Tuberculosis (cont.)

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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. Traditionally, sputum is collected for three successive mornings and then examined. A recent study in Africa and the Middle East suggested that these specimens could be collected on the first visit and then the next morning. The study suggests that collecting specimens in fewer visits will help identify a greater population in need of treatment.

A new technology, light emitting-diode fluorescence microscopy (LED-FM), a type of smear microscopy, is more sensitive than the standard Ziehl-Neelsen AFB stain to identify the bacteria. This test is faster to perform and again may help identify patients in need of therapy quicker.

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 48-72 hours after the injection. When only the skin test is positive, or evidence of prior TB is present on chest X-rays, the disease is referred to as "latent tuberculosis." This contrasts with active TB as described above.

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. The Xpert MTB/RIF assay detects 90% of pulmonary TB on a sample of a patient's sputum. In cases in which the smear of the sputum shows no organism, this assay detects TB 75% of the time. This assay also can determine if the organism is resistant to rifampicin, a commonly used drug for treatment.

Medically Reviewed by a Doctor on 1/15/2014

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Tuberculosis - Diagnosis Question: How was the diagnosis of your tuberculosis established?
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