The Tuberculin Skin Test (TST), also known as the Mantoux test, determines if a person has been infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis. A healthcare professional injects a small amount of testing fluid called tuberculin just beneath the skin’s surface, typically on the forearm. This creates a slightly raised bump called a wheal, confirming the fluid was correctly administered. The body’s reaction is a delayed-type hypersensitivity response, and following specific aftercare instructions is necessary to prevent false readings and maintain the accuracy of the result.
Keep the Area Free from Manipulation
After the injection, avoid any physical contact with the test site that could alter the skin’s reaction. Do not scratch, rub, or massage the area, even if it feels itchy, as this can cause inflammation that mimics a positive result. If you feel discomfort, applying a cold cloth or ice pack can help relieve the sensation without compromising the test integrity.
Do not cover the injection site with any type of dressing, including a bandage, tape, or gauze. Applying a covering can obscure the reaction or cause irritation from the adhesive, potentially leading to an inaccurate assessment by the clinician. Similarly, avoid applying any topical products like lotions, creams, or anti-itch ointments to the area. These substances can interfere with the skin’s natural response to the tuberculin.
Rules Regarding Water and Moisture
While you can resume most normal activities, protect the injection site from excessive moisture or scrubbing. Brief showering is acceptable, but avoid using harsh soaps directly on the site or rubbing it vigorously with a washcloth or towel. Gently pat the area dry after washing.
Do not submerge the arm in water; avoid activities like swimming, using a hot tub, or taking long baths until the test has been officially read. Excessive exposure to water can wash away any surface markings made to locate the site. Soaking the area can also alter the skin’s delayed hypersensitivity reaction, which could lead to an invalid or inconclusive result.
Do Not Miss the Reading Window
Adhering to the strict reading schedule is essential for the Mantoux test. You must return to a healthcare provider to have the test read between 48 and 72 hours after the injection. This precise time frame is necessary because the immunological reaction, which is a delayed response, peaks within this window.
If you miss the scheduled appointment and return after the 72-hour mark, the test result becomes invalid and must be repeated. The induration—the hardened, raised area indicating a positive reaction—begins to subside outside of this three-day window, making accurate measurement impossible. Missing this short period means the entire testing process must start over, requiring a second injection.
Avoid Interpreting Your Own Results
After the 48-hour mark, you may notice a reaction at the injection site, but resist the urge to self-diagnose. Do not attempt to measure the reaction yourself with a ruler or any other tool. Clinicians are specifically trained to read the test and use precise methods to measure the reaction across the forearm.
A common mistake is assuming that any redness (erythema) indicates a positive result. The healthcare provider looks for induration, which is a firm, palpable swelling, not just visible redness. The final interpretation depends on the exact size of the induration in millimeters, assessed based on your specific risk factors for tuberculosis exposure. Only a trained professional can officially declare the result and discuss any necessary follow-up steps.