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Diagnosing Tuberculosis

The signs and symptoms of tuberculosis depend on the site of infection. Generally symptoms and signs such as fatigue, anorexia, weight loss, and persistent low-grade fever occur with any body part that is infected with tuberculosis.

Some respiratory symptoms such as cough are a predominate sign that tuberculosis involves the lungs. The presence of these symptoms may lead the physician to order several types of tests to confirm whether tuberculosis is present.

The doctor may order a PPD (purified protein derivative) skin test in which the purified protein derivative is an antigen used to aid in the diagnosis of tuberculosis infection. This test is based on the observation that infection with the bacteria that causes tuberculosis frequently leads to a sensitivity to these antigens. The test site (usually the forearm) is cleansed. The PPD extract is then injected into the most superficial layer under the skin resulting in a transient bleb on the skin similar to a mosquito bite. The test will take 48-72 hours to develop then you must return to your health care provider at that time for a proper evaluation of the test site to determine whether you have had a significant reaction to the PPD test. A reaction of slight hardening and swelling at the site of injection is measured in millimeters.

The physician may also order an x-ray of the chest to look for evidence of TB in the lungs. The physician may also ask for a person to cough up a sample of sputum and have it tested for the presence of the tuberculosis bacterium.

While the PPD skin test and chest x-ray are not definitive tuberculosis tests, the “gold standard” for tuberculosis diagnosis is the presence of the tuberculosis bacterium in the sample you cough up from your lungs.

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© 2017 American Association for Respiratory Care