An intradermal injection is a shallow, precise method of administering a substance directly into the dermis layer of the skin. Unlike injections that target muscle or fat, this technique deposits medication where a localized reaction or a very slow rate of absorption is desired. The small volume of fluid used makes this route distinct from subcutaneous or intramuscular injections.
Anatomy and Target Layer
The skin has three main layers: the outer epidermis, the dermis beneath it, and the hypodermis (subcutaneous fat). The dermis is the specific target for an intradermal injection, sitting just below the epidermis. This layer is rich in blood vessels and contains a high concentration of immune cells, such as dermal dendritic cells and Langerhans cells.
Targeting the dermis is intentional because these immune cells make it highly responsive, often leading to a stronger localized immune response for certain vaccines. The relative lack of blood vessels compared to muscle tissue contributes to the slowest absorption rate of all parenteral routes. This slow absorption allows the body’s reaction to be easily visualized and assessed on the skin’s surface.
Specific Medical Applications
Intradermal injections are primarily employed for diagnostic and sensitivity testing requiring a localized, visible reaction. The most common use is the Tuberculin Skin Test (TST), or Mantoux test, which screens for exposure to tuberculosis. A small amount of purified protein derivative (PPD) is injected to assess the body’s cell-mediated immune response.
Allergy testing is another frequent application, where tiny amounts of potential allergens are introduced to observe for a localized hypersensitivity reaction. This helps identify specific substances that trigger an allergic response. The route is also used for certain vaccinations, such as the Bacillus Calmette-Guérin (BCG) vaccine, and for administering local anesthetics.
Technique and Procedure Details
Administering an intradermal injection requires a shallow angle and specific handling to ensure correct delivery into the dermis. A fine, short needle (typically 26 or 27 gauge) is used, and the dosage volume is usually very small, often less than 0.5 mL. The injection is administered at a shallow angle, generally between 5 and 15 degrees, keeping the needle almost flat against the skin.
The needle must be inserted with the bevel (the slanted opening at the tip) facing upward. This orientation facilitates smooth entry and proper placement just beneath the epidermis. The needle is inserted only far enough to cover the entire bevel, penetrating the skin by about 3 to 6.4 millimeters.
As the solution is injected slowly, the confirmation of correct technique is the appearance of a pale, raised bubble on the skin surface, known as a wheal or bleb. This distinct elevation indicates successful deposition into the dermal layer. If a wheal does not form, the injection may have been placed too deeply into the subcutaneous tissue, compromising the intended effect.
Post-Injection Monitoring and Expectations
After the wheal has formed, the needle is withdrawn gently at the same shallow angle it was inserted. Patients are advised not to rub or massage the injection site, as this action can push the substance out of the dermis into the deeper subcutaneous tissue. Moving the fluid interferes with the localized reaction necessary for diagnostic purposes.
For diagnostic tests, the wheal fades shortly after injection, but the site must be monitored for a specified period, typically 48 to 72 hours. A healthcare professional examines the site to measure any hardened, raised area, called induration, which indicates the test results. Slight redness or a small bruise is a common and expected reaction that resolves on its own.