TB Skin Test

The TB skin test is used to screen particular populations who are at risk of TB:

• 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 in the evaluation of how well a person’s immune system is working, and 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, but there are some risks involved that should be discussed with the doctor.

The TB skin test is used to help diagnose latent TB infection before it progresses to active disease. If your doctor suspects that you have active tuberculosis disease other tests, such as chest X-rays and AFB cultures may be used to confirm the diagnosis.

TB skin tests are requested when the doctor wants to screen their patient for a latent TB infection. They 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 not used as a general population screen, but may be performed prior to a person joining an at risk population, such as: going to college, or becoming a teacher or healthcare professional.

Since TB is airborne and passed through respiratory secretions, TB skin tests may be requested when someone has been in close contact with a patient who has an active case of TB (although it is usually about 6 weeks after contact and initial infection before a positive result would emerge), or when you have been in a foreign country where TB may be more common.

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.

At 48 or 72 hours after the start of the test a healthcare professional will usually interpret your test results by looking at the injection site on your forearm. 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 or not it is significant and likely due to a latent TB infection. 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 or that it is too early, it takes about 6 weeks after infection before a positive test can be determined. If your doctor wants to confirm a negative result they may repeat the TB skin test.

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 Bacillus Calmette-Guerin (BCG) vaccination. This is a vaccine that is not generally used in the United Kingdom but is often routinely administered in other countries that have a higher incidence of TB or to healthcare professionals who are in close contact with TB patients.

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.

If you have had a positive TB skin test, usually you should not have another one done. A positive will remain positive and the skin reaction of any subsequent TB skin tests will likely become increasingly severe - to the point that they may require medical attention. Even a negative test may still result in moderate pain, itching, or redness. If the test is not done according to guidelines, however, it will need to be repeated.

Limitations of the test include:

• Variability of interpretation: Reading of the results can be subjective and because of this, the measurement of the diameter of the raised reaction can vary.
• False positives: Can be due to the presence of environmental Mycobacterium species or because patients have been given the BCG vaccine. False positives can also if the wrong antigen is administered or when the test is interpreted incorrectly.
• False negatives: If the patient has been exposed within 6 weeks, they may produce a false negative result. You also may not respond to a TB skin test if you have had a recent viral infection or a "live" vaccine for conditions such as measles, mumps, chickenpox, influenza, or if you have overwhelming tuberculosis, another bacterial infection, or if you have a primary immunodeficiency or are taking immune suppressive drugs such as corticosteroids that weaken the immune response.
• Limited shelf life of the purified protein derivative (PPD): Shelf life varies between manufacturers but generally is 6 months to 1 year if freeze dried in original packaging and less (1-30 days) if reconstituted (made into a liquid form) and stored correctly. If the PPD is used past the shelf life or is stored incorrectly, this can also lead to false negative results.
• Unwillingness of some patients to return for test interpretation.

This is called the 'tine' test, or 'Heaf' test and is rarely used anymore. 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.