The Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, screens for Mycobacterium tuberculosis infection by injecting a small amount of purified protein derivative (PPD) under the skin, usually on the forearm. The intended result is a delayed-type hypersensitivity reaction, which indicates prior exposure to the bacteria. While a mild, localized reaction is expected, some individuals may experience a stronger, potentially allergic response to the PPD solution itself.
Differentiating Expected Responses from Allergic Reactions
A normal, positive TST reaction is characterized by a firm, raised swelling called induration, peaking between 48 and 72 hours after the injection. Healthcare providers measure the diameter of this hardened area, not the surrounding redness, to determine the test result. This reaction is a function of the immune system’s T-cells recognizing the injected protein.
An allergic reaction presents differently in timing and severity. Mild allergic symptoms may appear quickly, sometimes within hours, involving intense itching, excessive redness, or hives confined to the injection area. Severe allergic responses can include blistering, ulceration, or necrosis at the injection site, or symptoms extending beyond the arm. The presence of a rash that spreads widely or feeling generally unwell suggests a systemic issue.
Managing Localized Symptoms with Home Care
For mild allergic symptoms localized to the forearm, simple home care measures provide relief. The primary goal is to minimize irritation and reduce swelling without interfering with the test site’s evaluation, if possible. Applying a cold compress or a washcloth wrapped around an ice cube helps soothe intense itching and alleviate localized swelling.
Avoid scratching the area, as this can cause further irritation and potentially lead to infection. Over-the-counter topical treatments, such as hydrocortisone cream, may be gently applied to reduce inflammation and redness. Oral antihistamines, like diphenhydramine or cetirizine, can manage itching and minor hive formation. Keep the site clean and dry, and avoid covering it with an adhesive bandage, which can worsen local skin reactions.
Identifying Severe Reactions and Professional Treatment
A severe allergic reaction requires immediate professional medical attention, as it may signal anaphylaxis. Symptoms of anaphylaxis include difficulty breathing, wheezing, swelling of the throat or tongue, dizziness, or a rapid, widespread rash (urticaria). Any sign of a severe systemic reaction necessitates calling emergency services or going to the nearest emergency department.
For severe local reactions, such as blistering, necrosis, or ulceration, contact the administering clinic immediately. Medical professionals manage these reactions with specific interventions. Treatment for anaphylaxis involves an immediate injection of epinephrine, which rapidly counteracts the reaction.
For less immediate but severe symptoms, a healthcare provider may administer systemic corticosteroids to suppress the widespread immune response or prescribe stronger antihistamines. These treatments manage the body’s exaggerated reaction to the PPD solution. A documented severe reaction to the TST prevents future use of the test, as the risk of a repeat reaction is too high.
Alternatives for Future TB Screening
Individuals who have experienced a significant allergic reaction to the TST should not use this method again. Alternatives are available for future tuberculosis screening, primarily Interferon Gamma Release Assays (IGRAs).
The two most common IGRAs are the QuantiFERON-TB Gold Plus (QFT-Plus) and the T-SPOT.TB test. These are blood tests, not skin tests, which means they are not affected by a prior allergic reaction to the PPD solution. The IGRA measures the release of interferon-gamma from immune cells exposed to specific TB antigens in a laboratory setting. Since the test is performed outside the body, it bypasses the risk of a repeat skin reaction and provides reliable screening for M. tuberculosis infection.