An intradermal injection is a shallow delivery of a substance into the dermis, the layer of tissue situated just below the epidermis. This method is distinct because it targets this specific layer of skin, where the small volume of fluid is absorbed very slowly. This characteristic makes the technique useful for specific medical purposes.
Common Uses for Intradermal Injections
The slow absorption rate of intradermal injections is useful for diagnostic testing where a localized and visible reaction is necessary. They are the standard method for administering tuberculin (TB) skin tests, also known as the Mantoux test. When a small amount of tuberculin is injected into the dermis, any potential immune response is contained within that area, making it easy to observe and measure.
Allergy sensitivity tests also rely on this injection method. Small amounts of various potential allergens are injected into the skin to see if a localized reaction like redness or swelling occurs. The slow diffusion allows clinicians to identify which substances trigger a response. Some vaccines and local anesthetics are also administered this way.
The Intradermal Injection Procedure
A healthcare provider will select a suitable injection site, most commonly the inner surface of the forearm due to its thin skin and easy accessibility. Other potential sites include the upper back, just under the shoulder blade. The chosen area must be free of any rashes, moles, scars, or sores that could interfere with the injection or the reading of a subsequent reaction.
After selecting the site, the provider will clean the skin with an antiseptic swab and allow it to air dry. They will then stretch the skin taut and insert a very fine, short needle at a shallow angle, between 5 and 15 degrees relative to the skin’s surface. As the liquid is injected, a pale, elevated bump, known as a bleb or wheal, will form at the site. The appearance of this bleb is a visual confirmation that the injection was administered correctly into the dermal layer.
Comparison to Other Injection Types
The primary difference between injection types is the depth of the injection and the resulting absorption speed. Intradermal injections target the dermis just below the skin’s surface, while subcutaneous injections are administered into the fatty tissue layer just beneath the dermis. Intramuscular injections go deeper, delivering medication directly into a muscle.
This difference in depth directly affects how quickly the body absorbs the substance. Intradermal injections have the slowest absorption time of all parenteral routes. Subcutaneous injections are absorbed more quickly than intradermal ones but slower than intramuscular injections. The volume of fluid administered intradermally is the smallest, usually less than 0.5 milliliters.
Post-Injection Care and Reactions
The small bleb that forms during the injection will fade on its own. Mild itching or redness at the injection site can be a normal reaction. To ensure proper results and healing, you should:
- Avoid rubbing, scratching, or applying pressure to the area.
- Avoid covering the site with a tight bandage.
- Monitor the area for signs of infection, such as increasing pain, swelling, warmth, or pus.
- Contact a healthcare provider if signs of infection or a severe allergic reaction develop.
For TB tests, the site will be circled and must be evaluated by a professional within three days.