Tuberculosis (TB) is a bacterial infection primarily affecting the lungs. Testing is not a standard annual requirement for the general population because the risk of infection in many countries is low. Instead, the frequency of TB testing depends entirely on an individual’s specific risk of exposure or their weakened immune status. Understanding the circumstances that necessitate testing and the correct follow-up procedures is important for preventing the spread of the disease.
Identifying Scenarios That Require TB Testing
An initial TB test is necessary when a specific event or circumstance increases the likelihood of recent exposure to the bacteria. The most common trigger is having spent time in close proximity with a person known to have active, infectious TB disease. This type of contact often requires immediate testing, followed by re-testing several weeks later.
Another significant risk factor is being born in or having frequently traveled to countries where TB is common. Healthcare workers and others who work in specific congregate settings, such as correctional facilities or homeless shelters, typically require baseline testing upon hire. Finally, anyone experiencing persistent symptoms like a cough lasting three weeks or more, unexplained weight loss, night sweats, or fever should seek evaluation for active TB disease.
Recommended Testing Frequency for High-Risk Groups
The frequency of repeat TB testing is determined by the ongoing nature of a person’s risk. For most healthcare personnel, routine serial testing is no longer recommended due to the declining incidence of TB in the United States. They are typically screened upon hiring, and they only require re-testing if there is a known exposure or ongoing transmission within their facility.
If a healthcare worker is exposed to an infectious case, they should be tested immediately, and then re-tested approximately eight to ten weeks after the last known exposure. Healthcare workers diagnosed with latent TB infection who decline treatment are advised to undergo an annual symptom screen and risk reassessment instead of repeat testing. For employees in high-risk congregate settings, such as correctional facility personnel, annual screening and testing may still be recommended due to the increased rate of exposure.
Individuals with compromised immune systems, such as those with HIV, should be screened for TB infection at the time of their initial diagnosis. For these individuals, annual screening is recommended to check for new risk factors for TB exposure or progression to active disease. People with low risk for continued exposure only need to be re-tested if a new, specific exposure event occurs. Similarly, residents of long-term care facilities are usually tested upon admission, but not routinely thereafter unless a case of TB is identified within the facility.
Understanding the Types of TB Tests
The Tuberculin Skin Test (TST) and the Interferon-Gamma Release Assays (IGRA) are the primary methods used to determine if a person has been infected with the TB bacteria. The TST, often called the Mantoux test, involves injecting a small amount of purified protein derivative (PPD) just beneath the skin of the forearm. The healthcare provider must then examine the injection site within 48 to 72 hours to measure the size of the resulting raised, hardened area, known as induration.
One limitation of the TST is that it can yield a false-positive result in people who have received the Bacillus Calmette-Guérin (BCG) vaccine. The IGRA is a blood test that measures the immune system’s response to TB-specific antigens. Because it is not affected by the BCG vaccine, the IGRA is the preferred test for individuals who have been BCG-vaccinated.
The IGRA requires only a single visit for the blood draw, whereas the TST requires two visits—one for the injection and one for the reading. Both tests are accurate for detecting TB infection, but the choice often depends on the patient’s history, the likelihood of a false-positive TST due to BCG vaccination, and the resources available.
Interpreting and Following Up on Test Results
A negative TB test result indicates that the person does not have TB infection. However, if the person had a very recent exposure, they might be in a “window period” where the immune response has not yet developed. In cases of documented exposure, a negative result should be followed by a re-test several weeks later to confirm the absence of infection.
A positive result from either a TST or an IGRA signifies that the Mycobacterium tuberculosis bacteria is present in the body. This positive result does not distinguish between Latent TB Infection (LTBI), where the bacteria is inactive, and Active TB Disease, where the bacteria is multiplying and causing illness. Further diagnostic steps are required to make this distinction, typically including a chest X-ray and evaluation of symptoms.
If the chest X-ray is normal and the person shows no symptoms, a diagnosis of LTBI is made, and treatment is strongly recommended to prevent the infection from progressing to active disease. If the X-ray shows abnormalities or the person has symptoms, sputum samples are collected to confirm active TB disease, which requires a more comprehensive treatment regimen.