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Active tuberculosis is diagnosed using a microscope or a molecular genetic test to rapidly identify these MTB bacteria in a sputum sample that’s coughed up from the lungs. MTB bacteria can also be grown from sputum in culture after several weeks. A positive culture helps confirm the results of these rapid microscopic or molecular tests and help make sure that these bacteria are susceptible to standard drug regimens. Similar tests may be applied to other types of samples to diagnose extrapulmonary TB, such as might affect the brain, bones, or spine. A chest x-ray is commonly performed to screen for TB in individuals with symptoms and to determine the location and severity of pulmonary tuberculosis. Latent TB in contrast is diagnosed with immunological tests, and the most common of these may be known to people as the tuberculosis skin test or TST. The TST involves injecting purified protein derivative also called PPD into the skin, and then assessing after about 48 to 72 hours whether there’s been a response. Another newer test called an interferon gamma release assay requires a blood draw and gives a similar result in about 24 hours without having to be returned to the doctor.
