Contact allergy to p‐tert‐butylphenol‐formaldehyde resin (PTBP‐FR) was first reported from a shoe glue in the late 1950s.1 The prevalence of PTBP‐FR contact allergy has been reported as between 1% and 3%,2, 3 but from 2004 to 2013/14 the prevalence declined by ~50% in Europe.4 Contact allergy to PTBP‐FR has been associated with leather‐shoe‐related foot dermatitis.4-6 Leather watch straps, domestic glues, handicraft adhesives, artificial nail adhesives, athletic tapes, neoprene orthopaedic knee braces, and amputation prostheses have also been identified as souces.7 Contact allergy to PTBP‐FR is mostly non‐occupational,8 and occupational cases have only been reported in the shoe manufacture and car industry.6, 9 The aim of this study was to analyze the sources of sensitization, clinical relevance, and the prevalence of contact allergy to PTBP‐FR among general dermatology patients.

METHODS

The European baseline series including PTBP‐FR 1% pet. (Chemotechnique Diagnostics, Vellinge, Sweden) and additional series were tested in 7084 patients with suspected contact allergy from June 2005 to December 2018. Patch tests were applied to the upper back in Finn Chambers (Epitest, Tuusula, Finland) and the chambers were fastened with Scanpor tape (Norgeplaster, Vennesla, Norway) and removed at day (D) 2. The patch tests were read on D3 to D5 after the removal of the chambers, according to criteria of the International Contact Dermatitis Research Group. The relevance of the positive PTBP‐FR reactions and sources of sensitization were retrospectively evaluated from patient records.

RESULTS

The patients were 5160 (73%) females and 1924 (27%) males. Positive PTBP‐FR patch test reactions were seen in 46 patients (0.65%) (age range 17–89 years; 10 [22%] male and 36 [78%] female). Among the 10 male patients, eight had ++ reactions and two had a + reaction. Four of the 36 female patients had +++ reactions (11%), seven ++ reactions (19%), and 25 (70%) patients a + reaction to PTBP‐FR. Of the 46 of PTBP‐FR positive patients 12 (26%) had active atopic dermatitis and 19 (41%) had atopic constitution or previous atopic dermatitis. Thirty‐three (72%) of the PTBP‐FR positive patients also had concurrent patch test reactions.

Facial dermatitis was reported in nine (20%) patients, hand dermatitis in 16 (35%), and hand and/or feet dermatitis in 32 (70%). Four (9%) patients had dermatitis of the feet only. Most of the patients had dermatitis in multiple areas. One patient had dermatitis under a leather watch strap. Leather shoes or sandals were regarded as relevant sources in seven patients and handicraft glues in nine patients. Artificial nails and athletic tape had been used by one patient each. A leg support padding was the most likely sensitization source in one case. Earlier exposure to PTBP‐FR was considered in 36 (78%) of the patients. Current exposure with allergic contact dermatitis symptoms was present in 10 (22%) patients (six exposed to leather shoes or sandals, two to glues, one to artificial nails, and one to padding in a leg support). Detailed information on the 21 patients with either proven or suspected sensitization sources to PTBP‐FR is presented in Table S1. In the remainder of the patients, clinical relevance was unknown.

DISCUSSION

According to a large meta‐analysis, the average prevalence of contact allergy to PTBP‐FR has been 1.3% in both male and female patients.3 In North America, the prevalence of PTBP‐FR contact allergy was between 1.0% and 1.5% during 2015–2016.10 In our study only 0.65% patients reacted to PTBP‐FR, which is less than in these previous studies: PTBP‐FR contact allergy was seen in only 0.7% of female patients and 0.5% of male patients. Since male patients made up only 27% of the tested patients, PTBP‐FR seems not to be a very important contact allergen in the Finnish male population (0.1% of all the tested patients). By contrast, 36 PTBP‐FR‐positive female patients (0.5% of the tested patients) were found.

Only seven patients (15%) were suspected to be sensitized from leather shoes or sandals. It is possible that shoe dermatitis is less common in Finland, as socks are worn more often, and perhaps also, shoe‐related sweating is less frequent in our cold climate. On the other hand, this finding may also be indicative of a reduced importance of PTBP‐FR containing adhesives in footwear, as reported.4 Domestic glues in handicraft work were the most common source of sensitization, as nine patients had either current or earlier history of hand dermatitis. Therefore, PTBP‐FR should be considered when studying hand dermatitis patients with a history of glue and adhesive exposure.

Only one patient had occupational contact allergy to PTBP‐FR. This case has been published previously and, later, the patient was tested for other reasons.11 This patient had been sensitized to PTBP‐FR following exposure to glass wool at work. Another patient, a car factory worker, was suspected of having occupational contact allergy to PTBP‐FR. It was possible that he had used a glue containing PTBP‐FR at work, but PTBP‐FR could not be traced from that particular glue.

Most of the patients (ie, 25/46 [54%]), did not have any proven or suspected source for their PTBP‐FR contact allergy. These patients may have been asymptomatic, or they have experienced only very mild symptoms upon exposure to PTBP‐FR‐containing material. Many patients also had other contact allergies that were often currently clinically relevant. Leather shoes, sandals, or wrist‐watch straps, and domestic glues seem to represent the most common sources of sensitization. The patients´ own materials were not tested and the suspected PTBP‐FR exposure could not be traced in many cases; these are limitations of the study. Further studies are needed to obtain more detailed exposure information on PTBP‐FR, especially in non‐occupational cases.

In conclusion, the prevalence of PTBP‐FR contact allergy seems to be rather low and occupational contact allergy to PTBP‐FR is rare. Therefore, perhaps PTBP‐FR should be tested in more specialized series, for example, glues and/or shoes series, rather than in the European baseline series.