The Purified Protein Derivative (PPD) test, also known as the Tuberculin Skin Test (TST) or Mantoux method, screens for infection with Mycobacterium tuberculosis, the bacteria responsible for tuberculosis (TB). This test determines if a person has been exposed to the bacteria and developed an immune response, indicating latent tuberculosis infection (LTBI). The procedure involves injecting a small amount of PPD solution, an antigen derived from the bacteria, into the skin to elicit a localized reaction. Precise technique is required, as the results depend entirely on the correct placement of the substance within the skin layers.
Supplies and Site Preparation
The necessary materials include the tuberculin syringe (a small 1-milliliter syringe typically fitted with a 26- or 27-gauge needle) and the PPD solution, usually containing 5 tuberculin units (TU) in 0.1 mL of fluid. Other items needed are an alcohol wipe, gloves for infection control, a pen for marking the location, and documentation materials.
The preferred injection site is the volar (inner) surface of the forearm, about two to four inches below the elbow joint, where the skin is thin and relatively hairless. The chosen area must be free of any existing skin conditions, such as scars, rashes, dermatitis, or prominent veins, which could interfere with the injection technique or the subsequent reading. Clean the site thoroughly with an alcohol wipe and allow the skin to air dry completely before the injection.
Detailed Intradermal Injection Technique
The injection must be placed precisely within the dermis, the layer just below the epidermis. Draw exactly 0.1 mL of the PPD solution into the syringe, ensuring no air bubbles are present, as this precise volume is necessary for a standardized test. Hold the patient’s forearm taut to stabilize the skin and facilitate shallow needle insertion.
Insert the needle slowly with the bevel facing upward at a very shallow angle, ideally between five and fifteen degrees, almost parallel to the skin surface. This shallow angle ensures the injection remains intradermal, placing the solution between the skin layers rather than too deep into the subcutaneous tissue. Once the entire bevel is just beneath the skin surface, slowly inject the 0.1 mL of solution.
A properly performed intradermal injection immediately produces a distinct, pale, raised bump, known as a wheal or bleb, measuring approximately six to ten millimeters in diameter. The appearance of this wheal confirms the solution was delivered into the correct skin layer. If no wheal forms or if significant leakage occurs, the test is invalid and must be repeated immediately on the opposite arm or at least two inches away from the initial site.
Immediate Post-Injection Patient Instructions
After the needle is withdrawn, the patient must receive clear instructions for site care to ensure the test remains valid. Advise the patient not to rub, scratch, or press on the site, as mechanical irritation can compromise the accuracy of the final reading. Covering the site with a bandage or dressing is also discouraged, as this can influence the development of the reaction.
Patients are informed that a localized bump, slight redness, or itching is common and expected. The patient must return to the clinic for a follow-up visit between 48 and 72 hours after administration, because the delayed-type hypersensitivity reaction peaks during this timeframe. Failing to return within this window means the test is invalid, and a new test must be administered.
Evaluation and Measurement of the Reaction
The evaluation of the PPD test reaction is performed 48 to 72 hours post-injection and is based solely on the measurement of induration, which is the hard, palpable swelling at the site. Redness (erythema) or bruising is not measured, as these are not reliable indicators of a true immune response. The health care worker first visually inspects the site and then gently palpates the area to locate the boundaries of the induration.
A standard ruler is used to measure the induration’s diameter across the forearm, perpendicular to the long axis, and the measurement is recorded in millimeters. The interpretation of the result depends on this measurement in conjunction with the patient’s individual risk factors for tuberculosis.
Interpretation Thresholds
The threshold for a positive result varies based on the patient’s risk profile:
- An induration of five millimeters or more is considered a positive result for people who are immunocompromised, such as those with HIV, organ transplant recipients, or individuals in recent close contact with a person who has active TB.
- An induration of ten millimeters or more is considered a positive result for other high-risk groups, including recent immigrants from high-prevalence countries, residents of high-risk settings like correctional facilities, and healthcare workers.
- For people with no known risk factors for tuberculosis, a larger induration of fifteen millimeters or more is typically required to be considered a positive reaction.
This tiered system of interpretation ensures that the result is clinically relevant based on the patient’s likelihood of progression from latent infection to active disease.