TB Skin Test
 A TB skin test looks for TB infection. TB
proteins are injected under the surface of the
skin. If a person is infected with TB, a hard
swelling (induration) will develop at the site of
the injection in 48 to 72 hours. A health care
professional must measure the size of the reaction
and tell you if the test is positive.
A positive test
A positive TB skin test
usually means that you have TB infection. More
tests should be done to make sure you don't have
TB disease.
Your doctor may order a chest x-ray or a test of your
sputum (phlegm) to look for TB bacteria. A positive
test without TB infection can happen in people who
have been vaccinated with BCG or who have been infected
with other TB-like mycobacteria.
A negative test
A negative TB skin test
usually means a person is not infected with TB. A
negative test can happen in a person who has been
recently infected. It usually takes 2 to 12 weeks
after exposure to a person with infectious TB disease
for the skin test to become positive.
A negative test can also happen in an infected person
who has a weakened immune system.
If you have TB and HIV
infection or TB disease, your skin
test may be negative even if the TB germ is in your
body.
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