Tuberculosis (TB) Facts (cont.)

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How do physicians diagnose tuberculosis?

Because TB may occur as either a latent or active form, the definitive diagnosis of active TB depends on the culture of mycobacteria from sputum or tissue biopsy. However, it may take weeks for these slow-growing bacteria to grow on specialized media. Since patients with latent TB do not require isolation or immediate drug therapy, it is useful to determine if a person is either not infected, has a latent infection, or is actively infected with transmissible TB bacteria. Consequently, doctors needed a presumptive test(s) that could reasonably assure that the person was infected or not so therapy could begin. After getting a patient's history and physical exam data, the next usual test is the skin test (termed the Mantoux tuberculin skin test or the tuberculin skin test or TST). The test involves injecting tuberculin (an extract made from killed mycobacteria) into the skin. In about 48-72 hours, the skin is examined for induration (swelling) by a qualified person; a positive test (induration) strongly suggests the patient has either been exposed to live mycobacteria or is actively infected. Another test, IGRA (interferon-gamma release assays) can measure the immune response to Mycobacterium tuberculosis. Other quick tests are useful; chest X-rays can give evidence of lung infection while a sputum smear stained with certain dyes that are retained mainly (but not exclusively) by mycobacteria can show the presence of the bacterium. These tests, when examined by a doctor, are useful in establishing a presumptive diagnosis of either latent or active TB, and most doctors will initiate treatment based on their judgment of these tests. In addition, some of these tests are useful in the U.S. and elsewhere only in people who are not vaccinated with a TB vaccine (see below) but are less useful in vaccinated people. For some patients, culture studies still should be completed to determine the drug susceptibility of an infecting TB strain.

Other tests have been developed. For example, a PCR test (polymerase chain reaction) to detect TB antigens and the LED-FM microscopic technique to identify TB organisms with microscopy may be used. Two other TB blood tests (also called interferon-gamma release assays or IGRAs) have been approved by the FDA and measure how strongly the body's immune system reacts to TB bacteria. IGRAs are recommended in testing patients who have been vaccinated against TB (see prevention section below).

People with positive symptoms, positive blood tests, sputum smear, or culture positive are considered infected with TB and contagious. Physicians diagnose and treat people with latent TB infections (LTBI) according to the following 2013 CDC criteria:

  • No symptoms or physical findings suggestive of TB disease
  • TST or IGRA result is usually positive
  • Chest radiograph is typically normal
  • If done, respiratory specimens are smear and culture negative
  • Cannot spread TB bacteria to others
  • Should consider treatment for LTBI to prevent TB disease (strongly advised by the CDC)
Medically Reviewed by a Doctor on 1/24/2014

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