How is it used?
The TB skin test is not used as a general population screen but is used to screen particular populations at high-risk for
TB exposure, such as:
- those with diseases or conditions that weaken their immune systems, such as those with HIV or AIDS, that make them more vulnerable to a TB infection;
- those who are in confined living conditions such as nursing homes, schools, and correctional facilities;
- healthcare workers and others whose occupations bring them in close contact with those who may have active TB;
- those who have been in close contact with someone who has an active case of TB;
- those who come from or have lived for a period of time in a foreign country where TB may be more common.
The TB skin test is also used sometimes as part of a routine examination prior to starting school or a new job. Since mothers can pass TB to their unborn children, pregnant women are sometimes screened.
The TB skin test is used to help diagnose latent TB infection or active disease. If your doctor suspects that you have active tuberculosis, other tests, such as chest X-rays and AFB cultures, are used to confirm the diagnosis.
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When is it ordered?
TB skin tests are ordered when the doctor wants to screen his patient for a exposure to
TB. The TB skin test may be done yearly in those that are part of a high-risk group - either because they have a disease that weakens their immune system or because they work or live around others in high-risk groups. TB skin tests are frequently done prior to a person joining an at-risk population, such as going to college or becoming a teacher or health care worker.
Since TB is airborne and passed through respiratory secretions, TB skin tests may be ordered when someone has been in close contact with a patient who has an active case of TB or when they have been in a foreign country where TB is more common. This would be done a few weeks after suspected exposure as it usually takes about 6 weeks after contact and initial infection before a positive result would emerge.
TB skin tests should not be done when a person has had a previous positive reaction as they are more likely to have a severe local reaction.
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What does the test result mean?
A healthcare worker will interpret your test results by looking at the injection site on your forearm at 48 or 72 hours (in most cases). A positive result will form a red and swollen circle at the site of the injection. The size (diameter) of the swollen raised circle determines whether exposure to
TB has occurred. The size that is considered positive varies with the health status and age of the individual. Even when infected, children, the elderly, and patients who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the TB skin test.
Negative results may mean that you have not been exposed to TB, that your immune system has not responded to the test, or that it is too early to detect exposure. It takes about 6 weeks after infection before a person demonstrates a positive reaction to the PPD. If your doctor wants to confirm a negative result, he may repeat the TB skin test, or in some cases, perform another type of test called QuantiFERON® - TB Gold test (QFT-G).
Positive results may be due to a latent or active TB infection, or occasionally due to a false positive. Positive results may also be seen in those who have received a BCG (Bacille Calmette-Guérin) vaccination. BCG is a vaccine that is not used in the United States but is often routinely administered in other countries that have a higher incidence of TB.
Positive results must be followed up by other tests such as chest X-rays to look for signs of active TB disease. If active TB disease is suspected, AFB cultures may be used to confirm the diagnosis.
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Is there anything else I should know?
Once you have had a positive TB skin test, it is not necessary to have another one the next time there is a question of your exposure to TB. Your PPD reaction will usually remain positive, and the skin reaction to subsequent TB skin tests may become increasingly severe.
A negative TB skin test may cause mild itching or discomfort at the injection site. You may not respond to a TB skin test (even if you have TB) if you have had a recent viral infection, a "live" vaccine (such as measles, mumps, chickenpox, influenza), or if you have overwhelming tuberculosis, another bacterial infection, or are taking immune suppressive drugs such as corticosteroids.
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