A tuberculosis (TB) test determines if an individual has been infected with Mycobacterium tuberculosis, the bacteria causing tuberculosis. Testing is often conducted due to potential exposure, job or travel requirements, or if a person belongs to a higher-risk group. These tests identify the bacteria’s presence, prompting further evaluation if infection is detected.
Common Testing Locations
Individuals seeking a TB test have several accessible options. Primary care physician offices are a common starting point, where doctors assess medical history and determine the appropriate test. Local public health departments and clinics frequently offer TB testing as part of their community health services, sometimes at a lower cost.
Urgent care centers provide another convenient option, often accommodating walk-in appointments for TB testing. Some pharmacies, such as MinuteClinic locations within CVS, also offer walk-in TB testing. For students, university health services may offer testing by appointment. Occupational health clinics are also common sites for testing, particularly when required for employment purposes. Check specific requirements and costs, as these can vary among locations.
What to Expect During Testing
The two primary methods for TB testing are the tuberculin skin test (TST) and the TB blood test, known as an Interferon-Gamma Release Assay (IGRA). The TST involves injecting a small amount of fluid called tuberculin, or PPD, just under the skin on the inner forearm. A small, raised bump forms at the injection site, which typically flattens within minutes.
For the TST, a follow-up visit is necessary within 48 to 72 hours for a healthcare professional to examine the injection site. They measure any firm swelling to interpret the result. In contrast, the IGRA is a single-visit blood test where a sample is drawn and sent to a laboratory for analysis. This blood test measures the immune system’s response to TB antigens. Blood tests are preferred for individuals who have received the BCG vaccine or who may have difficulty returning for a second visit.
Understanding Your Results
A positive result from either a TB skin test or a TB blood test indicates infection with Mycobacterium tuberculosis bacteria. However, a positive result does not automatically mean a person has active tuberculosis disease. For a TST, a positive reaction is indicated by a firm, raised bump of a certain size at the injection site, with specific size interpretations varying based on individual risk factors. A positive IGRA means certain white blood cells in the blood sample recognized TB antigens.
Following a positive test result, healthcare providers recommend further evaluations to determine if the infection is a latent TB infection (LTBI) or active TB disease. These additional tests include a chest X-ray and sometimes sputum (phlegm) tests. If active TB disease is ruled out, a positive result signifies latent TB, where bacteria are present but not causing illness or spreading. Latent TB is treated to prevent its progression to active disease. Discuss all test results and any subsequent steps with a healthcare provider for proper guidance and treatment.