What is the test for tuberculosis?

There are many tests used to check for TB. The most common, which tests for exposure to TB organisms, is called the PPD, purified protein derivative, also known as the Mantoux test. It is done with an intradermal injection of a small 0.1 ml bubble of the PPD substance. A positive result in a healthy person is a 15mm induration around the test site 48 to 72 hours after testing. This test only shows that TB exposure and subsequent antibody formation has occurred. If the result is positive follow up tests like a chest x-ray or acid-fast bacilli test will be done based on presence of symptoms to find whether there is an active or latent infection. Note that it is possible to have a positive PPD result and not have a TB infection. There is a somewhat newer test that works to test for infection but is usually less often used than the PPD because it is more expensive and without appropriate history it may not be necessary; it is called the quantiFERON-TB test, QFT. Results are available in a few hours. This test is often the test of choice for patients who have had the BCG vaccine, common in Asia, or anyone with a history of a positive PPD. Since antibodies stay in the body for life any subsequent PPD test will be positive and therefore would not be effective to show any recent exposure. The QFT test will show that there are TB organisms present, not just anti-TB antibodies from a previous exposure. The most definitive diagnostic test is still the acid-fast bacilli test, done by taking sputum samples over 3 consecutive days, then growing the cultures to confirm for certain the presence of TB. The down side is that the cultures need 8 weeks to grow before a conclusive result for the presence of tuberculosis organisms and therefore obtain a definitive diagnosis of TB infection. Like the QFT test, the acid-fast bacilli test not for exposure and resulting antibody production but the presence of the organisms themselves. Local health departments usually will offer free TB screening to anyone who has had exposure to the organism. Early treatment with the proper drug regimen will reduce risk for long term side effects and reduce the spread of disease.