Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis, which typically attacks the lungs but can affect other parts of the body. TB testing determines if a person has been exposed to the bacteria. The primary goal is to identify latent TB infection (LTBI), where the bacteria are present but inactive, distinguishing it from active TB disease, where the bacteria are multiplying and causing illness. Determining testing frequency relies on understanding results and personal risk profile.
Understanding the Differences Between TB Tests
The two primary methods for tuberculosis screening are the Tuberculin Skin Test (TST) and the Interferon Gamma Release Assays (IGRAs). The TST, often called the PPD test, involves injecting a small amount of purified protein derivative beneath the skin. A healthcare professional must examine the injection site 48 to 72 hours later to measure the size of any resulting firm, raised area, known as induration.
The IGRA is a blood test that measures the immune system’s reaction to TB-specific proteins. Examples include the QuantiFERON-TB Gold Plus. This method requires only a single visit for a blood draw and provides results quickly, eliminating the need for a return visit to interpret the result.
A significant difference between the two tests lies in their susceptibility to the Bacillus Calmette-Guérin (BCG) vaccination, which is administered in many countries outside the United States. Since the BCG vaccine can cause a false-positive result on the TST, the IGRA is the preferred method for vaccinated individuals. Conversely, the TST is often used for children under five years old, as IGRA accuracy data is less extensive in this age group.
Establishing Your Testing Frequency Based on Risk
The frequency of tuberculosis testing depends entirely on an individual’s risk of exposure. For the majority of the low-risk general public, routine annual testing is not recommended. Testing is instead targeted toward specific populations based on their likelihood of exposure or potential for developing active disease.
Occupational Exposure
Healthcare personnel (HCP), laboratory workers, and emergency responders are often subject to specific workplace testing protocols due to potential exposure risks. Guidelines recommend a baseline TB test (TST or IGRA) when a person begins employment in a healthcare setting. Following a negative baseline result, routine serial testing, such as annual screening, is no longer broadly recommended for all HCP.
Instead of routine annual testing, many facilities now focus on annual symptom screening for all personnel. Serial testing is only considered for those who have a continued, elevated occupational risk of exposure, which is determined by the facility’s risk assessment. This shift from mandatory annual testing to targeted screening reduces unnecessary testing.
New Baseline Screening
Individuals entering environments with a higher prevalence of tuberculosis are typically required to have a baseline screening. This includes new immigrants or refugees from countries where TB is common. Initial testing is also standard for new entrants to correctional facilities, homeless shelters, and residential care settings to establish infection status upon entry.
Immediate Exposure
A specific testing protocol is followed if a person has known close contact with someone who has active, infectious TB disease. Initial testing should occur as soon as the exposure is identified, using either a TST or IGRA. Because the immune system takes time to develop a measurable response, a negative result at this point does not rule out infection.
If the initial test is negative, a follow-up test is recommended eight to ten weeks after the last known exposure. This delayed retest ensures the body has had sufficient time to mount an immune response, providing a more accurate result regarding recent infection.
Interpreting Results and Implications for Future Testing
A positive result from either a TST or an IGRA indicates infection with M. tuberculosis bacteria. A positive test result means the person has been exposed and developed a latent TB infection (LTBI), not necessarily active TB disease. Further evaluation, typically including a chest X-ray and symptom review, is necessary to determine if the infection is latent or active.
If active disease is ruled out, the positive test confirms latent TB infection, which can be treated to prevent it from progressing to active disease. Once a person has a documented positive result, they generally do not need routine serial TB testing again. The test is designed to detect a previous immune response and is likely to remain positive for the rest of the person’s life.
Repeatedly performing the test is unnecessary and could lead to confusion. Instead of retesting, individuals with a known positive result are monitored for the development of active disease symptoms. This usually involves an annual symptom review by a healthcare provider to catch early signs of progression from latent infection to active disease.