Tuberculosis (TB) testing screens for exposure to Mycobacterium tuberculosis, the bacterium causing the disease. This test is important for public health, helping identify and control TB transmission. Understanding the testing process and result interpretation is important for individuals and healthcare providers.
Why the TB Test is Administered
The primary purpose of a TB test is to screen for latent tuberculosis infection (LTBI). In LTBI, Mycobacterium tuberculosis bacteria are present but not actively causing symptoms, meaning the individual is not contagious. Identifying LTBI is important because it can progress to active TB disease, which is symptomatic and transmissible.
Testing is recommended in various scenarios to prevent this progression. Healthcare professionals are routinely screened due to their exposure risk. Individuals with compromised immune systems, such as those with HIV or undergoing immunosuppressive therapy, are also frequently tested.
People in close contact with someone diagnosed with active TB disease are advised to undergo testing. Testing is also relevant for those who have recently traveled from or reside in regions with high TB prevalence. Early detection and treatment of LTBI reduce the likelihood of developing active TB, contributing to public health efforts.
The TB Skin Test Administration Process
The Mantoux tuberculin skin test (TST) is a common TB screening method using Purified Protein Derivative (PPD). The procedure begins by selecting the inner surface of the forearm. The skin is cleansed with an alcohol swab and allowed to air dry.
A small, sterile needle attached to a tuberculin syringe is used for injection. The healthcare professional holds the syringe with the needle’s bevel facing upwards. The needle is inserted into the superficial intradermal layer of the skin at an angle of 5 to 15 degrees.
A precise amount of PPD solution, typically 0.1 mL, is slowly injected. This creates a distinct, raised, pale bump on the skin, called a wheal or “bleb,” indicating correct intradermal administration. The wheal should measure between 6 to 10 mm in diameter. After injection, the needle is withdrawn, and the site should not be rubbed or covered. Patients should allow the area to air dry and avoid irritating it until read by a healthcare professional.
Interpreting Your TB Test Results
After the TB skin test, individuals must return for a reading within 48 to 72 hours. This timeframe is important because the immune response develops and becomes measurable within this window. Waiting too long beyond 72 hours can lead to an inaccurate or “false negative” reading as the reaction may fade.
The test is interpreted by measuring the diameter of any induration, a firm, raised area of swelling, not just redness, at the injection site. The healthcare professional measures its widest diameter across the forearm using a ruler. Redness or bruising without a palpable raised area is not a positive reaction.
A positive result varies based on individual risk factors. An induration of 5 millimeters or more may be positive for individuals with recent close contact to active TB, those with HIV, or people with chest X-ray findings suggestive of previous TB. An induration of 10 millimeters or more may be positive for recent immigrants from high-prevalence countries, healthcare workers, or residents of high-risk settings. An induration of 15 millimeters or more is generally positive for anyone, regardless of risk factors. A negative result indicates the individual has not been infected with Mycobacterium tuberculosis.
Next Steps After Your TB Test
For individuals with a negative TB skin test result, no further action is typically necessary, unless there is ongoing exposure to active TB or high suspicion of recent infection. In such cases, a repeat test or further evaluation might be considered.
A positive TB skin test result indicates a likely infection with Mycobacterium tuberculosis, though it does not differentiate between latent and active disease. A positive result requires further evaluation to determine the infection’s precise status. This includes additional diagnostic tests to confirm TB bacteria and ascertain if the infection is latent or active.
These additional tests often include a chest X-ray for signs of active disease. Blood tests, specifically Interferon Gamma Release Assays (IGRAs) such as the QuantiFERON-TB Gold Plus or T-SPOT.TB test, may also be performed. IGRAs measure the immune response to specific TB antigens and help distinguish between true infection and a false positive from a prior BCG vaccination. If active TB disease is ruled out, treatment for latent TB infection is often recommended to prevent its progression, using medications like isoniazid, rifampin, or rifapentine.