Improving Patch Test Efficacy: Part 2
Allergic contact dermatitis (ACD) is a common disease that affects over 13 million Americans each year.1 The societal impact of ACD is significant, with an estimated prevalence in the United States of almost 20% and an annual cost approaching $2 billion.1,2 ACD can also have a significant negative physical and emotional impact on patients’ quality of life. With thousands of potential allergens lurking amidst innumerable products, isolating relevant allergens can be a daunting and often frustrating task. However, with appropriate testing, education, and diligence, remission of ACD is an attainable goal.
This 2-part article discusses improving patch test efficacy. Part 1 provided information on patch test kits, selecting the right patient, selecting an adequate screen and suspect allergens, accepting the limits of the test, and supplemental panels. Part 2 discusses performing the test correctly, use in pediatric populations, relevance determination, and education.
Instructions for the patient should include avoidance of topical corticosteroids or topical immune modulators at the site of patch application for 1 week prior to patch testing if possible.3 Ideally, the patient should not have exposure to systemic corticosteroid 1 to 2 weeks (or sun exposure/phototherapy 3 weeks) before patch testing, as they are known to inhibit patch reactivity..3-5 Feuerman’s 1972 study of prednisone’s effects evaluated 43 patients who were given different doses of prednisone 48 hours prior to patch testing: 40 mg (n=12), 30 mg (n=15), 20 mg (n=16).4 They found that 20 mg given 48 hours prior had “almost no effect,” while 30 mg and 40 mg suppressed or diminished reactions.4 The authors also cited a previous study by O’Quinn and Isbell in 1969 in which patients were given 40 mg of prednisone 24 hours prior to patch testing, and noted that 6 of 20 patients had suppressed reactions.4 Patients on other immunosuppressant drugs, such as methotrexate, tumor necrosis factor-α inhibitors, and mycophenolate mofetil, have been shown to have positive patch tests, although data is limited to date.6-8
Antihistamines may be administered during the patch test procedure as they have not been reported to decrease the patient’s ability to elicit an allergic reaction.3,9 The antihistamine exception is if contact urticaria is suspected, in which case they should not be taken.3 Lastly, as reasonably as possible, patients should be advised to have the test performed at a time when they can avoid wetting or loosening the patches, excessive exercise, and sweating.
The uninvolved skin on the upper back is generally the primary site of panel application, avoiding (when possible) areas of preexisting dermatitis, sunburn/tan, or erythema. The spine should also be avoided (Figure 1). At-home shaving of back hair (to improve contact with the stickers) prior to the day of testing is recommended for patients with significant back hair. The sites should be marked and numbered appropriately and secured with hypoallergenic adhesive tape (eg, Hypafix ScanPor, patchProtect). Photodocumentation of the patch test placement should occur at the time of application. Per patient preference, a photograph may be taken on their personal phone, as a reference point, for the patient to observe for late-delayed reactions after 7 days.
The second reading should be performed after 72 to 120 hours of initial application.10,11 This delayed reading is critical to detect a crescendo of a reaction, and it also helps in distinguishing irritant from allergic reactions, as irritant reactions generally appear early and improve over the 96 hours. Thus, a response that appears positive on the first reading but is resolving by the second reading may indicate an irritant reaction.
Recently, experts have advocated the use of delayed patch test readings (day 7 or beyond) to maximize the sensitivity of the test. One study noted that upward of 8% of positive patch test (PPT) reactions would be missed if reading on day 6 or 7 was not conducted.12 Late delayed reading applies in particular if metals, topical antibiotics, formaldehyde-releasing preservatives, cocamide diethanolamine, p-phenylenediamine (PPD), or topical corticosteroids are suspected as the causative allergen (Table 1).10,12-14
Considerations in Pediatric Populations
Until recently, there was no federally indicated patch test for children younger than 18 years. However, throughout the ACD provider community, there had been increasing off-label use of patch testing in children and adolescents due to the recognized benefits of patch testing in this population. In August 2017, the FDA approved the Thin-Layer Rapid Use Epicutaneous (T.R.U.E.) Test (SmartPractice) for patients 6 years and older.15 It is the first and only commercially available patch test indicated for children and adolescents aged 6 to 17 years in the US. (Figure 3). In 2017, Health Canada (the Canadian Federal department responsible for national public health) approved the 550 comprehensive Chemotechnique hapten range for use in children, with certain recommended precautions in children younger than 8 years (Table 2), making it the first commercially available patch test indicated in Canada for children and adolescents younger than 18 years.16-18
Published data suggests that pediatric populations have a significant prevalence of sensitization to cocamidopropyl betaine, propylene glycol, glucosides, propolis, α-tocopherol (vitamin E), carmine, and fragrance mix.19 Therefore, these allergens should be considered for inclusion in supplemental patch testing in pediatric patients (Table 3).19 Conversely, gold sodium thiomalate and thimerosal are less likely to be relevant in pediatric populations, and at the discretion of the patch tester might be manually removed (eg, excised) from ready-to-use kits, creating space for additional allergens. Likewise, PPD and epoxy resin are not often tested in pediatric populations, unless they are reasonably suspect.
Notably, the German Contact Dermatitis Research Group recommends that patches be removed after 24 hours of application to minimize the incidence of irritant reactions.20 This modification in practice has been corroborated by US patch testers testing children younger than 8 years (as observed by Dr Jacob) and is recommended in the Health Canada precautions. Historically, practitioners diluted some patch test concentrations in half in children younger than 5 years.21
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