What are the criteria for patch testing in individuals with a history of skin allergies or dermatitis?

Patch Testing Criteria

Patch testing should be performed in any patient with chronic or persistent dermatitis lasting more than a few weeks, since clinical features alone cannot reliably distinguish allergic contact dermatitis from irritant or endogenous causes. 1

Who Should Undergo Patch Testing

Primary Indications

• Any chronic or persistent dermatitis regardless of suspected etiology, particularly involving the hands or face where clinical distinction between allergic, irritant, and endogenous dermatitis is unreliable 1

• Previously well-controlled atopic or endogenous dermatitis that becomes difficult or impossible to control with the same topical treatments that previously worked 1

• Suspected allergic contact dermatitis based on history or physical examination findings, including disease aggravated by topical medications or emollients 1

Clinical Patterns Suggesting Need for Testing

• Distribution patterns suggesting contact with specific items: marked facial/eyelid involvement, increased severity at neck flexures, vesicular lesions on dorsal hands and fingertips, or unusual patterns like sides of feet 1, 2

• Occupational dermatitis with symptoms improving on weekends and holidays, then recurring upon return to work 2

• Recalcitrant or therapy-resistant hand eczema in working-age patients 1

• Adult- or adolescent-onset atopic dermatitis where allergic contact dermatitis may be masquerading as or complicating atopic dermatitis 1

• Generalized dermatitis that may involve contact allergens like fragrances, preservatives, cleansers, or textiles 1

Timing Requirements and Contraindications

Mandatory Waiting Periods to Avoid False-Negatives

• 6 weeks after natural sun exposure or artificial UV exposure (tanning beds) 1, 2

• 3 months after finishing systemic immunosuppressive agents 1, 2

• 6 months after finishing biological agents 1, 2

• 2 days after potent topical steroids on the back (test application site) 2

Special Populations

• Pregnancy: No evidence of harm exists, but no safety data are available; therefore, patch testing should be undertaken only when required and after informed consent is obtained 1

• Breastfeeding: No evidence that patch testing is harmful 1

• Patients on immunosuppression: Where immunosuppressive treatment cannot be stopped safely, patch testing can yield positive results which, while possibly suboptimal, may be preferable to not testing at all 1

Important Exception

• Antihistamines do NOT need to be stopped unless specifically testing for urticaria or contact urticarial reactions 1, 2

Technical Specifications

Application Method

• Standard occlusive application of antigens at standardized concentrations in appropriate vehicles for 48 hours 1, 3

• Application site: Back is most commonly used for convenience and available surface area, though outer upper arms can also be used 1

• Test sensitivity and specificity: 70-80% 1, 3

Reading Schedule

• Optimal timing: Day 2 and day 4 readings 1, 3

• Additional day 6 or 7 reading: Detects approximately 10% more positive reactions that were negative at days 2 and 4, particularly for neomycin, tixocortol pivalate, and nickel 1, 3

Allergen Selection

Baseline Series

• Standardized baseline screening series picks up approximately 80% of allergens 1

• Series vary by country (U.S. vs. Europe) and should be revised regularly to remove allergens diminishing in relevance and add emerging allergens 1

Supplemental Testing

• Extended series for specific anatomical sites, occupational groups, and chemical exposures when baseline series fails to identify less common allergens 1

• Patient's own products should be tested at non-irritant concentrations when suspected 1

Common Pitfalls to Avoid

• Do not assume all dermatitis is irritant or atopic without patch testing, as allergic contact dermatitis is at least as common in atopic dermatitis patients (6-60%) as in the general population 1

• Do not test during active widespread dermatitis or on skin with recent UV exposure, as this increases false-negative rates 1

• Do not stop antihistamines unless testing for urticaria 1

• Do not confuse irritant reactions with true allergic reactions during interpretation 3

• Remember that positive tests only indicate sensitization, not necessarily clinical relevance; correlation with active dermatitis and sometimes repeat open application testing is required for confirmation 1, 3