Positive TB skin test
PPD skin test, the acronym for purified protein derivative, is a method
used to diagnose tuberculosis (TB).
How the test is performed
clean the site (usually the forearm) with rubbing alcohol. Then the
extract is injected PPD skin beneath the top layer, causing the formation
of a welt on the skin, which usually disappears within a few hours.
The reaction will take 48 to 72 hours to appear and you should return to
the doctor in that time to have the area checked. This check will
determine if you have had a significant reaction to the PPD test. The
reaction is measured in millimeters of firm swelling (induration), no
flushing, at the site of infection.
Prepare for the Test
There is no special preparation for this test.
Tell your doctor if you have had a positive PPD skin test. If so, it
should not be repeated except under unusual circumstances.
Also tell your doctor if you have a medical condition or are taking
certain drugs such as steroids, which can affect your immune system.
These situations can lead to inaccurate results are presented in the
What you feel during the examination
You will feel a brief sting as the needle is inserted just below the skin
Why the Test is Performed
The PPD test is performed to find out if you have been infected with
Mycobacterium tuberculosis, the bacteria that cause TB.
A negative reaction (no induration) or a size of hard swelling that falls
below the threshold for each risk group may mean that you have not been
infected with the bacterium that causes tuberculosis. There are different
cutoffs for children, people with HIV and other risk groups.
This is not a perfect test and it is possible that up to 20% of people
infected with the bacteria that cause tuberculosis do not have a reaction
on the PPD skin test. In addition, certain conditions that affect the
immune system (cancer, recent chemotherapy, late-stage AIDS) may cause a
false negative test.
Abnormal Results Mean
The test results depend on the size of the skin reaction and the person
A small reaction (5 mm of firm swelling at the site) is considered
positive in people:
They have received an organ transplant.
Have a weakened immune system or are taking steroid therapy
(approximately 15 mg of prednisone per day for a month).
They have been in close contact with someone who has active
Changes occur in a chest radiograph that look like a past
Larger reactions (greater than or equal to 10 mm) are considered positive
People with a negative test known in the past two years.
People with diabetes, kidney failure or other conditions that increase
your chance of developing active tuberculosis.
Injection drug users.
mmigrants who have moved from a country with a high rate of tuberculosis
over the past 5 years.
Children under 4 years.
Infants, children and adolescents who are exposed to high-risk
Students and employees of certain group life environments such as
prisons, nursing homes and homeless shelters.
In people with no known risks for tuberculosis, 15 mm or more of firm
swelling at the site are evidence of a positive reaction.
There is a very small risk of severe redness and swelling of the arm in
people who have previously undergone a PPD test yielded a positive result
and those who undergo the test again. Also have been a few cases of this
reaction in people who had not been tested before.
A positive skin test does not necessarily mean that a person has active
tuberculosis. They have to do more tests to check for active disease.
Many people born outside the United States may have had a vaccine called
"BCG" (BCG), which can lead to a false positive test. However, most
experts say that past BCG vaccination should not change the outcome of
the PPD test when it is done on people with increased risk of
tuberculosis disease or infection.