Tuberculin Skin Test
How It Is Done
For a tuberculin skin test, you sit
down and turn the inner side of your forearm up. The skin where the test is
done is cleaned and allowed to dry. A small shot of the TB antigen (purified
protein derivative, or PPD) is put under the top layer of skin . The fluid makes
a little bump (wheal) under the skin. A circle may be drawn around the test
area with a pen.
Do not cover the site with a bandage. You must see
your doctor 2 to 3 days after the test to have the skin test checked.
How It Feels
You may feel a quick sting or pinch from
There is a very slight risk of having a severe
reaction to the tuberculin skin test, especially if you have had tuberculosis
allergic reaction can cause a lot of swelling and pain
at the site. A sore may be present.
You cannot get a TB infection
from the tuberculin skin test, because no live bacteria are used for the
After the test
Some redness at the skin test site
is expected. The site may itch, but it is important that you do not scratch it,
since this may cause redness or swelling that could make it hard to read the
skin test. If itching is a problem, put a cold washcloth on the site and then
A strong positive reaction may cause mild pain. Talk to
your doctor if you have:
A tuberculin skin test is done to see if
you have ever had
tuberculosis (TB) (infection with Mycobacterium tuberculosis).
Redness alone at the skin test
site usually means you have not 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
determine the result. Your doctor will consider your chance of having TB when
looking at the skin test site.
Results of the test depend on your risk for TB. If you are in a high-risk group, a smaller bump is considered a sign of infection. People at low risk for having TB need to have a larger bump to be diagnosed with a TB infection.
Three levels of risk have been defined:
High-risk group includes
people who have
HIV, those who have had
close recent contact with a person who has an active TB infection, and those
who have symptoms or a chest
X-ray that shows TB. Other people at high risk for
tuberculosis include those who take medicines that contain
corticosteroids for a long time or people taking tumor
necrosis factor-alpha antagonists (used to treat
rheumatoid arthritis and
Moderate-risk group includes people who have recently moved
from or traveled in a country with a high rate of TB; those who use illegal
drugs by injection (intravenous drug users); people who
live in nursing homes; workers in hospitals, nursing homes, schools, and
prisons; children younger than 4 years old; children (ages 4 to 18) who are
exposed to high-risk adults; and homeless people. Others at moderate risk for
having tuberculosis include people who are 10% or more below their ideal body
weight and people who have
leukemia, cancer, or those who have had part of their
stomach removed (gastrectomy).
Low-risk group includes people who do not have any possible exposure to TB listed
in the other risk groups.