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The Tuberculosis Skin Test
How is the Mantoux PPD skin test given?
A small amount of a harmless substance called PPD Tuberculin is placed
just under the top layer of the skin on the forearm with a small needle
and syringe. It is not a shot, only a small prick is felt.
What is the reason for the skin test?
The test can show if a person has ever been "infected by" tuberculosis
(TB) germs. There are several kinds of TB skin tests, but the most
reliable is the Mantoux PPD skin test.
What happens after the skin test is given?
The test is read 48 to 72 hours after it has been given. Generally, a
swelling of 10 mm. or more is considered positive.
- If the PPD test is negative, no further action needs to be taken.
- If the PPD test is positive, a chest X-ray will be needed to see
if the germs have damaged the lungs.
What does a positive test mean?
It means that TB germ is in the body or the person is "infected".
However, it does not necessarily mean that he/she has TB disease. If the
chest X-ray is normal and there are no symptoms, the TB germs are
probably inactive and TB disease is not present. There is, however, a
10% chance that in the future active TB disease may develop,
particularly when the person's resistance becomes low. TB medication may
be given to prevent the development of TB disease in the future.
How can a person be infected and not have TB disease?
After TB germs enter the body, in most cases, body defenses control the
germs by building a wall around them. The germs can stay alive inside
these walls for years in an "inactive" state. While they are inactive,
they cannot harm the person and they cannot spread to other people. The
person is "infected" but does not have TB disease. He/she is not sick
and does not know that he/she is infected.
Will BCG vaccination cause a positive PPD skin test?
BCG vaccination is given in many developing countries to protect against
TB. After BCG vaccination, the PPD skin test may become positive.
However, the swelling is usually less than 10 mm. and wanes after 3-5
years. Swelling 10 mm. or more in a person from a country with a high
prevalence of TB indicates likely infection with TB and necessitates
further evaluation and, usually, treatment.
Differences in approaches to positive PPD in the U.S. and
developing countries
In developing countries there is a high prevalence of positive PPD
tests. Patients are treated only when they have abnormal chest X-rays.
In the U.S., the practice is different:
(a) If they have normal chest X-rays, they have TB infection
without having the disease. Only one medication is used for 6-9 months
to prevent TB disease later on in life.
(b) If they have abnormal Chest X-rays or positive sputum
culture for TB, they have the disease. Then 3-4 kinds of medications
will be used for 6-9 months. There will also be concern then they may
spread the disease to family members or close contacts.
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