An intradermal (ID) injection is a shallow procedure that delivers a small volume of a substance directly into the layers of the skin. This specific method of administration is different from common injections that place medication into the muscle or fat beneath the skin. The technique is used primarily for diagnostic purposes, allowing medical professionals to observe a localized reaction to a substance.
Anatomy and Mechanism of Intradermal Delivery
The target for an ID injection is the dermis, which is the layer of skin situated just beneath the thin, outermost epidermis. The dermis is rich in blood vessels, lymphatic vessels, and various immune cells, including dendritic cells and macrophages. The presence of these immune cells makes this layer ideal for sensitivity testing, as they can rapidly process and react to foreign substances introduced at this site.
When a small amount of liquid is correctly injected into this layer, the substance is absorbed very slowly compared to other injection routes. This slow, localized absorption is a desired feature, as it confines the reaction to a specific area and prevents a rapid, systemic response throughout the body. The most distinctive feature indicating a successful intradermal injection is the immediate formation of a raised, pale bump on the skin, often called a wheal or bleb.
The Specific Technique for Administration
Intradermal injections require a highly controlled technique to ensure the solution is delivered to the correct, shallow depth. Healthcare providers use specialized equipment, typically a tuberculin syringe and a short, fine needle, often 25- to 27-gauge and between 1/4 to 5/8 inch in length. The small size of the needle helps to precisely penetrate the skin without going too deep.
The injection itself is performed at a very shallow angle, usually between 5 and 15 degrees, with the needle almost parallel to the skin’s surface. Before insertion, the skin at the injection site, commonly the inner forearm, is held taut to stabilize the layers. The needle is inserted only far enough for the entire bevel, or slanted tip, to be covered beneath the skin.
Once the needle is correctly positioned within the dermis, the solution is slowly injected, and the wheal should immediately begin to form. The appearance of this characteristic bump serves as a visual confirmation that the substance has been delivered to the intended site. It is important not to massage or rub the injection site after the needle is withdrawn, as this action could disperse the substance, leading to inaccurate test results.
Primary Diagnostic Applications
The most frequent medical use for an intradermal injection is for specific types of diagnostic testing that rely on observing a localized immune response. This method is the standard for the Tuberculin Skin Test (TST), often called the Mantoux test, which screens for exposure to Mycobacterium tuberculosis. A small amount of purified protein derivative (PPD) is injected, and the patient returns days later for the healthcare provider to measure the size of the localized swelling, which indicates a reaction.
Allergy testing procedures also heavily rely on the intradermal method to determine sensitivity to various allergens, such as pollens, foods, or medications like penicillin. The slow absorption rate is particularly beneficial in allergy testing because it allows for an accurate, localized observation of the body’s reaction without triggering a severe, rapid systemic allergic response. Typically, the inner surface of the forearm or the upper back is chosen as the injection site due to the thinness of the skin and ease of observation. The resulting reaction provides the necessary information for diagnosis.