The additive value of patch testing non‐commercial test substances and patients' own products in a clinic of occupational dermatology

BACKGROUND: Commercial patch test substances do not cover all occupational contact allergens. Workplace materials and in‐house test substances are tested to complement the investigation of occupational skin disease (OSD). OBJECTIVES: To quantify the additional value of testing workplace materials and non‐commercial in‐house test substances in the diagnosis of OSD. MATERIALS AND METHODS: Patients files of 544 patients patch tested at the Finnish Institute of Occupational Health in 2015–2019 were reviewed for occupation, diagnoses and patch test results. RESULTS: OSD was diagnosed in 353 (64.9%) of the patients. A total of 206 (37.9%) patients had occupational allergic contact dermatitis (OACD). In 19 (3.5%) patients, the only clues to the diagnoses of OACD were positive reactions to workplace materials, and in 20 (3.7%) patients, the diagnosis of OACD was based on commercially unavailable test substances. In 167 OACD cases diagnosed by commercial test substances, additional causes were found in 17 by testing patients' own and non‐commercial test substances. In 43 (7.9%) cases, positive reactions to workplace materials reinforced diagnoses based on commercial test substances. The overall additive value of testing own products was 16.7% (91 cases). CONCLUSION: We would have missed 39 (18.9%) of our 206 OACD cases if we had solely used commercial test substances.