The Tuberculosis Skin Test (TST), commonly known as the Mantoux PPD (Purified Protein Derivative) test, is a standard procedure used worldwide to determine if a person has been infected with Mycobacterium tuberculosis. The test involves an intradermal injection of a small amount of PPD, a protein derivative, which triggers a delayed-type hypersensitivity reaction in individuals previously exposed to TB. The TST screens for latent TB infection and prior exposure; it does not diagnose active tuberculosis disease. Accurate administration is necessary because flawed technique can lead to incorrect results, potentially delaying treatment or causing unnecessary follow-up.
Necessary Supplies and Site Preparation
Proper preparation requires gathering specific materials, including a vial of tuberculin PPD solution, typically standardized at 5 Tuberculin Units (TU) per 0.1 milliliters (ml). The injection requires a single-dose tuberculin syringe fitted with a short, 1/4- to 1/2-inch, 27-gauge needle with a short bevel. Alcohol swabs, sterile gloves, and a puncture-resistant sharps container must also be readily available.
Before drawing the solution, review the patient’s medical history for contraindications, such as a severe reaction to a previous TST or a confirmed TB diagnosis. The liquid PPD must be drawn into the syringe immediately before administration, ensuring precisely 0.1 ml remains after expelling air bubbles. The solution’s potency can diminish if it is preloaded or exposed to light for extended periods.
The correct anatomical location is the inner surface of the forearm, positioned two to four inches below the elbow joint. The selected site must be free from barriers that could interfere with placement or reading, such as excessive hair, scars, sores, or visible veins. Cleanse the area with an alcohol swab using a circular motion, moving outward from the center, and allow it to air dry completely before injection.
Step-by-Step Injection Technique
The PPD solution must be administered as an intradermal injection, deposited within the top layer of the skin (the dermis). Place the patient’s forearm palm-side up on a firm surface, and stretch the skin taut with the non-dominant hand. Hold the needle almost parallel to the skin surface, with the bevel facing upward.
Insert the syringe into the skin at a shallow angle, ideally between 5 and 15 degrees, until the entire bevel is covered and visible beneath the surface. Maintaining this shallow angle ensures the solution is delivered into the correct skin layer. Once positioned, depress the plunger slowly and steadily to inject the 0.1 ml of PPD solution.
Resistance should be felt during the injection, confirming the needle is within the dense dermis layer. Successful injection is confirmed by the immediate formation of a distinct, pale, raised wheal or “bleb,” measuring approximately six to ten millimeters (mm) in diameter. If the wheal does not form or if solution leaks out, the injection was too deep, and the test must be repeated at a new site at least two inches away.
Withdraw the needle smoothly at the same shallow angle. Instruct the patient not to rub, scratch, or cover the site with a bandage. If a small drop of blood appears, gently blot it with a cotton ball without applying pressure, which could force the PPD out. Immediately discard the needle and syringe into a designated sharps container.
Reading and Documenting the Test Results
The assessment of the TST reaction is a separate procedure that must occur within a specific window of time—between 48 and 72 hours after the injection. If the patient does not return for the reading within 72 hours, the test results are considered invalid, and the procedure must be repeated. The reading process involves a trained professional who first visually inspects the site under good light and then uses palpation to check for induration.
Induration is the hard, dense, raised swelling indicating a localized immune response, and it is the only reaction that must be measured; surrounding erythema (redness) is disregarded. To accurately locate the margins of the induration, gently sweep fingertips over the area to feel the edges of the raised tissue. A ballpoint pen can be used to lightly mark the widest borders of the induration, as the tip will stop at the edge of the hardened area.
A flexible, millimeter-scale ruler is used to measure the diameter of the induration across the forearm, perpendicular to the long axis of the arm. If the measurement falls between two lines on the millimeter scale, the smaller of the two numbers is recorded. The final measurement, recorded in millimeters, is interpreted based on the patient’s individual risk factors for tuberculosis.
5 mm or Larger Induration
This threshold is considered positive for high-risk individuals. This includes those who are immunocompromised, have recent close contact with a person who has active TB, or have evidence of previous TB on a chest X-ray.
10 mm or Larger Induration
This threshold is considered positive for people with a higher likelihood of exposure. This group includes recent immigrants from high-prevalence areas or those working in high-risk settings.
15 mm or Larger Induration
In persons with no known risk factors for TB exposure, an induration of 15 mm or more is required to be considered a positive reaction.
Accurate documentation involves recording the exact measurement in millimeters, even if the result is 0 mm, rather than simply writing “positive” or “negative.” The record must also include the date and time the test was read, the name of the person who performed the reading, and any adverse reactions observed, such as blistering. This detailed documentation ensures the result is precise and provides a clear baseline for future TSTs or follow-up care.