The Tuberculin Skin Test (TST), commonly called the Mantoux test, screens individuals for infection with Mycobacterium tuberculosis. The test detects a delayed-type hypersensitivity reaction, indicating the immune system has been sensitized by previous exposure, suggesting latent TB infection. The substance injected is purified protein derivative (PPD), an extract from the bacteria’s culture filtrate. Accurate results depend entirely on precise administration and correct interpretation of the reaction.
Preparing the Patient and Materials
Before administration, a detailed patient screening must be completed to ensure the test is appropriate and to aid in interpretation. Check for any history of prior TST reactions, especially severe allergic responses like blistering or necrosis. The patient’s history of Bacillus Calmette-Guérin (BCG) vaccination should also be noted, as it can cause false-positive TST results. The standard dose is 5 Tuberculin Units (TU) of PPD solution delivered in 0.1 milliliters (mL).
Gathering the correct materials ensures a sterile procedure. Materials include the PPD solution, a tuberculin syringe with a short (quarter- to half-inch) 26- or 27-gauge needle, alcohol wipes, and gloves. Select the injection site on the volar (inner) aspect of the forearm, 2 to 4 inches below the elbow bend. This area must be free of visible veins, scars, sores, or excessive hair that could interfere with the injection or reading.
Step-by-Step Injection Technique
Clean the injection site thoroughly with an alcohol wipe and allow it to dry completely. Draw the PPD solution (0.1 mL of 5 TU strength) into the syringe, carefully expelling air bubbles to ensure the precise dose is administered. Stretch the skin taut between the thumb and forefinger to create a firm surface for the injection.
Insert the needle slowly with the bevel facing upward, positioned almost parallel to the skin at a shallow angle of 5 to 15 degrees. The goal is an intradermal injection, depositing the PPD just beneath the epidermis. The needle should be visible through the skin surface just after the entire bevel is covered.
Inject the PPD slowly; a slight resistance should be felt as the fluid enters the skin. Success is confirmed by the immediate formation of a distinct, pale, raised wheal or bleb, measuring 6 to 10 millimeters (mm) in diameter. If a wheal does not form, or if fluid leaks, the injection was likely too deep (subcutaneous) and must be repeated at a separate location at least 2 inches away. Do not rub, massage, or cover the site with a bandage afterward, as this can disperse the solution and invalidate the test.
Reading and Interpreting the Test Results
A trained professional must read the TST result within 48 to 72 hours following administration. Readings taken outside this timeframe are unreliable and require the test to be repeated. The reaction measured is induration (a firm, raised, hardened area), not erythema (redness), which has no interpretive value.
To measure induration accurately, support the forearm and slightly flex the elbow. The provider visually inspects the site and gently palpates it to locate the outer edges of the hardened area. A common method involves gently pushing the tip of a ballpoint pen toward the site; the tip will stop at the edge of the induration. Measure the diameter in millimeters across the forearm, perpendicular to the long axis.
Criteria for Positive TST Results
The interpretation of a positive result depends on the patient’s individual risk factors for TB exposure or progression to active disease.
- An induration of >= 5 mm is positive for immunocompromised persons (e.g., those with HIV) or individuals with recent close contact with active TB disease.
- An induration of >= 10 mm is positive for recent immigrants from high-prevalence countries, injection drug users, residents or employees of high-risk congregate settings, or children under four years of age.
- An induration of >= 15 mm is the threshold for a positive result for persons with no known risk factors for TB infection.
Action Based on Test Outcomes
A negative TST result (induration size below the applicable positive threshold) generally means no further action is necessary. If the patient had a recent high-risk exposure, re-testing may be recommended 8 to 10 weeks after exposure. This accounts for the time required for the immune system to mount a response, as the TST cannot detect very recent infections.
A positive TST indicates Mycobacterium tuberculosis infection but does not distinguish between latent TB infection (LTBI) and active TB disease. Any patient with a positive TST must undergo a thorough medical evaluation, including a review of symptoms and a chest X-ray. The chest X-ray helps rule out active TB disease, which is contagious and requires immediate multi-drug therapy.
An Interferon-Gamma Release Assay (IGRA), a blood test, may be used as a second, confirmatory test, especially in individuals who have received the BCG vaccine. If the medical evaluation and chest X-ray are normal, the patient is diagnosed with LTBI and can be offered treatment to prevent progression to active TB disease. If active TB disease is suspected or confirmed, the patient is immediately started on an appropriate multi-drug regimen, and LTBI treatment is not appropriate.