The Tuberculin Skin Test (TST), also known as the PPD test (Purified Protein Derivative), screens for exposure to the bacterium Mycobacterium tuberculosis, which causes tuberculosis. This test detects latent TB infection, where the bacteria are present but are not causing illness or symptoms. A small amount of tuberculin protein is injected just under the skin. This allows healthcare providers to see if the immune system recognizes the substance from a previous exposure.
Immediate Post-Injection Reactions
The goal of the TST is to deliver the tuberculin solution into the dermis, the superficial layer of the skin, using an intradermal injection technique. This layer contains the immune cells necessary for the reaction. Ideally, there should be minimal or no bleeding at the injection site immediately following the procedure.
If blood appears, it signals that the needle may have gone too deep, penetrating into the subcutaneous tissue. This deeper injection is a technical error that compromises accuracy. When the solution is deposited too deeply, the tuberculin antigens are not presented to the correct immune cells, which can lead to a false-negative result.
A minor drop of blood is sometimes seen and is not alarming, but significant bleeding or the absence of the expected skin reaction suggests improper placement. If the test is potentially compromised, the provider will document the incident. They may repeat the test immediately on the opposite arm or at a location at least two inches away from the original site.
Proper TST Administration Technique
The correct administration of the TST, using the Mantoux method, requires precision to ensure the solution is delivered to the correct skin layer. The procedure uses a small gauge needle, such as a 27-gauge needle, attached to a tuberculin syringe. The standard dose is a measured 0.1 mL of tuberculin purified protein derivative (PPD).
The injection is performed on the inner surface of the forearm, typically four inches below the elbow, in an area free of veins and scars. The needle is inserted slowly into the skin at a shallow angle, usually between 5 and 15 degrees, with the bevel facing upward. This shallow angle ensures the solution remains within the dermis.
The most important sign of a correctly placed injection is the immediate formation of a distinct, pale, and raised bubble-like area called a wheal or bleb. This wheal, typically 6 to 10 millimeters in diameter, confirms that the fluid was successfully deposited intradermally. If this wheal does not form, or if the solution leaks out, the test is considered invalid and must be repeated.
Reading and Interpreting the Results
Patients must return to the healthcare provider to have the test site assessed 48 to 72 hours after the injection. Reading the result involves a careful examination of the injection site under good lighting. The provider looks for a specific reaction indicating a delayed-type hypersensitivity response, which reflects the body’s immune memory of the bacteria.
The feature that determines the test result is induration, a firm, raised, and hardened area of swelling. Redness, or erythema, is irrelevant for interpreting the test. The provider uses a standardized ruler to measure the diameter of the induration across the forearm, perpendicular to the long axis.
The measurement is recorded in millimeters, and 0 mm means no reaction occurred. The threshold for a positive result varies based on the person’s risk factors for TB exposure or progression.
Positive Thresholds
An induration of 5 millimeters or greater is considered positive for individuals who are immunosuppressed, such as those with HIV, or who have recently had close contact with an active TB case.
A reaction of 10 millimeters or greater is considered positive for individuals with certain medical conditions, those recently arrived from a country with high TB prevalence, or people who work in high-risk congregate settings.
For individuals with no known risk factors, the induration must measure 15 millimeters or greater to be considered positive. A positive TST indicates infection with M. tuberculosis but does not distinguish between latent infection and active disease, requiring further evaluation like a chest X-ray.
Alternatives to the Tuberculin Skin Test
The TST has limitations, leading to the development of alternative testing methods. The primary alternative is a class of blood tests known as Interferon-Gamma Release Assays (IGRAs). These tests, which include products like the QuantiFERON-TB Gold Plus and T-SPOT.TB, measure the immune system’s response to specific TB-related antigens in a blood sample.
IGRAs offer a significant advantage because they are less likely to produce a false-positive result in individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine. The BCG vaccine, used widely outside the United States, can cause a false-positive TST because it shares similar antigens with the tuberculin solution. IGRAs are also preferred for patients who may not return for the TST reading, as they require only a single blood draw.
IGRAs provide more objective results, relying on laboratory measurements rather than the healthcare provider’s visual assessment and palpation of a skin reaction. If a TST or IGRA test is positive, subsequent steps involve a medical evaluation. This evaluation often includes a chest X-ray and a symptom assessment. These follow-up measures are necessary to rule out active TB disease and determine if treatment for latent infection is appropriate.