TB Skin Test
TB may cause an inactive (latent) infection or an active, progressive disease. The immune systems of about 90% of the people who become infected with TB manage to control its growth and confine the TB infection to a few cells in the body. The bacteria in these cells are inactive but still alive. The person does not have any symptoms, and they are not infectious, but they do have a "latent TB infection." If the person's immune status is later compromised, the dormant mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. This active TB does cause illness in the person, and it can be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, or breathing.
If the TB skin test is positive, the person may have an active case of TB or the latent form of infection. Further testing will be done to diagnose the presence of an active tuberculosis infection.
How is the sample collected for testing?No sample is required. The test is performed on the patient's skin. A purified protein derivative (PPD) solution that contains Mycobacterium tuberculosis antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red raised bump) in those who may have been infected by TB.
A healthcare worker will wipe your inner forearm with alcohol and let the skin dry. Using a 1cc syringe and a tiny needle, he will inject a small amount of PPD solution just under the first layer of your skin. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed. The site must be examined by a healthcare worker at 48 and/or 72 hours to see if a local skin reaction has occurred.
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.
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.
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.
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.
I’ve heard of another test called QuantiFERON- TB Gold test. Can this be used to test for latent TB infection?Yes, the FDA approved a blood test called the QFT-G (QuantiFERON® - TB Gold Test). It can be used to help confirm or rule out a latent or active tuberculosis infection. This test is not affected by previous QFT-G or TB skin tests or by BCG vaccination. It does not require the patient to return in 48 to 72 hours to have their results read by a healthcare worker and there is no potential for a local skin reaction due to a PPD injection because there is no injection. The QFT-G requires that a blood sample be collected from the patient and processed (centrifuged and stored) in the lab within 16 hours of collection, which complicates the clinical utility and the availability of the test in many situations. There are limited data as yet on the usefulness of this test in certain populations, such as children, those who have suppressed immune systems, and those who have been recently exposed to TB.
Should I get a TB skin test if I am pregnant?Only under your doctors supervision and only if there is a need to do so. There have not been enough studies done to clearly document the safety of the PPD solution during pregnancy. On the other hand, since TB can be passed from mother to child through the amniotic fluid during pregnancy, if you are at an increased risk of contracting TB, your doctor may want you to have a TB skin test done.
What about the multiple puncture prong test for TB?This is called the tine test and is rarely used any more. It involved the use of a device with multiple prongs/pins that were either dipped into a tuberculin solution and then pricked the skin, or pricked the skin through a drop of tuberculin that had been applied to the surface of the skin. The tine test was not considered as accurate because the amount being delivered could not be controlled. Any positive tine tests had to be followed up with the regular TB/PPD skin test.