What laboratory tests should be ordered to evaluate a patient with recurrent dermatitis?

Laboratory Testing for Recurrent Dermatitis

Patch testing with a baseline allergen series is the primary laboratory investigation for recurrent dermatitis, as clinical features alone cannot reliably distinguish allergic contact dermatitis from irritant or endogenous causes. 1, 2

Primary Diagnostic Approach

The cornerstone of laboratory evaluation is patch testing, not traditional blood work or serology. 1, 2

Patch Testing Indications

• Offer patch testing to any patient with chronic or persistent dermatitis lasting more than a few weeks, as visual inspection cannot differentiate between allergic contact, irritant, and endogenous dermatitis types. 1, 2
• Patch testing becomes particularly important when previously well-controlled dermatitis becomes refractory to treatments that previously worked, suggesting a new allergic contact component. 2
• Distribution patterns that warrant patch testing include marked facial/eyelid involvement, unusual patterns like lateral foot involvement, or vesicular lesions on dorsal hands and fingertips. 2, 3

Baseline and Extended Allergen Series

• Start with a standardized baseline screening series, which detects approximately 80% of relevant allergens. 1, 2
• The British Society for Cutaneous Allergy revises its baseline series regularly, removing obsolete allergens and adding emerging ones like methylisothiazolinone. 1
• Add supplemental allergen series based on exposure patterns (occupational chemicals, fragrances, rubber chemicals, cosmetics) when the baseline series fails to identify the culprit. 1, 2
• Test the patient's own products at non-irritant concentrations when specific products are suspected. 1, 2

Timing Considerations to Avoid False-Negatives

Critical timing requirements exist to ensure valid results:

• Wait 6 weeks after natural sun exposure or artificial UV exposure before performing patch tests. 2, 3
• Defer testing for 3 months after finishing systemic immunosuppressive agents and 6 months after finishing biological agents. 1, 2, 3
• Avoid potent topical steroids on the back for 2 days before testing. 3
• Antihistamines do not need to be stopped unless specifically testing for urticaria or contact urticarial reactions. 3

Reading Schedule and Interpretation

• Standard readings occur at days 2 and 4, but consider additional readings at day 6 or 7 if results are unexpectedly negative at day 4, as this picks up approximately 10% more positive reactions, including allergens like neomycin and tixocortol pivalate. 1
• Patch tests should be read in natural daylight and rated as positive (+, ++, +++), negative, or irritant. 1
• Metal salts (nickel, cobalt, potassium dichromate), fragrances, and carba mix frequently cause irritant reactions that can be misclassified as positive allergic reactions. 1

Specialized Testing When Indicated

Photopatch Testing

• Perform photopatch testing when photoallergic contact dermatitis is suspected, applying a photoallergen series in duplicate on either side of the upper back, with one side irradiated with 5 J/cm² UVA after 2 days. 1
• The incidence of true photoallergy is low at <5%, though additional readings after day 4 increase detection rates. 1

Open Patch Testing

• Use open patch testing for potential irritants, contact urticaria, or protein contact dermatitis by applying the suspected agent "as is" on the forearm with readings at 30-60 minutes and again at 3-4 days. 1
• Repeated open-application testing (applying product twice daily for 5-10 days on the volar forearm) helps assess cosmetics and personal-care products where irritancy may interfere with standard patch testing. 1

Serum Testing Has Limited Role

While total serum IgE and allergen-specific IgE panels can be elevated in atopic dermatitis patients and may show sensitization patterns, these blood tests do not diagnose contact dermatitis and should not replace patch testing. 4, 5 Serum IgE testing may help distinguish atopic dermatitis from pure contact dermatitis but does not identify contact allergens causing recurrent flares. 4

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 (prevalence 6-60%) as in the general population. 2, 6
• Positive patch tests only indicate sensitization, not necessarily clinical relevance; correlation with the distribution and timing of active dermatitis is required for confirmation. 2
• Testing during active widespread dermatitis or on recently UV-exposed skin increases false-negative rates. 2
• Many skin diseases can mimic dermatitis, including signs of systemic disease or malignancies, so consider the differential diagnosis carefully. 7