Phototherapy
Phototherapy can result in local cutaneous immune suppression; therefore, it is recommended that it not be applied to the patch test area either during the patch test process or for 1 to 2 weeks prior to patch test application. In addition, if heat or sweating are generated during phototherapy, they can affect the success of patch testing by poor patch adherence and/or disruption of allergen distribution.
Systemic Medications
Oral antihistamines do not affect patch testing and can be continued during the patch test process.
It is ideal to avoid systemic immunomodulatory agents during the patch test process, but they occasionally are unavoidable, either because they are necessary to manage other medical conditions or because they are needed to achieve clear enough skin to proceed with patch testing. If it is required, prednisone is not recommended to exceed 10 mg daily.12,13 If intramuscular triamcinolone acetonide has been administered, patch testing should occur at least 1 month after the most recent injection.12 Oral methotrexate can probably be continued during patch testing but should be kept at the lowest possible dose and should be held during the week of testing, if possible. Adalimumab, etanercept, infliximab, and ustekinumab can be continued, as they are unlikely to interfere with patch testing.12 There are reports of positive patch test reactions on dupilumab,14,15 and some authors have described the response as variable and potentially allergen dependent.16,17 We believe that it generally is acceptable to continue dupilumab during patch testing. Data on cyclosporine during patch testing are mixed, and caution is advised as higher doses may suppress a positive patch test. Azathioprine and mycophenolate should be avoided, if possible.12
Pearls and Pitfalls
A few tips along the way can help assure your success in patch testing.
- Proper patient counseling determines a successful test. Provide your patient with verbal and written instructions about the patch test process, patch care, and any other necessary information.
- A simple sponge bath is permissible during patch testing provided the back stays dry. One of the authors (A.R.A.) advises patients to sit in a small amount of water in a bathtub to bathe, wash only the front of the body in the shower, and wash hair in the sink.
- No sweating, swimming, heavy exercise, or heavy physical labor. If your patient is planning to run a marathon the week of patch testing, they will be sorely disappointed when you tell them no sweating or showering is allowed! Patients with an occupation that requires physical labor may require a work excuse.
- Tape does not adhere to areas of the skin with excess hair. A scissor trim or electric shave will help the patches stay occluded and in place. We use an electric razor with a disposable replaceable head. A traditional straight razor should not be used, as it can increase the risk for folliculitis, which can make patch readings quite difficult.
- Securing the patches in place with an extra layer of tape provides added security. Large sheets of transparent medical dressings work particularly well for children or if there is difficulty with tape adherence.
Avoid application of patches to areas of the skin with tattoos. In theory, tattooed skin may have a decreased immune response, and tattoo pigment can obscure results.18 However, this is sometimes unavoidable, and Fowler and McTigue18 described a case of successful patch testing on a diffusely tattooed back.
- Avoid skin lesions (eg, scars, seborrheic keratoses, dermatitis) that can affect tape application, patch adherence, or patch readings.
Final Interpretation
The first step to excellent patch testing is understanding the patch test process. Patch test systems include T.R.U.E. and the chamber method. There are several allergen screening series, and the best series for each patient is determined based on geographic region, exposures, and allergen prevalence. The timing and practice of the patch test reading is vital, and physicians should be cognizant of medications and phototherapy use during the patch test process. An understanding of common pearls and pitfalls makes the difference between a good and great patch tester.
Now that you are an expert in performing the test, watch out for part 2 of this series on patch test interpretation, relevance, education, and counseling. Happy testing!