Patch Testing

Patch testing identifies delayed (type IV) hypersensitivity to contact allergens. Allergens are applied to unbroken skin on the upper back under occlusion for 48 hours, then read at 48 hours and again at 72 to 96 hours. Late readings identify some reactions that appear only after several days.

A positive reaction is an eczematous response at the allergen site. It shows sensitisation, which does not always produce clinical disease.

Chronic or recurrent eczema that has not responded to treatment, and dermatitis with a pattern suggesting contact exposure, are the main reasons for testing. Many people with positive reactions have no current symptoms from that allergen, so each positive result is matched to the exposure history.

Photopatch testing. Photo allergic contact dermatitis is assessed by applying duplicate allergen sets, with one set irradiated with ultraviolet A after removal. A reaction only in the irradiated site indicates photosensitivity.

Indications. Chronic hand, facial, eyelid or genital dermatitis; dermatitis with an unusual distribution; occupational dermatitis in healthcare, hairdressing, construction and catering; suspected allergy to cosmetics, topical medicines, fragrances, rubber or metals; and dermatitis that persists after avoidance of a suspected source.

Preparation and technique. Topical corticosteroids on the back and systemic immunosuppressants reduce reactions, so they are stopped where clinically safe, usually for at least one week before application. Phototherapy and sun exposure to the back are avoided in the preceding weeks. Oral antihistamines do not usually interfere with results. Active dermatitis over the back lowers reliability, so the test site is chosen from clear skin.

A screening series covers the most frequent allergens. It is extended with materials chosen from the exposure history, such as products brought in by the patient. Patches are applied to the back, and the patient avoids heavy sweating and wetting for 48 hours.

Contraindications and precautions. Recent systemic corticosteroid use reduces reliability, and results obtained during treatment are interpreted with caution. Testing is individualised in pregnancy.

Readings are graded as negative, doubtful, weak (+), strong (++) or extreme (+++). Doubtful and weak reactions carry a higher false positive rate and are reported with caution.

Interpretation.

• Positive reactions to fragrances, preservatives, nickel, cobalt, potassium dichromate, topical antibiotics, rubber accelerators and formaldehyde releasers are the most common true sensitisations.
• Relevance is assessed for each positive allergen. A positive result matters only when the exposure matches the clinical history.
• Equivocal results are repeated after a period of avoidance and with a clear skin site, or referred to a specialist service.

Causes of a false positive reaction. Irritant reactions from strong irritants or overly concentrated allergens. These are non-specific and often appear as blistering or sharply demarcated erythema.

Causes of a false negative reaction. Recent corticosteroid or immunosuppressant use, an allergen not included in the series, an inadequate concentration, patches removed too early, or misreading at the reading visit.

Critical values and findings needing immediate action.

• A strong or extreme reaction with spreading dermatitis or systemic symptoms requires review of the patch site and prompt management.
• Any reaction in a patient with a previous severe reaction to an allergen warrants specialist review before further exposure.

Limitations and artefacts. Extensive active dermatitis on the back lowers reliability. Heavy sweating, wetting and friction loosen patches and produce false negatives. Positive results should not be reported without relevance.

Follow up tests. Equivocal or doubtful results may require repeat patch testing, photopatch testing where photosensitivity is suspected, and specific allergen testing when a relevant product is identified. Avoidance advice and a written list of cross-reactive products accompany each confirmed allergy.

Special groups. Infants and young children are tested with a smaller allergen set and only with specialist input. Older adults with thin skin or chronic dermatitis may show prolonged reactions, which require careful reading. Nickel allergy is common in women, and reactions may be relevant to oral exposure as well as skin contact. Testing for systemic contact dermatitis is a separate specialist question.