What Does Positive Vs. Negative Look Like on a TB Test?

Tuberculosis is a disease of the lungs caused by a bacteria. People get TB when they inhale this bacteria. It then resides in the lungs where it can become activated and become destructive to lung tissue. The most common way to test for TB is a skin test.

Test

• The TB skin test injects a fluid called purified protein derivative under the skin on the arm to see if there is a reaction.

Time Frame

• After being injected, you will wait for two to three days then return your doctor. At that time, your doctor will look at your skin to see if you are positive or negative for the TB bacterium.

Result

• The test is determined positive or negative based on the amount of swelling around the site of the injection.

Positive

• If a person is healthy, 15 mm or more of swelling is the indication of a positive test, according to Medicinenet.com. Risk factors are also considered in a diagnosis. Blisters also indicate a positive reaction.

Negative

• If the swelling is less than 15 mm, the test is generally considered negative. However, according to Medicinenet.com, up to 25 percent of people with new infections will test negative.