In addition to a properly selected standard or screening series, supplemental patch testing with additional allergens can increase the diagnostic yield. Numerous supplemental series exist, including cosmetic, dental, textile, rubber, adhesive, plastics, and glue, among many others. In the NACDG 2015-2016 patch test cycle, it was found that 23% of 5597 patients reacted to an allergen that was not present on the NACDG screening series.2
In some situations, it is appropriate to patch test patient products, or nonstandard allergens. An abundance of caution, understanding of patch testing, and experience is necessary; for example, some chemicals are not recommended for testing, such as cleaning products, certain industrial chemicals, and those that may be carcinogens. We frequently consult De Groot’s Patch Testing8 for recommended allergen test concentrations and vehicles.
Patch Test Readings
The timing of the patch test reading is an important component of the test. Most North American comprehensive patch test clinics perform both first and delayed readings. After application, patches remain in place for 48 hours and then are removed, and a first reading is completed. Results are recorded as +/− (weak/doubtful), + (mild), ++ (strong), +++ (very strong), irritant, and negative.2 Many patch test specialists use side lighting to achieve the best reading and palpate to confirm the presence of induration; panel alignment devices commonly are utilized. There are some scenarios where shorter or longer application times are indicated, but this is beyond the scope of this article. A second, or delayed, reading should be completed 72 to 144 hours after initial application. We usually complete the delayed reading at 96 to 120 hours.
Certain patch test reactions may peak at different times, with fragrances often reacting earlier, and metals, topical antibiotics, and textile dyes reacting later.9 In the scenario of delayed peak reactions, third readings may be indicated.
Neglecting to complete a delayed reading is a potential pitfall and can increase the risk for both false-positive and false-negative reactions.10,11 In 1996, Uter et al10 published a large study of 9946 patients who were patch tested over a 4-year period. The authors compared patch test reactions at 48 and 72 hours and found that 34.5% of all positive reactions occurred at 72 hours; an additional 15.1% were positive at 96 hours. Importantly, one reading at 48 hours missed approximately one-third of positive patch test reactions, emphasizing the importance of delayed patch test readings.10 Furthermore, another study of 9997 consecutively patch tested patients examined reactions that were either negative or doubtful between days 3 or 4 and followed to see which of those reactions were positive at days 6 or 7. Of the negative reactions, the authors found that 4.4% were positive on days 6 or 7, and of the doubtful reactions, 9.1% were positive on days 6 or 7, meaning that up to 13.5% of positive reactions can be missed when a later reading is not performed.11
Medications During Patch Testing
Topical Medications
Topical medications generally can be continued during patch testing; however, patients should not apply topical medications to the patch test application site. Ideally, there should be no topical medication applied to the patch test application site for 1 to 2 weeks prior to patch test placement.12 Use of topical medications such as corticosteroids, calcineurin inhibitors, and theoretically even phosphodiesterase 4 inhibitors can not only result in suppression of positive patch test reactions but also can make patch adherence difficult.