Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis, which primarily affects the lungs but can attack any part of the body. The condition exists in two forms: latent TB infection (LTBI) and active TB disease. In latent infection, the bacteria are dormant, cause no symptoms, and cannot be spread to others. Active TB disease means the bacteria are multiplying, causing illness, and the person may be contagious, particularly if the infection is in the lungs. Screening is important for early detection, preventing progression to active disease, and limiting its spread.
Who Needs Routine TB Screening
Routine screening is generally not necessary for the average person but is strongly recommended for specific high-risk populations. Testing should be done for those who have had recent close contact with a person known to have active TB disease. Individuals born in or who frequently travel to countries where the disease is common also have a higher risk of exposure.
People living with conditions that compromise the immune system are at a higher risk of developing active TB if infected. This includes individuals who are HIV-positive, those with diabetes mellitus, or patients receiving immunosuppressive therapies like high-dose corticosteroids. Healthcare personnel are also considered a high-risk group due to occupational exposure. Screening is also important for residents and employees of congregate settings like correctional facilities, homeless shelters, and nursing homes.
Comparing the Different TB Tests
Two primary methods are available for testing for TB infection: the Tuberculin Skin Test (TST) and the Interferon Gamma Release Assay (IGRA). The TST, also known as the Mantoux test, involves injecting a small amount of purified protein derivative (PPD) under the skin. A healthcare worker must observe the injection site 48 to 72 hours later to measure the size of any raised, firm area, called induration, which indicates a positive result.
The IGRA is a blood test that requires only a single visit, making it more convenient. This test measures the release of interferon-gamma from white blood cells when exposed to specific TB antigens in a laboratory setting. The IGRA is often preferred for individuals who have received the Bacille Calmette-Guérin (BCG) vaccine because the vaccine can cause a false-positive result with the TST. Both tests are effective at identifying TB infection, but the choice depends on a person’s history, immune status, and test availability.
Guidelines for Testing Frequency
For the general, low-risk population, routine or annual TB testing is not recommended unless a specific exposure occurs. If a known exposure happens, testing should be performed immediately, followed by a re-test eight to ten weeks after the last known contact. This waiting period allows enough time for the immune system to develop a detectable response to the bacteria.
Testing frequency is often dictated by occupational or institutional requirements in high-risk environments. Healthcare personnel must undergo baseline TB screening, including a risk assessment and a test, upon employment. Routine annual testing for healthcare workers is generally no longer recommended unless they work in a setting with ongoing transmission or known exposure.
Individuals with untreated latent TB infection should receive annual screening for symptoms of active disease, rather than a repeat TB test. People who frequently travel to or work in high-prevalence countries may also be advised to undergo annual testing if their exposure is prolonged or repeated.
Interpreting a Positive TB Result
A positive result from a TST or IGRA indicates that Mycobacterium tuberculosis bacteria have entered the body and the immune system has responded. A positive screen does not definitively mean a person has active TB disease, so further evaluation is necessary to distinguish between LTBI and the active form.
The next step is typically a medical evaluation that includes a symptom assessment and a chest X-ray. If the X-ray is normal and the person has no symptoms, the diagnosis is usually latent TB infection. If the X-ray shows abnormalities or if the person has symptoms like a prolonged cough, additional tests, such as a sputum sample, are ordered to confirm active disease. Treatment for LTBI is encouraged to prevent progression to active, transmissible disease.