Understanding the Intradermal Injection
An intradermal (ID) injection is a specialized and shallow injection technique where a substance is administered directly into the dermis [1.2.2]. The dermis is the layer of skin situated between the epidermis (the outermost layer) and the hypodermis or subcutaneous tissue [1.2.2]. This method is distinct from more common injections like subcutaneous (into the fatty tissue) and intramuscular (into the muscle) because it targets a highly specific and relatively thin layer of skin [1.5.1]. The dermis is rich in blood vessels and immune cells, including dermal dendritic cells [1.2.1, 1.5.1]. Injecting into this layer allows for a slow absorption of the substance, which is a key reason for its use in specific medical applications. The body's reaction to the substance is also more easily visible on the skin's surface [1.4.4]. A successful intradermal injection is characterized by the formation of a small, raised, pale elevation of the skin called a "wheal" or "bleb," which confirms the medication has been delivered to the correct location [1.4.2, 1.10.1].
Primary Purpose and Common Uses
The primary advantage and purpose of intradermal injections lie in their slow absorption rate and the localized, visible immune response they can produce [1.4.2, 1.4.4]. This makes them exceptionally well-suited for diagnostic testing rather than systemic drug delivery.
- Tuberculosis (TB) Testing: The most well-known use is the Mantoux tuberculin skin test [1.4.4, 1.10.3]. A small, standardized amount (0.1 ml) of purified protein derivative (PPD) is injected into the forearm [1.8.1]. If a person has been exposed to TB bacteria, their immune system will recognize the protein, causing a localized hypersensitivity reaction (a firm, raised area called an induration) at the site within 48 to 72 hours [1.8.1, 1.8.2].
- Allergy Testing: ID injections are a highly sensitive method for allergy testing [1.7.2]. A small amount of a potential allergen is injected into the skin, often on the upper back or forearm [1.4.1, 1.7.4]. If an allergy is present, a red, swollen hive-like reaction will appear at the injection site, typically within about 20 minutes [1.7.2]. This method is often used when a standard skin prick test is negative but an allergy is still suspected [1.7.3].
- Vaccinations: While less common than intramuscular injections for vaccines, some are administered intradermally. This can include vaccines for monkeypox, rabies, and Bacillus Calmette-Guérin (BCG) for tuberculosis [1.4.4, 1.5.2]. Intradermal vaccination can sometimes produce a stronger immune response with a smaller dose of the vaccine, a concept known as dose-sparing [1.6.3, 1.6.4].
- Aesthetics and Dermatology: In cosmetic medicine, intradermal injections are used to deliver substances like hyaluronic acid or Natural Growth Factors (NGF) to rejuvenate skin, reduce fine lines, and improve texture by stimulating collagen production [1.2.1].
Step-by-Step Intradermal Injection Procedure
Administering an ID injection requires precision and is considered more technically challenging than other injection types [1.10.4]. The following steps outline the standard procedure, often called the Mantoux technique [1.2.2]:
- Preparation: First, gather supplies, including a tuberculin syringe (calibrated in tenths of a milliliter) with a small, short needle (typically 25- to 27-gauge and 1/4 to 1/2 inch long), alcohol swabs, and gloves [1.4.2, 1.9.2]. The medication dosage is very small, usually under 0.5 mL, and often around 0.1 mL for TB tests [1.9.2, 1.8.1].
- Site Selection and Cleaning: The most common site is the inner (ventral) surface of the forearm due to its thin skin and lack of hair [1.4.1, 1.4.3]. The upper back or under the shoulder blade are also used, especially for allergy testing [1.4.3]. The site must be free of lesions, scars, moles, or tattoos [1.4.2]. Clean the area with an alcohol swab and allow it to air dry completely [1.3.1, 1.3.5].
- Positioning and Skin Tautness: Position the patient's arm comfortably. With the non-dominant hand, use the thumb and forefinger to gently pull the skin at the injection site taut [1.3.2, 1.3.5].
- Needle Insertion: Hold the syringe in the dominant hand, almost parallel to the forearm, with the needle's bevel (the slanted opening) facing up. Insert the needle slowly at a very shallow 5 to 15-degree angle, advancing it only about 1/8 inch (3 mm) until the entire bevel is just under the skin [1.2.2, 1.3.4].
- Injecting the Medication: Slowly inject the medication. As you inject, a small wheal or bleb, about 6 to 10 mm in diameter, should form at the site [1.8.1, 1.10.3]. The appearance of this wheal is the key indicator of a successful intradermal injection [1.3.5].
- Withdrawal and Post-Injection Care: Withdraw the needle quickly at the same angle it was inserted [1.3.5]. Do not massage the site, as this can cause the medication to disperse into the underlying subcutaneous tissue and interfere with test results [1.3.4]. Do not apply a bandage over a TB test site. Safely dispose of the needle in a sharps container [1.3.5].
Comparison of Injection Types
Understanding the differences between injection routes highlights the unique role of the ID method.
| Feature | Intradermal (ID) | Subcutaneous (SC) | Intramuscular (IM) |
|---|---|---|---|
| Target Tissue | Dermis (just below epidermis) [1.5.1] | Adipose (fatty) tissue below the dermis [1.5.1] | Muscle tissue [1.5.1] |
| Angle of Insertion | 5 to 15 degrees [1.9.3] | 45 to 90 degrees [1.5.5] | 90 degrees [1.5.5] |
| Needle Gauge | 25–27 gauge [1.9.1] | 25–30 gauge [1.5.5] | 20–25 gauge [1.5.5] |
| Needle Length | 1/4 to 1/2 inch [1.9.1] | 3/8 to 5/8 inch [1.5.5] | 1 to 1.5 inches [1.5.5] |
| Max Volume | < 0.5 mL [1.9.2] | < 1 mL [1.5.5] | 1 mL (deltoid) to 3 mL (gluteal) [1.5.5] |
| Absorption Rate | Slowest [1.4.2] | Slow | Fast [1.5.3] |
| Common Uses | TB & allergy tests, some vaccines [1.4.4] | Insulin, certain hormones, blood thinners [1.5.2] | Most vaccines, antibiotics, steroids [1.5.2, 1.5.3] |
Conclusion
In summary, an intradermal injection is a highly precise medical procedure used to introduce small amounts of a substance into the dermal layer of the skin. Its utility is defined by slow absorption and the ability to create a visible, localized reaction, making it an invaluable tool for diagnostic purposes like tuberculosis and allergy testing. While it requires more skill to perform correctly than other injection types, its effectiveness in specific diagnostic and therapeutic scenarios makes it a crucial technique in modern medicine.
For more information on the administration of vaccines, you can visit the Centers for Disease Control and Prevention (CDC).
