The Tuberculin Skin Test (TST) is a standardized diagnostic procedure used to determine if a person has been infected with Mycobacterium tuberculosis, the bacterium responsible for tuberculosis (TB). The test involves injecting a small amount of purified protein derivative (PPD) into the skin to elicit a delayed-type hypersensitivity reaction. This reaction helps healthcare professionals screen individuals for latent TB infection. Because proper administration and interpretation are technique-dependent, the procedure must be performed by a trained medical professional.
Essential Preparation and Site Selection
The first step is gathering the necessary supplies, including a tuberculin syringe with a short, small-gauge needle (typically 26- or 27-gauge), and the PPD solution. The PPD is usually standardized to five tuberculin units (5 TU) in a 0.1 mL dose. Before drawing the solution, the provider must confirm the PPD’s expiration date and ensure the vial has not been open for longer than 30 days, as the antigen’s potency diminishes.
The preferred injection site is the volar surface (inner aspect) of the forearm, about two to four inches (5 to 10 cm) below the elbow. This area is chosen because it has relatively little hair and is easily accessible for both the injection and the subsequent reading. The skin must be free of scars, lesions, tattoos, or visible veins that could interfere with the injection or result interpretation. The site is cleaned with an alcohol swab and allowed to air dry completely to prevent the alcohol from inactivating the PPD solution.
Technique for Administering the Injection
After preparing the site, the patient’s forearm should rest on a firm surface, palm up, with the elbow slightly flexed. The syringe, containing the precise 0.1 mL dose of PPD, is held almost parallel to the skin surface, and the needle’s bevel must face upward. The skin of the forearm is stretched taut with the non-dominant hand to stabilize the tissue for the injection.
The needle is inserted slowly into the most superficial layer of the skin at a very shallow angle, between 5 and 15 degrees. The insertion should be just deep enough so that the entire bevel is covered, but the needle tip remains visible through the epidermis, confirming the needle is placed intradermally. The 0.1 mL of PPD is injected slowly, and the provider should feel slight resistance as the solution enters the skin.
A successful injection is immediately confirmed by the formation of a distinct, pale, raised area called a wheal or bleb at the injection site. This wheal should measure approximately 6 to 10 millimeters (mm) in diameter. If no wheal forms, or if a significant amount of the PPD leaks out, the test must be repeated immediately at a separate site on the same arm, or on the opposite forearm, at least two inches away. After the injection, the needle is withdrawn, and the patient is instructed not to rub, scratch, or cover the site with a bandage, as this could disrupt the wheal and invalidate the test.
Proper Reading and Interpretation of Results
The reading of the TST must occur between 48 and 72 hours following the injection to ensure the immune response has peaked. The reaction is assessed by measuring the induration, which is the palpable, firm swelling caused by the localized immune response. The surrounding redness or erythema is not measured. The arm should be slightly flexed at the elbow and examined under good lighting.
To accurately measure the induration, the healthcare provider uses a gentle sweeping motion with their fingertips to locate the firm edges of the raised area. A ruler is then used to measure the diameter of the induration across the forearm, perpendicular to the long axis of the arm. The measurement is recorded in millimeters.
Interpretation depends on the measured induration and the patient’s specific risk factors for TB infection and progression to active disease. An induration of 5 mm or greater is considered a positive result for individuals at the highest risk. This includes those with HIV, recent close contacts of a person with active TB, or immunosuppressed patients, such as organ transplant recipients.
An induration of 10 mm or greater is considered positive for intermediate-risk groups. This threshold applies to:
- Recent immigrants from countries with a high prevalence of TB.
- Injection drug users.
- Residents or employees of high-risk congregate settings like correctional facilities or hospitals.
- Persons with clinical conditions that increase the risk of TB, such as diabetes or chronic renal failure.
For persons with none of the known risk factors for TB, an induration of 15 mm or greater is required for a positive result. A result that is less than the relevant threshold for a patient’s risk category is considered negative.