How Long Before a TB Test Shows Results?

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Identification

  • TB, which is short for tuberculosis, can be fatal if it's not treated properly. In fact, according to the Centers for Disease Control, it was once the No. 1 cause of death in the United States. TB is tricky because once you breathe in the bacteria, which is the only way to get it, your immune system can stop the bacteria from spreading through your body, but not actually get rid of the TB. That means that years later, when your immune system is compromised in some way, you can develop the disease. You can't give anyone the disease if you've got latent TB. You become contagious only when the bacteria multiplies in your body, giving you TB disease. When you have a latent infection, you won't even have symptoms.

When to Get Tested

  • If you have been around a person with TB disease, you have to get a test to make sure you haven't inhaled the bacteria. You also should get tested if you have HIV or another cause of a weakened immune system, come from a place with high TB disease rates, live or work in a place where TB disease is active or are experiencing TB symptoms, such as coughing, coughing up blood, chest pain, weakness, loss of appetite, weight loss, fever, night sweats or chills.

Contact a Health Professional

  • To get a TB test, you have to visit a doctor, clinic or the health department. It's easier to treat latent TB than TB disease, so don't put it off. Health professionals will need to know how and why you think you've been exposed and when the exposure happened. They'll order a skin test or a blood test, but these tests will show only whether you've been exposed to TB bacteria; they won't indicate if the bacteria has multiplied and given you the disease.

Skin Test

  • The tuberculin skin test, or TST, is the most common way to test for TB exposure. The test is performed on the forearm by injecting tuberculin into the skin. It will cause a welt 6 mm to 10 mm in diameter, which means the tuberculin was properly injected. You then have to wait at least 48 hours for the results.

Skin Test Results

  • You have to go back to the doctor to get the test properly read, and this should be done within 48 to 72 hours of taking the test. If you don't get there in time, you have to take another test. The health professional will measure the diameter of the reaction at the site of the injection in millimeters across the forearm (not in the direction running the length of the arm). They won't measure redness. Interpretation of whether the test is positive depends not only on the measurement, but also on your risk factors. For instance, if you've recently been exposed to someone with TB disease, a measurement of 5 mm or more is considered positive, while if you're a recent immigrant, a measurement of 10 mm or more is considered positive. A measurement of at least 15 mm is considered a positive test result in anyone.

Blood Test

  • TB blood tests--interferon-gamma release assays--are not available everywhere, because they are a recent development. They have a few advantages over skin tests, including that only one trip to doctor is needed to draw a blood sample. Another advantage of TB blood tests is that the test is less open to interpretation. TB antigens are added to the blood to see if your white blood cells react. The amount of reaction alerts your doctor to the presence of the TB bacteria. Test results come back within 24 hours.

After the Test

  • If either the skin or blood test comes back positive, you must have further testing to confirm the results and to determine if you have latent TB or TB disease. Some vaccinations can cause a false positive, so this is an important step, if only to make sure you actually have TB. Confirmation tests include an AFB smear, which typically tests sputum samples, and chest X-rays.

Xpert MTB/RIF

  • As of spring 2009, a new test was available in Europe that detects TB in two hours and does it much more accurately. Not only does the Xpert MTB/RIF test detect TB, but it also can tell if there is a resistance to rifampicin (RIF), which is the drug commonly used to treat TB disease. This new test is awaiting approval by the FDA for use in the United States.

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