Intradermal Skin Test Procedure

The intradermal skin test is a specialized diagnostic procedure used to identify hypersensitivity reactions, particularly in cases of suspected allergies or certain infectious diseases. Unlike scratch or prick tests, which are performed on the surface of the skin, the intradermal test involves injecting a small amount of allergen or antigen just beneath the skin’s surface. This method is highly sensitive and is often used when a more definitive result is needed. Below is a comprehensive guide to the intradermal skin test procedure, its applications, and considerations.

What is an Intradermal Skin Test?

The intradermal skin test is a diagnostic tool that evaluates delayed or immediate hypersensitivity reactions by introducing a small quantity of a specific substance (allergen or antigen) into the dermal layer of the skin. It is commonly used in allergy testing, tuberculosis (TB) screening (e.g., the Mantoux test), and other immunological assessments.

Procedure Overview

The intradermal skin test is performed in a controlled clinical setting by trained healthcare professionals. Here’s a step-by-step breakdown of the procedure:

1. Preparation

• Patient Assessment: The healthcare provider reviews the patient’s medical history, including known allergies, medications, and previous reactions to skin tests.
• Informed Consent: The patient is informed about the procedure, potential risks, and benefits.
• Supplies: Sterile syringes, fine-gauge needles (typically 26–27 gauge), the test antigen/allergen solution, and a measuring ruler are prepared.

2. Skin Selection

• The volar surface of the forearm is the most common site for the test, as it provides a flat, accessible area with minimal hair and visible reactions.
• The skin is cleaned with alcohol or antiseptic wipes to reduce the risk of infection.

3. Injection Technique

• Needle Insertion: The needle is inserted at a 5–15 degree angle into the dermal layer of the skin. The bevel of the needle should face upward to ensure proper placement.
• Injection: A small volume (0.02–0.1 mL) of the test solution is slowly injected, creating a wheal (a small, raised bump) in the skin. If the wheal is not visible, the injection may need to be repeated.
• Control Injection: A control solution (e.g., saline or glycerin) is often injected nearby to differentiate between a true reaction and nonspecific skin irritation.

4. Observation

• Immediate Reactions: For allergens, the skin is observed for 15–20 minutes for signs of an immediate hypersensitivity reaction, such as redness, swelling, or itching.
• Delayed Reactions: For tests like the Mantoux TB test, the skin is examined after 48–72 hours for induration (firm swelling) or erythema (redness).

5. Measurement and Interpretation

• Immediate Reactions: The diameter of the wheal and surrounding erythema is measured and compared to the control site.
• Delayed Reactions: The size of the induration is measured using a ruler, and results are interpreted based on established criteria (e.g., for the Mantoux test, induration ≥ 10 mm is considered positive in high-risk individuals).

Applications of Intradermal Skin Tests

1. Allergy Testing: Identifies hypersensitivity to allergens such as pollen, insect venoms, or medications.
2. Tuberculosis Screening: The Mantoux test uses an intradermal injection of tuberculin purified protein derivative (PPD) to detect latent TB infection.
3. Vaccine Testing: Assesses immune responses to vaccines, such as the DPT (diphtheria, pertussis, tetanus) vaccine.
4. Autoimmune Disease Diagnosis: Used in some cases to evaluate immune responses in conditions like sarcoidosis.

Advantages and Limitations

Advantages

• High Sensitivity: Detects even mild hypersensitivity reactions.
• Small Volume Required: Uses minimal amounts of the test substance, reducing the risk of systemic reactions.
• Versatility: Applicable to a wide range of antigens and allergens.

Limitations

• False Positives/Negatives: Results can be influenced by factors like medications, skin conditions, or technical errors.
• Risk of Reactions: Although rare, systemic allergic reactions (e.g., anaphylaxis) can occur.
• Skill-Dependent: Requires precise technique for accurate results.

Potential Risks and Complications

• Local Reactions: Redness, swelling, or itching at the injection site.
• Systemic Reactions: Rarely, severe allergic reactions (e.g., anaphylaxis) may occur, especially with allergen testing.
• Infection: Risk of skin infection if sterile technique is not followed.

Patient Considerations

• Medications: Certain drugs (e.g., antihistamines, corticosteroids) can suppress skin test reactions and should be discontinued before testing, if possible.
• Skin Conditions: Eczema, psoriasis, or other skin disorders may interfere with test results.
• Pregnancy: Intradermal tests are generally safe during pregnancy but should be performed with caution.

FAQ Section

Conclusion

The intradermal skin test is a valuable diagnostic tool for identifying hypersensitivity reactions and immune responses. Its precision and sensitivity make it indispensable in allergy testing, tuberculosis screening, and other immunological assessments. However, the procedure requires skilled execution and careful interpretation of results to ensure accuracy and patient safety. As with any medical test, it should be performed by trained professionals in a controlled environment.