The Tuberculin Skin Test (TST), often called the Mantoux test, is a common screening method used to determine if a person has been exposed to the bacteria that causes tuberculosis (TB). This test involves injecting a small amount of purified protein derivative (PPD) into the top layer of skin on the inner forearm. The goal is to trigger a localized immune response if TB exposure has occurred. While the injection is shallow and quick, it is normal to experience minor reactions at the site, such as mild discomfort, slight redness, or a temporary wheal, which is a pale, raised bump that appears immediately after the injection.
Why Bruising Occurs After a TST
Bruising following a TST is a benign side effect related to the mechanical act of the injection. The TST requires an intradermal injection, meaning the needle must be inserted just below the epidermis. This technique deposits the PPD solution into the dermal layer to create a small, visible bleb or wheal.
Despite the healthcare provider’s skill, the skin’s surface is rich with superficial capillaries. The fine needle may inadvertently nick one of these delicate blood vessels during insertion or withdrawal. When this happens, a small amount of blood leaks into the surrounding tissue, resulting in the discoloration known as a bruise, or hematoma.
Factors like individual skin fragility, the slight angle of the needle, or minor patient movement can increase the likelihood of capillary damage. The pressure required to inject the PPD solution can also sometimes cause minimal bleeding under the skin. Bruising is a recognized, temporary reaction that typically fades within a few days and has no connection to the test’s result.
Distinguishing Bruising from a Positive Reaction
Bruising and a positive TST result are entirely separate phenomena. Bruising is discoloration from blood, which is a mechanical issue, while a positive reaction is an immunological event called induration. Induration is the diagnostic finding a healthcare provider measures to interpret the test result.
Induration is a palpable, raised, and hardened area at the injection site, caused by an immune response where specialized white blood cells gather in the skin. This firm swelling signifies that the body’s immune system has recognized the TB protein, suggesting a past exposure to the bacterium. A positive TST is determined exclusively by measuring the diameter of this elevated firmness in millimeters, not by the extent of any surrounding redness or bruising.
A healthcare worker must feel the site between 48 and 72 hours after the injection to accurately measure the induration. This measurement is the only factor considered for the result. A person can have a significant bruise and a negative test, or no bruise and a positive test, because the two are unrelated.
Aftercare and Signs of Potential Issues
Aftercare focuses on protecting the injection site to ensure an accurate reading. It is important not to scratch or rub the area, as irritating the site can interfere with the measurement of induration. You should avoid covering the site with any type of bandage, lotion, or cream, and you can gently wash the area with water without scrubbing.
If the site is itchy, applying a cold compress can provide relief. Gentle cold application can also help minimize the discoloration and swelling of a bruise. You should contact a healthcare provider immediately if you notice signs of a severe reaction or infection. These signs include pain that worsens over time, pus draining from the site, spreading redness and warmth that extends well beyond the injection area, or the development of a fever.