Optimizing the patch‐test concentration of para‐tertiary‐butylcatechol: results of a prospective study with a dilution series

Para‐tertiary‐butylcatechol (PTBC), which has long been patch tested internationally at 1% and 0·5% concentrations, was recently shown to induce patch‐test sensitization at the 1% patch‐test concentration. In order to determine a safe patch‐test concentration, we performed a prospective study with lowered patch‐test concentrations. A dilution series of PTBC 0·25%, 0·1%, 0·01% and 0·001% (petrolatum, pet.) was tested on the upper back. Additionally, 0·25% PTBC was tested on the left upper arm to allow patients to carry out self‐examination daily. Patch tests were read on D1–3, D7, D14 and D21 after patch‐test application. Patients who were not able to return for all scheduled readings were telephoned and asked to report any reaction at the patch‐test sites. 65 out of the 101 patients included completed the study. A positive patch‐test reaction was observed in 4 patients during D1–D3, indicating previous sensitization. 1 patient had a doubtful reaction at D3 reading. Negative patch‐test results were noted in 60 patients. None of the patients developed a positive patch‐test reaction during the late readings (D7–D21). Thus, patch‐test sensitization was not observed in any case. Para‐tertiary‐butylcatechol 0·25% pet. is recommended for patch testing internationally.