What additional testing is recommended for patients presenting with skin sensitivity?

Additional Testing for Skin Sensitivity

Offer patch testing with a baseline allergen series to all patients with suspected contact dermatitis, as this is the gold standard diagnostic test that will identify approximately 80% of relevant allergens. 1

Primary Diagnostic Approach: Patch Testing

When to Perform Patch Testing

• Offer patch testing for patients with chronic or persistent dermatitis, as clinical features alone are unreliable in distinguishing allergic contact dermatitis from irritant and endogenous dermatitis, particularly with hand and facial eczema 1
• Patch testing should be performed when contact allergy is suspected or cannot be ruled out in patients with persistent eczematous eruptions 1
• The test has sensitivity and specificity of 70-80% for detecting allergic contact dermatitis 1

Baseline and Supplemental Series

• Start with a standardized baseline allergen series, which will detect approximately 80% of relevant allergens 1
• Consider testing with additional allergen series (fragrances, rubber chemicals, cosmetics, occupational products) based on the patient's specific allergen exposure patterns and occupational history 1
• The British Society for Cutaneous Allergy regularly revises its baseline series to include emerging allergens like methylisothiazolinone 1

Timing and Reading Protocol

• Apply patches for 2 days, then read at day 2 and day 4 as the optimum timing 1
• Consider an additional reading at day 6 or 7, which will detect approximately 10% more positive reactions that were negative at earlier readings 1
• Late-reacting allergens include neomycin, tixocortol pivalate, and nickel 1

Patient Preparation

• Provide a patient information leaflet on patch testing as part of the counseling process, including information on potential side effects, and obtain informed patient consent 1
• Consider deferring patch testing for 3 months after finishing systemic agents and 6 months after finishing biological agents to minimize false-negative reactions 1

Specialized Contact Testing

Photopatch Testing

• Perform photopatch testing when photoallergic contact dermatitis is suspected 1
• Apply photoallergen series in duplicate on either side of the upper back 1
• After 2 days, irradiate one side with 5 J/cm² UVA and read both sides in parallel after a further 2 days 1
• The incidence of true photoallergy is low (<5%), but additional readings after day 4 increase detection rates 1

Open Patch Testing

• Use open patch testing for potential irritants or sensitizers, and for investigating contact urticaria and protein contact dermatitis 1
• Apply the suspected agent "as is" on the forearm, assess within the first 30-60 minutes, and perform a further reading after 3-4 days 1

IgE-Mediated Allergy Testing (For Immediate Hypersensitivity)

Skin Prick Testing

• Skin prick testing is the primary diagnostic tool for confirming IgE-mediated allergic diseases, with sensitivity and specificity typically exceeding 80% 2, 3
• This is appropriate for patients with suspected allergic rhinitis, asthma, or immediate-type reactions rather than contact dermatitis 2, 4
• Discontinue antihistamines and some antidepressants several days before testing, but oral or inhaled steroids do not need to be stopped 2
• Read results at 15-20 minutes, measuring wheal and erythema with positive (histamine) and negative (saline) controls 2

When to Use Blood Testing Instead

• Use allergen-specific IgE immunoassays when skin testing is contraindicated: widespread eczema, severe dermatographism, inability to discontinue antihistamines, β-blocker use, or unusually high anaphylaxis risk 3
• Blood testing has lower sensitivity (70-75%) compared to skin prick testing but carries no anaphylaxis risk 3

Intradermal Testing

• Consider intradermal testing when additional sensitivity is needed or when skin prick tests are negative despite high clinical suspicion 2, 3
• This carries higher risk, with six historical fatalities reported from intradermal inhalant testing, five in asthmatics who did not receive prick testing first 3

Clinical Correlation and Relevance Assessment

Interpreting Results

• Assess the relevance of each positive patch test reaction to the patient's presenting dermatitis 1
• Classify reactions as: current relevance (exposure during current episode), past relevance (past episode only), relevance unknown, cross-reaction, or exposed without dermatitis 1
• Correlation with clinical history is essential, as more patients are sensitized than have corresponding symptoms 4, 5

Common Pitfalls

• Metal salts (nickel, cobalt, potassium dichromate), fragrances, and carba mix often cause irritant reactions that can be misinterpreted as positive allergic reactions 1
• Avoid testing during active widespread dermatitis, as this can lead to false results 1