Tuberculosis (TB) remains a significant global health challenge. Periodic TB refers to regular screening for this infectious disease. Consistent screening identifies infections early, which helps control its spread and prevent severe health outcomes. Early detection allows for timely intervention, benefiting individuals and public health.
Understanding Tuberculosis
Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis. This bacterium primarily attacks the lungs but can also affect other body parts, including the kidney, spine, and brain. It transmits through the air when an infected person coughs, sneezes, or speaks, releasing airborne droplets.
Latent TB infection (LTBI) differs from active TB disease. In LTBI, the bacteria are present but inactive, causing no symptoms and not spreading. The immune system often contains the bacteria, preventing multiplication. However, individuals with latent TB risk developing active disease if their immune system weakens, potentially leading to severe illness and transmission.
Active TB disease occurs when Mycobacterium tuberculosis bacteria multiply and cause symptoms. Common symptoms include a persistent cough lasting three weeks or longer, chest pain, coughing up blood or sputum, unexplained weight loss, fever, and night sweats. People with active TB in their lungs or throat can transmit the bacteria to others through airborne particles.
Why Regular TB Screening Matters
Regular TB screening identifies individuals infected with Mycobacterium tuberculosis before they develop active disease. Identifying latent infections allows for preventative treatment, significantly reducing the risk of future disease development.
Specific populations benefit from routine screening due to increased risk of exposure or progression to active disease. Healthcare workers are screened due to frequent patient contact. Individuals with compromised immune systems, like those with HIV or on immunosuppressive therapy, are prioritized as their bodies are less able to contain latent infection. People with close contact to active TB cases and immigrants from high-burden countries also undergo screening.
Methods for TB Testing
Two primary methods are used for tuberculosis screening: the Tuberculin Skin Test (TST) and Interferon-Gamma Release Assays (IGRAs). The TST involves injecting a small amount of tuberculin purified protein derivative (PPD) into the skin, typically on the forearm. A healthcare professional measures the induration, or hardened raised area, at the injection site 48 to 72 hours later to interpret the result. The size of this induration, not redness, determines the test outcome.
IGRAs are blood tests measuring the immune response to specific TB proteins. Commonly used IGRAs include QuantiFERON-TB Gold Plus (QFT-Plus) and the T-SPOT.TB test. These tests involve drawing a blood sample for laboratory analysis. Blood cells are exposed to Mycobacterium tuberculosis antigens, and the test measures interferon-gamma released by T cells. IGRAs require only one patient visit for blood collection, unlike the TST which requires two.
Both TST and IGRAs have advantages and disadvantages. The TST is less expensive and widely available, but its interpretation can be influenced by prior BCG vaccination, leading to false-positive results. IGRAs are less affected by BCG vaccination, offering greater specificity, but they are more expensive and require laboratory processing, which may not be available everywhere. The choice of test depends on patient factors, local TB prevalence, and resource availability.
Understanding Your Test Results
Interpreting the results of a TB test requires careful consideration, as the meaning can vary based on the specific test used and individual risk factors. For the Tuberculin Skin Test (TST), a positive result is determined by the size of the induration, or firm swelling, at the injection site. For example, an induration of 5 millimeters or more is considered positive for individuals with weakened immune systems or recent close contact with an active TB case. In contrast, an induration of 10 millimeters or more may be positive for people from high-burden countries or those who work in high-risk settings, while 15 millimeters or more is generally considered positive for individuals with no known risk factors.
Interferon-Gamma Release Assays (IGRAs) provide results as positive, negative, or indeterminate. A positive IGRA result indicates that the person has been infected with Mycobacterium tuberculosis and has developed an immune response to the bacteria. A negative result suggests that the person has not been infected. An indeterminate result means the test could not definitively determine infection, often due to technical reasons or a weak immune response from the individual, and may require retesting.
Several factors can influence test results. For the TST, prior vaccination with Bacillus Calmette-Guérin (BCG), a vaccine sometimes given in countries with high TB prevalence, can cause a false-positive result. Prior infection with non-tuberculous mycobacteria can also lead to a false-positive TST. While IGRAs are less affected by BCG vaccination, certain medical conditions or medications that suppress the immune system can lead to false-negative results for both TST and IGRAs, as the body may not mount a sufficient immune response to the test.
Managing a Positive TB Test
A positive TB test result, whether from a TST or an IGRA, indicates that an individual has been infected with Mycobacterium tuberculosis. The immediate next step involves a thorough medical evaluation to determine if the infection is latent TB infection (LTBI) or has progressed to active TB disease. This evaluation includes a medical history, a physical examination, and a chest X-ray. The chest X-ray helps to identify any changes in the lungs that might suggest active TB disease, such as infiltrates or cavities.
If the chest X-ray is abnormal or if symptoms suggestive of active TB are present, additional tests are performed. These include sputum smear microscopy and culture, where samples of mucus coughed up from the lungs are examined for the presence of TB bacteria. Nucleic acid amplification tests (NAATs) may also be used for rapid detection of Mycobacterium tuberculosis DNA. These tests confirm an active TB diagnosis and guide treatment.
For individuals diagnosed with latent TB infection (LTBI) after ruling out active disease, treatment is recommended to prevent the progression to active TB. Common treatment regimens for LTBI involve taking specific antibiotics for several months, such as isoniazid daily for six to nine months, or rifampin daily for four months. Shorter regimens, like isoniazid and rifapentine weekly for three months, are also available and often preferred for improved adherence. If active TB disease is diagnosed, a more intensive course of multiple anti-TB drugs is prescribed, typically for six to nine months, to effectively cure the disease and prevent further transmission.