors rely on symptoms and other physical signs, the person's history of exposure to TB, and X-rays that may show evidence of M. tuberculosis infection.

The health care provider also will take sputum and other samples to see if the TB bacteria will grow in the lab. If bacteria are growing, this positive culture confirms the diagnosis of TB. Because M. tuberculosis grows very slowly, it can take 4 weeks to confirm the diagnosis. An additional 2 to 3 weeks usually are needed to determine which antibiotics to use to treat the disease.

What Happens When Someone Gets Infected with M. Tuberculosis?

Between 2 to 8 weeks after being infected with M. tuberculosis, a person's immune system responds to the TB germ by walling off infected cells. From then on the body maintains a standoff with the infection, sometimes for years. Most people undergo complete healing of their initial infection, and the bacteria eventually die off. A positive TB skin test, and old scars on a chest X-ray, may provide the only evidence of the infection.

If, however, the body's resistance is low because of aging, infections such as HIV, malnutrition, or other reasons, the bacteria may break out of hiding and cause active TB.