The Tuberculosis Skin Test (TST), also known as the Mantoux or PPD test, screens for infection with Mycobacterium tuberculosis. The procedure involves a healthcare professional injecting a small amount of purified protein derivative (PPD) just beneath the skin’s surface, typically on the inner forearm. This injection triggers a localized, delayed-type hypersensitivity reaction in people who have been previously exposed to the bacteria. To ensure the test accurately reflects the body’s immune response, strict adherence to post-test instructions is necessary, as certain actions can interfere with the biological reaction and compromise the validity of the result.
Physical Restrictions on the Injection Site
Physical interference with the injection site is the most common way a patient can invalidate a TST result. Avoid rubbing, scratching, or massaging the area where the tuberculin solution was administered. Scratching or aggressive scrubbing creates irritation, leading to localized redness and swelling unrelated to the immune response, potentially causing a false-positive reading. If the site becomes itchy, applying a cold cloth or an ice cube is the recommended method to relieve the sensation.
Patients must not apply any creams, lotions, ointments, or topical medications over the test area. Products like hydrocortisone can interfere with the skin’s reaction to the PPD solution. The use of bandages, tape, or other dressings to cover the injection site is prohibited. Adhesives can cause skin irritation or an allergic reaction, which may obscure the true reaction or cause a false reading.
While patients can generally shower or bathe as normal, they should avoid scrubbing the area when washing and gently pat the arm dry afterward. The PPD solution is injected into the upper layer of the skin. Excessive pressure or scrubbing could physically disperse the solution or irritate the tissue. Any physical action that irritates the skin could lead to a result that does not accurately represent the body’s immune status regarding tuberculosis.
Attempts to Interpret the Results Yourself
A common mistake is trying to measure or judge the result of the TST at home. The immune response that signals a positive result is a raised, hardened area called induration, not simply discoloration or redness. The erythema, or redness, that frequently surrounds the injection site is not measured and does not determine the test result. Redness may occur as a minor, non-specific reaction to the injection itself.
Only a trained healthcare professional can accurately interpret the test because they must palpate the area to feel and measure the diameter of the induration in millimeters. The interpretation criteria are complex, as a positive result is defined by different induration sizes depending on the patient’s risk factors and medical history.
Skipping the Required Follow-Up Appointment
Failing to return for the scheduled follow-up appointment within the specific timeframe is a logistical error that invalidates the test. The TST must be read precisely between 48 and 72 hours after the injection. This window is necessary because the cell-mediated immune response that creates the induration takes time to develop, peaking within this period. If the appointment is missed and the reading occurs after the 72-hour mark, the result is considered invalid, and the test must be repeated. Reading the test later than 72 hours can lead to an underestimation of the induration size.