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Tuberculosis (TB) Diagnosis

To identify those who may have been exposed to Mycobacterium tuberculosis (Mtb), healthcare 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 Mtb
  • Some people exposed to bacteria related to Mtb
  • Some people born outside the United States who were vaccinated with the TB vaccine 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, healthcare providers rely on symptoms and other physical signs, the person's history of exposure to TB, and X-rays that may show evidence of Mtb infection.

The healthcare provider also will take sputum and other samples to see if the TB bacteria will grow in the laboratory. If bacteria are growing, this positive culture confirms the diagnosis of TB. Because Mtb grows very slowly, it can take four weeks to confirm the diagnosis. An additional two to three weeks usually are needed to determine which antibiotics to use to treat the disease.

What Happens When Someone Gets Infected With Mtb?

Between two and eight weeks after being infected with Mtb, 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.

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Last Updated May 08, 2007