TB Screening Tests

How is it used?

When is it ordered?

What does the test result mean?

Is there anything else I should know?

How is it used?
TB screening tests are not used as a general population screens but are used to screen certain people who are at high risk for TB exposure, such as:

Those who have signs and symptoms consistent with active tuberculosis
Those with diseases or conditions that weaken the immune system, 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 homeless shelters, migrant farm camps, nursing homes, schools, and correctional facilities
Health care 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
Those who inject illegal drugs
TB screening tests are 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.

Testing is used to help diagnose a Mycobacterium tuberculosis infection, either a latent TB infection or active disease. If the doctor suspects that a person has active tuberculosis, other tests, such as chest X-rays and AFB smears and cultures, are used to confirm the diagnosis.

Either a tuberculin skin test or an IGRA may be performed. IGRA tests are relatively new and are not performed by all laboratories. Since the test requires viable white blood cells, the IGRA blood sample must be received and tested by the laboratory within a designated window of time.

When is it ordered?

TB screening tests are ordered when a doctor wants to screen a person for a exposure to TB. They may be done yearly in those who 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. They 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 when a person speaks, sings, coughs or sneezes, TB screening tests may be ordered when someone has been in close contact with someone 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 be detected. 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.

Either a TB skin test (TST) or an IGRA may be ordered. Recommendations from the Centers for Disease Control and Prevention (CDC) list preferences for an IGRA test when those tested are less likely to comply with returning to have their TST evaluated or when the person being tested has received BCG (Bacille Calmette-Guérin). BCG is a TB vaccine that can interfere with the interpretation of a TST. BCG is not used as a vaccine in the United States, but it is often routinely administered in countries with a higher incidence of TB and it is used in the US as a treatment for some cancers.

The CDC lists a preference for the TST when testing children under 5 years of age and says that both tests might be useful when someone with a negative initial TST or IGRA test has an increased risk for TB infection and/or clinical suspicion of active tuberculosis exists. Sometimes the TST is given in a two-step process. If there is a risk that the first TST is a false negative reaction, a second skin test is given so that the TST may stimulate the immune system, causing a positive or boosted reaction in the second test. This is typically performed for health care workers at the start of their employment.

What does the test result mean?
A health care worker will interpret your tuberculin skin 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 people who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the TST. Positive TST results are also commonly seen in those who have received a BCG vaccination.

IGRA test results may be positive, negative, or indeterminate. Positive results mean that a person is likely to have been exposed to TB. They may be due to a latent or active TB infection, or occasionally due to a false positive. IGRA results are not affected by BCG.

Negative results for either test may mean that a person has not been exposed to TB, that the person is not infected with tuberculosis, that their immune system has not responded to the antigen in 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 TB screening tests. If a doctor wants to confirm a negative or indeterminate result, he may repeat the same test or do either the TST or IGRA as an alternate follow-up test.

Occasionally, a person infected with another species of Mycobacterium, for example Mycobacterium kansasii, will give a false positive IGRA result. 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 smears and cultures may be used to confirm the diagnosis and determine the drug susceptibility for the M. tuberculosis infecting the patient.



Is there anything else I should know?
Once someone has had a positive TB skin test, it is not necessary to have another one the next time there is a question of exposure to TB. A TST reaction will usually remain positive, and the skin reaction to subsequent tuberculin skin tests may become increasingly severe.

A negative TST may cause mild itching or discomfort at the injection site. A person may not respond to a TB skin test (even with TB exposure) if she has had a recent viral infection, a "live" vaccine (such as measles, mumps, chickenpox, influenza), or has overwhelming tuberculosis, another bacterial infection, or is taking immune suppressive drugs such as corticosteroids.

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