How is a TB Test Performed?

There are a couple of versions of the Tuberculosis (TB) test. One checks for exposure to TB and is much more commonly used to rule out tuberculosis exposure, which might mean contagion with the illness. The other test is generally only performed if TB is reasonably suspect, due to a positive tuberculosis exposure test, or other clear signs the illness is present.

The first TB test is also called the purified protein derivative standard or PPD, and sometimes the tuberculosis skin test. This check is normally performed in a doctor’s office or hospital. A doctor or nurse will clean the skin and often draws a circle on the arm, most typically the inner part of the arm’s lower half. Once the circle is drawn, a small amount of purified protein derivative is injected into the center of the circle. Some doctors and nurses don’t draw the circle, but it is an easy way to measure test results.

Between two to four days later, the arm is rechecked to see how it has reacted to the injected PPD, and size of reaction in millimeters can determine whether a person has had previous exposure to TB. People usually must return to their doctor in a stated period to have the test interpreted. Nurses or doctors may use a chart to check size of skin reaction in order to see if it is large enough to be read as positive.

The test is imperfect and there can be approximately 20% false negative results. People with certain autoimmune disorders may react in the opposite direction by showing a false positive. When doubt exists about results of the TB test, it might be performed again to see if the same result occurs. If a person has knowingly been exposed to tuberculosis, doctors may want to repeat the test in a few months if the first test is negative.

Once a person tests positively for TB they will likely always do so in the future. In fact, even after being successfully cured of the illness, most people will yield a positive test in the future. A person with previous positive test results or who has had tuberculosis should inform doctors who want to perform this test, since it isn’t particularly worthwhile.

It’s important to remember that when an accurate positive TB test occurs, this does not mean a person has TB. It means they have been exposed to it, and will not always go on to develop the symptoms of tuberculosis. This is actually not a condition that is easy to catch, though close exposure to others who have the disease may put people more at risk.

When the TB test is positive for the first time, doctors need to perform other tests to determine if the positive test indicates presence of the illness. The second round of involves collecting sputum samples that can be evaluated to look for evidence of active illness. Doctors may also order chest x-rays to help confirm diagnosis. If TB is present it is treated with several medications that have great success in curing the illness.