Intradermal injections are a specific method of delivering substances into the skin, distinct from other injection types like intramuscular or subcutaneous. This technique involves targeting a very shallow layer of the skin, making it suitable for particular medical applications.
Characteristics of Intradermal Injections
Intradermal injections are characterized by their delivery into the dermis, the layer of skin located just beneath the epidermis. This method involves administering a small volume of medication, typically ranging from 0.01 to 0.1 milliliters, using a specialized syringe and needle.
The injection is performed at a shallow angle, usually between 5 to 15 degrees, with the needle almost flat against the skin and the bevel (the slanted opening of the needle) facing upwards. The needle is inserted only about 1/4 inch, with the entire bevel covered by the skin.
A telltale sign of a correctly administered intradermal injection is the formation of a small, raised bump on the skin’s surface, often called a “wheal” or “bleb.” The wheal typically measures 6 to 10 mm in diameter when properly formed.
The dermis has a limited blood supply compared to deeper tissues. This characteristic leads to a slower absorption rate of the injected substance, making intradermal injections suitable for applications where a gradual release or localized reaction is desired. Common sites for these injections include the inner forearm and the upper back.
Primary Applications
Intradermal injections are primarily used for diagnostic tests and specific types of vaccinations that require localized and slow absorption of a substance. The dermis’s unique properties, including its slower absorption, make it suitable for observing reactions or eliciting a targeted immune response.
One common application is the tuberculin skin test, also known as the Mantoux test, used for tuberculosis (TB) screening. In this test, a small amount of tuberculin purified protein derivative (PPD) is injected into the forearm. The presence and size of a localized reaction (induration) are then measured after 48 to 72 hours to determine if a person has been exposed to TB bacteria.
Allergy skin tests also frequently utilize the intradermal route. Small amounts of suspected allergens are injected just beneath the skin’s surface to observe for a localized allergic reaction, such as redness or swelling. This method allows for a more sensitive and accurate assessment of allergies, particularly for certain substances like insect venom or penicillin, or when initial skin prick tests are inconclusive.
Another application is the administration of certain vaccines, such as the Bacillus Calmette-Guérin (BCG) vaccine for tuberculosis. The BCG vaccine is administered into the skin, often in the deltoid region of the arm. This method can lead to an improved immune response for some vaccines and potentially allows for dose sparing. The slow absorption in the dermis helps facilitate a prolonged immune response.