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Patch testing in the allergist’s office: useful and relevant

Published online: September 7, 2018

Allergic contact dermatitis (ACD) is a delayed (T-cell mediated) hypersensitivity reaction in response to an external chemical or agent resulting in significant skin inflammation. Patch testing is the gold standard diagnostic tool for investigating ACD. In the United Kingdom, patch testing has typically been confined to the dermatologist’s practice. While allergists commonly investigate for immediate hypersensitivity (ie, food allergy, allergic rhinitis) with skin prick or specific IgE testing, patients with suspected contact dermatitis are typically not investigated. It is now strongly recommended that a detailed assessment of both immediate and delayed-type hypersensitivity is undertaken in the same clinical setting. Detailed reports on patch testing by allergists are also underrepresented at the international level.

In a study recently published in The Journal of Allergy and Clinical Immunology: In Practice, Watts et al. undertook a comprehensive evaluation of a patch testing initiative from an adult allergy practice based at Guy’s and St Thomas’ Hospitals in the United Kingdom. They reported on various patient characteristics, contact allergy prevalence, and relevance data, in addition to the utility of standard/supplementary panels and immediate hypersensitivity testing. Finally, a comparative analysis with other patch test centers internationally was undertaken.

A total of 156 adult patients (18 years and older) suspected of having contact dermatitis between October 2016 and April 2018 were evaluated in total. Patients were patch tested to an extended British standard series alongside supplementary panels and/or patient’s own material. Testing for immediate hypersensitivity was also performed where clinically indicated (before patch testing). Of the 156 patients tested (mean age 36.9 years, female 88%, white ethnicity 71.8%, atopy 68.6%), allergic contact dermatitis was diagnosed overall in 49% and current relevance was assigned to 31%. The extended British standard series was able to detect the responsible allergen in 87% of patients. Most prevalent contact allergens were nickel (28.2%), p-phenylenediamine (8.3%), cobalt (8.3%), methylisothiazoline (5.8%), and hydroperoxides of limonene (4.5%) and linalool (4.5%). Patients with a history of dermatitis associated with their occupation (eg, healthcare workers) were more likely to have relevant positive patch test reactions. A history of atopic disease or positive immediate hypersensitivity testing was not associated with relevant positive patch test results.

ACD was diagnosed in almost 50% of the patient cohort, highlighting the importance and usefulness of patch testing in an allergy practice. The patch test results are broadly similar to other dermatology and allergy centers. Hydroperoxides of limonene and linalool (both fragrances) are notable additions to the prevalence data. The authors recommend that patch testing should be incorporated into more allergy practices and that a detailed relevance assessment should be performed in all patch test analyses.

The Journal of Allergy and Clinical Immunology: In Practice is an official journal of the AAAAI, focusing on practical information for the practicing clinician.

A patch testing initiative for the investigation of allergic contact dermatitis in a UK allergy practice: a retrospective study
By Timothy J. Watts, Sophia Watts, David Thursfield, Rubaiyat Haque