Understanding the Intradermal Injection
An intradermal (ID) injection is a specialized method for administering substances directly into the dermis, the layer of skin situated just beneath the epidermis [1.4.5, 1.5.6]. This route is distinct from other common injection types, such as subcutaneous (into the fatty tissue) and intramuscular (into the muscle) [1.4.1]. The key to a successful intradermal injection lies in its precision, specifically the angle of needle insertion. The correct degree of insertion for an intradermal injection is a very shallow 5 to 15 degrees, almost parallel to the skin [1.2.2, 1.2.5, 1.3.3]. This precise angle ensures the medication is delivered to the correct location for its intended purpose.
The Anatomy Behind the Angle
The skin consists of three main layers: the epidermis (outermost layer), the dermis (middle layer), and the hypodermis or subcutaneous tissue (deepest layer) [1.5.2]. The dermis is only a few millimeters thick and contains a rich network of capillaries, nerve endings, and immune cells called antigen-presenting cells [1.4.1, 1.5.3]. The 5 to 15-degree angle is critical because it allows the needle to penetrate the epidermis and deposit the medication directly into the highly vascular and immunologically active dermis [1.5.2, 1.8.2]. If the angle is too steep, the needle may enter the subcutaneous tissue, leading to incorrect absorption and potentially inaccurate test results [1.2.4].
Step-by-Step Intradermal Injection Procedure
Administering an ID injection requires careful attention to detail to ensure accuracy and patient safety.
1. Preparation:
- Verify Orders: Confirm the medication, dose, route, and patient identity [1.3.5].
- Gather Supplies: You will need a tuberculin syringe (1 mL), a small gauge needle (typically 25 to 27 gauge) that is 1/4 to 1/2 inch long, alcohol swabs, gauze, and non-sterile gloves [1.5.4, 1.9.1, 1.9.2].
- Select and Clean the Site: The most common sites are the inner surface of the forearm and the upper back, under the scapula [1.5.4, 1.7.2]. The site should be free of hair, moles, scars, tattoos, or lesions [1.3.1, 1.7.2]. Clean the area with an alcohol swab in a circular motion and allow it to air dry completely [1.3.4].
2. Administration:
- Position the Needle: Hold the syringe in your dominant hand, almost parallel to the patient's skin, at a 5 to 15-degree angle [1.2.1, 1.2.5]. Ensure the needle's bevel (the slanted opening) is facing up [1.2.3, 1.2.4].
- Taut the Skin: Use your non-dominant hand to gently pull the skin taut at the injection site [1.2.3, 1.3.1].
- Insert the Needle: Slowly advance the needle only about 1/8 inch (or 3 millimeters) into the skin, just until the bevel is fully covered [1.2.2, 1.3.3]. You should feel resistance [1.3.3].
- Inject the Medication: Administer the medication slowly. A successful ID injection will create a small, distinct, raised bump on the skin that resembles a mosquito bite. This is known as a wheal or bleb [1.2.4, 1.8.1, 1.8.4]. The formation of this bleb is visual confirmation that the medication is in the dermis [1.2.4].
3. Post-Injection:
- Withdraw and Dispose: Remove the needle at the same shallow angle it was inserted [1.2.4, 1.3.3]. Engage the safety device and immediately dispose of the syringe in a sharps container [1.2.5].
- Do Not Massage: It is critical not to rub or massage the site, as this can disperse the medication into the subcutaneous tissue and interfere with absorption or test results [1.2.4, 1.8.1].
- Document: Record the procedure, including the time, date, location, medication administered, and the appearance of the wheal [1.3.5]. If administering a TB test, you may circle the site with a pen for easy identification later [1.8.4].
Common Applications
The intradermal route has the longest absorption time of all parenteral routes, which makes it ideal for specific applications [1.5.6]:
- Tuberculosis (TB) Screening: The Mantoux tuberculin skin test (TST) is the most common use, where the reaction to the injected tuberculin is assessed after 48-72 hours [1.5.1, 1.5.6].
- Allergy Testing: Small amounts of various allergens are injected into the dermis to observe for a localized hypersensitivity reaction, indicated by a red, raised wheal [1.3.4, 1.5.1].
- Vaccinations: Some vaccines, like certain flu and rabies vaccines, can be administered intradermally, which may offer an enhanced immune response with a smaller dose [1.5.1, 1.5.3].
- Aesthetics: In cosmetic dermatology, intradermal injections are used to administer substances like Botox or natural growth factors to improve skin texture and reduce fine lines [1.5.2, 1.6.2].
Comparison of Injection Angles
The angle of insertion is a primary differentiator between injection types.
| Feature | Intradermal (ID) | Subcutaneous (SC) | Intramuscular (IM) |
|---|---|---|---|
| Angle of Insertion | 5–15 degrees [1.2.6] | 45–90 degrees [1.2.2] | 90 degrees [1.2.6] |
| Injection Site | Dermis [1.4.1] | Subcutaneous (fatty) tissue [1.4.2] | Muscle tissue [1.4.1] |
| Needle Gauge | 25–27 gauge [1.9.2] | 25–27 gauge [1.2.2] | 20–25 gauge [1.4.5] |
| Needle Length | 1/4 to 1/2 inch [1.9.2] | 3/8 to 5/8 inch [1.2.2] | 1/2 to 1.5 inches [1.4.5] |
| Absorption Rate | Slowest [1.5.6] | Slower than IM [1.4.2] | Fastest of the three [1.4.3] |
| Common Uses | TB tests, allergy tests [1.5.1] | Insulin, anticoagulants [1.2.2] | Vaccines, antibiotics [1.4.5] |
Potential Complications
While generally safe when performed correctly, intradermal injections can have localized side effects, including redness, itching, swelling, firmness, or pain at the injection site [1.6.1, 1.6.4]. In rare cases, complications such as bleeding, blistering, or scarring can occur [1.6.1]. During allergy testing, there is a risk of a systemic allergic reaction (anaphylaxis), which requires immediate medical attention [1.3.4].
Conclusion
Mastering the question, 'What degree of insertion is intradermal?' is fundamental for any healthcare professional. The precise 5 to 15-degree angle is not arbitrary; it is dictated by the anatomy of the skin and the pharmacological goal of depositing a substance into the dermis for slow absorption and a visible, localized reaction. This technique remains an indispensable tool in diagnostics for tuberculosis and allergies, underscoring the importance of procedural accuracy in patient care.
For authoritative guidelines on vaccination procedures, please refer to the Centers for Disease Control and Prevention (CDC).
