Tuberculosis (cont.)
How does a doctor diagnose tuberculosis?
TB can be diagnosed in several different ways, including chest x-rays, analysis of sputum, and skin tests. Sometimes, the chest x-rays can reveal evidence of active tuberculosis pneumonia. Other times, the x-rays may show scarring (fibrosis) or hardening (calcification) in the lungs, suggesting that the TB is contained and inactive. Examination of the sputum on a slide (smear) under the microscope can show the presence of the tuberculosis-like bacteria. Bacteria of the mycobacterium family, including atypical mycobacteria, stain positive with special dyes and are referred to as acid-fast bacteria (AFB). A sample of the sputum also is usually taken and grown (cultured) in special incubators so that the tuberculosis bacteria can subsequently be identified as tuberculosis or atypical tuberculosis.
Several types of skin tests are used to screen for TB infection. These so-called tuberculin skin tests include the Tine test and the Mantoux test, also known as the PPD (purified protein derivative) test. In each of these tests, a small amount of purified extract from dead tuberculosis bacteria is injected under the skin. If a person is not infected with TB, then no reaction will occur at the site of the injection (a negative skin test). If a person is infected with tuberculosis, however, a raised and reddened area will occur around the site of the test injection. This reaction, a positive skin test, occurs about 48 to 72 hours after the injection.
If the infection with tuberculosis has occurred recently, however, the skin
test can be falsely negative. The reason for a false negative test with a recent
infection is that it usually takes two to 10 weeks after the time of infection
with tuberculosis before the skin test becomes positive. The skin test can also
be falsely negative if a person's immune system is weakened or deficient due to
another illness such as AIDS or cancer, or while taking medications that can
suppress the immune response, such as cortisone or anticancer drugs.
Remember, however, that the TB skin test cannot determine whether the disease
is active or not. This determination requires the chest x-rays and/or sputum
analysis (smear and culture) in the laboratory. The organism can take up to six
weeks to grow in culture in the microbiology lab. A special test to diagnose TB called the PCR (polymerase chain reaction) detects the genetic material of the bacteria. This test is extremely sensitive (it detects minute amounts of the bacteria) and specific (it detects only the TB bacteria). One can usually get results from the PCR test within a few days.
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