Is it possible to get a false-positive TB skin test?
Some persons may react to the TST even though they are not infected with M. tuberculosis. The causes of these false-positive reactions may include, but are not limited to, the following: Previous TB vaccination with the bacille Calmette-Guérin (BCG) vaccine.
What is a negative TB skin test look like?
The test is “negative” if there is no bump (or only a very small bump) at the spot where the fluid was injected. A negative TB skin test usually means that you don’t have TB. In some situations, you may need to have another TB skin test later.
Can a TB test be done incorrectly?
People with recent TB infections and very old TB infections can also show false negative test results. If the test is done incorrectly, a false negative might occur.
How common are false positive TB skin tests?
In 18 studies involving 1,169,105 subjects, the absolute prevalence of false-positive TST from NTM cross-reactivity ranged from 0.1% to 2.3% in different regions. Conclusions: The effect on TST of BCG received in infancy is minimal, especially > or =10 years after vaccination.
Does BCG vaccine cause false positive TB test?
Vaccination with BCG may cause a false positive reaction to a TB skin test. A positive reaction to a TB skin test may be due to the BCG vaccine itself or due to infection with TB bacteria.
Is a TB skin test supposed to bruise?
There is very little risk to having a TB skin test or blood test. For a TB skin test, you may feel a pinch when you get the injection. For a blood test, you may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Is it normal for a TB test to be red?
Results. Redness alone at the skin test site usually means you haven’t been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to find out the result.
Can TB cause skin problems?
Skin – TB can cause certain rashes, including erythema nodosum – a red, lumpy rash on the legs – or lupus vulgaris which gives lumps or ulcers. Spread to many parts of the body – this is called miliary TB, and can affect many organs, including lungs, bones, liver, eyes and skin.