TB Screening Tests
When you have a high risk of exposure to TB; sometimes as part of an examination prior to starting school or a new job (such as a college student, teacher, or daycare employee); when you have signs and symptoms consistent with TB
For a tuberculin skin test, no sample is required. A small amount of purified protein derivative (PPD) solution is injected just under the first layer of skin of your inner forearm. For an interferon gamma release assay, a blood sample is drawn by needle from a vein in your arm.
Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. TB primarily targets the lungs but may affect any area of the body such as the urinary tract, central nervous system, bones, joints, and/or other organs. TB screening tests help to determine whether a person has become infected with Mycobacterium tuberculosis bacteria. The screening tests measure the body's immune response to antigens derived from these bacteria, either directly as a skin reaction to a tuberculin skin test (TST) or indirectly with an interferon gamma release assay (IGRA) blood test.
TB may cause an inactive (latent) infection or an active, progressive disease. The immune system of about 90% of people who become infected with TB manages 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 is not infectious but does have a "latent TB infection."
If, after some time, the immune system of an individual with an inactive infection becomes weakened (compromised), the mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. Active TB does cause illness and can be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, talking, singing, or breathing.
Both the tuberculin skin test and the IGRA blood test can detect M. tuberculosis infections, but neither can distinguish between latent and active infections. Additional tests, such as AFB testing, are required to help establish a diagnosis of an active TB infection.
How is the sample collected for testing?
For the tuberculin skin test, no sample is required. The test is performed on a person's skin. A purified protein derivative (PPD) solution that contains M. tuberculosis antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red, raised bump) in those who have been infected by TB.
A health practitioner will wipe the inner forearm with alcohol and let the skin dry. Using a 1cc syringe and a tiny needle, the health practitioner will then inject a small amount of PPD solution just under the first layer of the 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 health practitioner at 48 and/or 72 hours to see if a local skin reaction has occurred.
For an interferon gamma release assay, a blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Tuberculosis (TB) screening tests are not used as general population screens but are used to screen people who are at high risk for TB exposure, such as:
- People with diseases or conditions that weaken the immune system, such as HIV or AIDS, which makes 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
- 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
- People who have signs and symptoms consistent with active tuberculosis
- 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.
Either a tuberculin skin test (TST) or an interferon gamma release assay (IGRA) may be performed to screen for TB:
- The tuberculin skin test involves two steps: the injection of a small amount of purified protein derivative (PPD) solution under the first layer of skin of the forearm and an evaluation of the injection site conducted by a health practitioner at 48 and/or 72 hours to see if a local skin reaction has occurred.
- The IGRA test measures the release of a substance called gamma interferon by white blood cells in a sample of blood when the cells are exposed to specific TB antigens. The IGRA test is not performed by all laboratories. The test requires viable white blood cells, so the IGRA blood sample must be received and tested by a laboratory within a designated window of time.
Recommendations from the Centers for Disease Control and Prevention (CDC) list preferences for an IGRA test for people who are less likely to comply with returning to have their TST evaluated or when the person being tested has received the BCG (Bacille Calmette-Guérin) vaccine that might 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 U.S. as a treatment for some cancers.
The CDC lists a preference for the TST for testing children younger than 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 healthcare workers at the start of their employment.
When is it ordered?
TB screening tests may be ordered:
- On a yearly basis for 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
- Prior to a person joining an at-risk population, such as going to college or becoming a teacher or healthcare worker
- When someone has been in close contact with someone who has an active case of TB; this would be done a few weeks after a suspected exposure as it usually takes about 6 weeks after contact and initial infection before a positive result would be detected.
- When an individual has lived for an extended time in a country where TB is more common
- When a person has signs and symptoms of TB, such as a chronic cough that produces phlegm or sputum, sometimes with bloody streaks, fever, chills, night sweats, and unexplained weight loss
A TB skin test should not be done when a person has had a previous positive reaction. Once positive, a TST reaction will usually remain positive and the skin reaction to subsequent tuberculin skin tests may become increasingly severe.
What does the test result mean?
A health practitioner will interpret a tuberculin skin test result by looking at the injection site on the person's 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.
A positive TST or IGRA test result means that the person is likely to have been exposed to TB. It may be due to a latent or active TB infection. If a health practitioner suspects that someone has active tuberculosis, other tests, such as chest X-rays and AFB laboratory testing, are used to confirm the diagnosis.
A negative result for either test means that it is likely that the person tested does not have a TB infection. However, it does not entirely rule out tuberculosis. It may mean that the person's 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 suspicion of TB remains high and a health practitioner wants to confirm a negative or indeterminate result, he or she may repeat the same test or do either the TST or IGRA as an alternate follow-up test.
Occasionally, a person infected with or exposed to other Mycobacterium species, for example Mycobacterium kansasii, will give a false-positive TST or IGRA result for 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 testing including smears and cultures and sensitivity testing, may be used to confirm the diagnosis and determine the drug susceptibility for the M. tuberculosis infecting the person.
Is there anything else I should know?
Positive TST results are commonly seen in those who have received a BCG (Bacille Calmette-Guérin) vaccination. IGRA results are not affected by BCG.
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 the person 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.
A person should generally wait 4-6 weeks to do a TB skin test after having had a vaccination with a live-virus vaccine.
Does it matter whether I have a tuberculin skin test (TST) or a blood IGRA test?
In most cases, either test can be used. However, the results are not interchangeable so if you are at risk and your health practitioner is ordering TB screening on a periodic basis to monitor your exposure, then it may be preferable to do the same test each time for consistency and, if it is an IGRA, to send it to the same laboratory.
The Centers for Disease Control and Prevention (CDC) recommends an IGRA test for people who 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, but it is only routinely administered in countries with a higher incidence of TB and is not used in the U.S. The CDC advises using a TST for children younger than age 5.
Should I get a tuberculosis screening test if I am pregnant?
You may be tested under your health practitioner's supervision if there is a need to do so. Since TB can be passed from mother to child during pregnancy, if you are at an increased risk of contracting TB, your health practitioner may want you to have a TST or IGRA done. Either of these is considered safe during pregnancy.
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 as other tests 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.
What if I have a TST and it is more than 72 hours before I go back to have it evaluated?
In most cases, if you do not return within the designated 48-72 hours, then your test cannot be adequately evaluated and would need to be redone.
On This Site
Conditions: Tuberculosis, Nontuberculous Mycobacteria, Lung Diseases
Screening: Infants, Children (2-12), Teens (13-18), Young Adults (19-29), Adults (30-49), Adults (50 and Up)
In the News: Blood Tests Recommended Over Skin Tests to Check for TB in Most Cases (2017)