Understanding Intradermal Injections
An intradermal (ID) skin test is a diagnostic procedure used to determine a patient's sensitivity to specific substances [1.2.7]. It involves injecting a small amount of a substance, such as an allergen or tuberculin, into the dermis—the layer of skin just beneath the epidermis [1.7.1]. This route has the longest absorption time of all parenteral methods, which makes it ideal for sensitivity tests where the reaction needs to be easily visualized and assessed on the skin [1.2.7]. Common applications include testing for tuberculosis (the Mantoux test), diagnosing allergies to environmental factors, drugs, or insect venom, and checking for sensitivity to local anesthetics [1.2.2, 1.4.1].
Unlike food allergy testing, which typically uses skin prick tests, intradermal tests are considered more sensitive and are often used when a prick test yields a negative or inconclusive result, but an allergy is still suspected [1.3.1, 1.4.1]. The procedure must be performed by trained medical professionals, such as allergists, dermatologists, or nurses, who can correctly administer the test and manage any potential adverse reactions [1.5.1, 1.5.2, 1.5.5].
The Critical Angle: What Degree Is an Intradermal Skin Test?
The precision of an intradermal injection is paramount for accurate diagnostic results, and the most critical factor is the angle of needle insertion. An intradermal skin test must be administered at a 5 to 15-degree angle relative to the skin's surface [1.2.1, 1.2.2]. The needle should be held almost flat against the patient's arm or back, with the bevel facing upward [1.2.6].
This shallow angle ensures the needle penetrates the epidermis and deposits the testing substance directly into the dermis [1.2.4]. If the angle is too steep, the substance may be injected into the subcutaneous tissue, leading to an inaccurate result and preventing the formation of a proper wheal [1.2.7]. The successful administration of the test is marked by the appearance of a small, pale, raised bump called a "bleb" or "wheal" at the injection site, which indicates the fluid is in the correct skin layer [1.2.1, 1.2.6].
Step-by-Step Intradermal Test Procedure
A standardized procedure ensures safety and accuracy:
- Preparation: The healthcare provider gathers supplies, including a tuberculin syringe with a small-gauge needle (26 or 27 gauge), the test substance, alcohol swabs, and gloves [1.2.2, 1.2.7].
- Site Selection: A suitable injection site is chosen, typically the inner surface of the forearm, which is free of hair, scars, rashes, or moles [1.2.2, 1.2.7]. The upper back can also be used [1.2.7].
- Cleaning: The selected area is cleaned with an alcohol swab and allowed to air dry completely to prevent irritation [1.2.2].
- Injection: The skin is pulled taut [1.2.1]. The needle is inserted, bevel up, at a 5 to 15-degree angle until the bevel is just covered by the epidermis [1.2.2, 1.2.5].
- Administration: The substance is injected slowly. A small bleb or wheal, resembling a mosquito bite, should form [1.2.6].
- Withdrawal: The needle is removed at the same angle it was inserted [1.2.6]. The area should not be massaged, as this can disperse the substance into the underlying tissue [1.2.4].
- Observation: The patient is observed for 15-20 minutes for any immediate adverse reactions [1.2.1, 1.3.4]. For TB tests, the site is often circled, and the patient must return in 48-72 hours for a reading [1.2.3].
Interpreting the Results: The Wheal and Flare
For allergy tests, a positive result is typically indicated by the formation of a wheal (a raised, swollen bump) and a flare (surrounding redness) within 15 to 20 minutes [1.3.4, 1.6.4]. The size of the wheal is measured; a wheal that is at least 3-5 mm larger than the negative control is generally considered positive [1.6.1, 1.6.2]. A larger wheal often suggests a higher sensitivity to the allergen [1.6.5]. For a TB test, the result is based on the size of the induration (the hard, dense, raised area), not just the redness, measured 48 to 72 hours after the injection [1.2.3].
Comparison of Injection Types
Intradermal injections differ significantly from subcutaneous and intramuscular injections in their purpose, angle, and absorption rate.
| Feature | Intradermal (ID) | Subcutaneous (SC) | Intramuscular (IM) |
|---|---|---|---|
| Injection Angle | 5-15 degrees [1.2.2] | 45-90 degrees [1.2.2] | 90 degrees [1.7.6] |
| Target Tissue | Dermis [1.7.1] | Fatty Tissue (Subcutis) [1.7.2] | Muscle [1.7.1] |
| Absorption Rate | Slowest [1.2.7] | Slower than IM [1.7.2] | Fastest of the three [1.7.3] |
| Common Uses | Allergy tests, TB screening, sensitivity tests [1.2.2, 1.2.7] | Insulin, some hormones, blood thinners [1.7.2] | Vaccines, antibiotics, hormones [1.7.2] |
| Needle Gauge | 25-27 gauge [1.2.2] | 25-30 gauge [1.7.5] | 20-25 gauge [1.7.5] |
Conclusion
Mastering the intradermal skin test procedure is a fundamental clinical skill, with the central question being, "What degree is an intradermal skin test administered?" The answer—a precise 5 to 15-degree angle—is critical for depositing the test agent into the dermis and achieving a reliable diagnostic outcome. This technique remains a cornerstone for identifying tuberculosis and various allergies, guiding patient care through accurate and visualized results. The distinct wheal that forms is a clear indicator of a properly executed test, underscoring the importance of technique in modern pharmacology and diagnostics.
For more information on the JYNNEOS vaccine administered intradermally, you can visit the CDC's official guidance page. [1.2.1]
