TB Skin Test
If you have been exposed to a person with TB; if you have a clinical condition or risk factor that makes progression to active TB more likely
No sample is required. A small amount of purified protein derivative (PPD) solution is injected with a tuberculin syringe just under the first layer of skin of your inner forearm.
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. A TB skin test measures the body's immune response to antigens derived from these bacteria.
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.
A tuberculin skin test cannot 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?
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 healthcare practitioner will wipe the inner forearm with alcohol and let the skin dry. Using a 1cc syringe and a tiny needle, the healthcare 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 healthcare practitioner at 48 and/or 72 hours to see if a local skin reaction has occurred.
How is it used?
Tuberculosis (TB) skin 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
- 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
Either a tuberculin skin test (TST) or a blood test called an interferon gamma release assay (IGRA) may be performed to screen for TB, but in most cases the IGRA is now preferred over the TST. 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.
Recommendations from the Centers for Disease Control and Prevention (CDC), the Infectious Diseases Society, of America (IDSA), and the American Thoracic Society (ATS) list a preference for an IGRA test when:
- Someone is unlikely to return to have their TST evaluated
- The person is 5 years or older
- Is likely to be infected with TB
- Has a low to intermediate risk of progressing to active tuberculosis
- A test for latent tuberculosis infection (LTBI) is warranted
- And/or if 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 TST test is considered an acceptable alternative to the IGRA if the IGRA is not available or is considered too costly or burdensome.
These same agencies do NOT recommend testing people who are not likely to be infected with TB, or those who are considered at a low risk for TB infection and disease progression.
However, when testing for latent TB infection is required, such as for employment or a legal requirement, they suggest:
- An IGRA rather than a TST when someone is 5 years or older
- A second test, either an IGRA or TST, when the initial test is positive and to only consider the person positive if both tests are positive
- A TST rather than an IGRA when a person is less than 5 years old, healthy, and the TB screening is warranted
If there is a risk that the first TST is a false-negative reaction, a second skin test can be given so that the TST can have the chance to stimulate the immune system, causing a positive or boosted reaction in the second test.
When is it ordered?
TSTs are ordered less frequently than IGRA tests for TB screening but may still 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 healthcare workers
- 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 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 for life and the skin reaction to subsequent tuberculin skin tests may become increasingly severe.
What does the test result mean?
A healthcare 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 raised 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 and the person may have a latent or active TB infection. If a healthcare practitioner suspects that someone has active tuberculosis, a history and physical examination and other tests, such as chest X-rays and AFB laboratory testing, are used to confirm the diagnosis.
A negative result 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 healthcare practitioner wants to confirm a negative or indeterminate result, the practitioner may repeat the TST or do an 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 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?
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.
Should I get a tuberculosis skin test if I am pregnant?
You may be tested under your healthcare 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 healthcare practitioner may want you to have a TST or IGRA done. Either of these is considered safe during pregnancy.
What if I have a TST and it is more than 72 hours before I go back to have it evaluated?