The Tuberculin Skin Test (TST), also known as the Mantoux test, screens for tuberculosis (TB) bacteria infection. It identifies latent TB infection, where bacteria are present but inactive and not causing symptoms.
The Tuberculin Skin Test Procedure
The TST involves injecting a small amount of purified protein derivative (PPD) into the inner forearm. This intradermal injection, given just under the skin, creates a pale, raised bump, typically 6 to 10 millimeters in diameter.
The test site must be evaluated by a trained healthcare professional between 48 and 72 hours after administration. If the individual does not return within this 72-hour window, another skin test may be necessary for an accurate reading.
Step-by-Step Guide to Reading Your TB Test
When examining the test site, the healthcare professional looks for a raised, hardened area known as induration. This firm swelling indicates an immune system reaction to tuberculin. Redness around the injection site is common but does not indicate a positive result; only the induration is measured.
To measure induration, the healthcare provider gently feels the area to locate its edges. Using a ruler, they measure the diameter of this firm area across the forearm, perpendicular to its long axis. The measurement is recorded in millimeters; if no induration is present, it is recorded as “0 mm.”
Interpreting the Results
The interpretation of a TST result depends on the induration size and the individual’s risk factors for TB infection or progression to disease. A measurement of 0 mm or below the defined positive threshold is generally considered a negative result. A positive result indicates past TB bacteria infection, though it does not necessarily mean active TB disease.
Different induration sizes are considered positive based on specific risk categories. An induration of 5 millimeters or more is considered positive for individuals with weakened immune systems, such as those with HIV, organ transplant recipients, or those on immunosuppressive medications. It is also considered positive for recent contacts of people with infectious TB disease or those with chest X-ray findings suggestive of previous TB.
An induration of 10 millimeters or more is considered positive for people born in countries where TB is common, individuals who misuse drugs or alcohol, and those living or working in high-risk settings like nursing homes or correctional facilities. Healthcare workers, children younger than 5 years of age, and people with certain medical conditions like diabetes or severe kidney disease also fall into this category.
For individuals with no known risk factors for TB, an induration of 15 millimeters or more is a positive result.
Certain factors can influence TST results. Prior vaccination with the Bacille Calmette-Guérin (BCG) vaccine, commonly used in countries with high TB prevalence, can lead to a false-positive result. Infections with other non-tuberculosis mycobacteria can also cause false positives.
Conversely, a weakened immune system, recent TB infection (within 8 to 10 weeks), or recent live-virus vaccinations can lead to false-negative results.
What to Do After Your TB Test is Read
After your TB test is read, next steps depend on the result. If negative with no symptoms or recent exposure, no further action is typically needed. However, if you have TB symptoms or recent significant exposure despite a negative result, further evaluation may be recommended.
For a positive TST result, further evaluation by a healthcare professional is necessary to determine if you have latent TB infection or active TB disease. This evaluation commonly includes a chest X-ray to check for signs of active disease in the lungs. Sputum tests, where mucus is collected and tested for TB bacteria, may also be performed. Blood tests, such as Interferon-Gamma Release Assays (IGRAs), can confirm TB infection and are often preferred for those who have received the BCG vaccine. If active TB is ruled out, treatment for latent TB infection may be recommended to prevent active disease.