Understanding TB Skin Test Results: What to Expect

TB (Tuberculosis) is an infectious lung disease that spreads from person to person through the air by coughing or sneezing. A skin test for TB is needed for diagnosis. During a skin test, a small amount of testing fluid is injected under the skin. The injection site is examined in two to three days for any signs of reaction to the testing fluid.

Negative Skin Test

• If the TB test is negative, there is no indication of raised or red bumps at the site of the injection. A second test may be needed at a later time in some instances when exposure to active cases of TB are indicated.

False Negative Skin Test

• False negative results happen in some who may not react to the tuberculin skin test (TST) when exposure is recent or when a recent live-virus vaccine has be administered.

False Positive Skin Results

• False positive results happen from a Bacillus Calmette-Guérin (BCG) vaccine, which is a vaccine against tuberculosis that contains a weakened form of the virus.

Small Skin Test Reaction

• Small positive reaction to the skin with raised, red bumps 5 millimeters in size is considered a positive TB test results in people who have HIV or who are in close contact with someone who has active tuberculosis.

Large Skin Test Reaction

• Large positive reaction greater than 10 millimeters in size are considered to have positive results in those who have diabetes or in health care workers.