Intradermal Testing

Intradermal testing injects a small volume of allergen into the dermis, usually 0.02 to 0.05 mL. It is more sensitive than skin prick testing for some allergens, including penicillin, insect venoms and inhalant allergens, but it produces more false positive results.

It is used when a skin prick test is negative but the history strongly suggests allergy, and when an allergen cannot be tested by prick. It is performed by specialists in drug allergy, venom allergy and selected aeroallergen assessment.

Indications. Penicillin allergy evaluation, insect venom allergy, selected drug allergy, and aeroallergen confirmation when prick testing is negative.

Preparation and technique. The injection is made at a shallow angle into the dermis, raising a wheal of 3 to 5 mm. Each allergen is injected at a separate site, with a negative control and a positive control. Penicillin skin testing uses benzylpenicilloyl polylysine (the major determinant) and a minor determinant mix. A negative result is interpreted only where both are used, in line with local practice.

Contraindications and precautions. Severe uncontrolled asthma, previous severe anaphylaxis to the allergen where specialist opinion advises against testing, beta blocker use, and extensive dermatitis at the injection site. Testing is generally deferred in pregnancy unless the result would change management.

Interpretation.

• Readings are taken at 15 to 20 minutes.
• A positive result is a wheal at least 3 mm greater than the negative control, with surrounding erythema.
• Delayed reactions to some drugs require separate reading at a later time.

Causes of a false positive result. Dermographism, higher allergen concentrations, and non-specific irritation at the injection site.

Critical values and findings needing immediate action.

• Systemic reactions and anaphylaxis can occur. Resuscitation equipment and adrenaline must be immediately available.
• The patient is observed after testing, and any early systemic sign requires immediate treatment.

Limitations and artefacts. False positive reactions are more common than with prick testing, and a positive intradermal result still requires clinical correlation before avoidance advice. A negative intradermal test does not exclude allergy in every setting.

Follow up tests. Oral challenge or drug provocation testing, depending on the allergen, confirms or excludes allergy after a positive or discordant result.

Special groups. Children require additional caution with dosing and observation. Testing in infants is performed only by specialists. Older adults with cardiovascular disease need caution, because a systemic reaction and its treatment carry added risk.