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Improve Allergen Diagnosis for Eczema, Contact Dermatitis


Learning to identify some well-disguised drivers of eczema and contact dermatitis can speed patients toward clear or almost clear, according to a presentation at Maui Derm NP+PA Fall 2021.

However common an allergen is, it may present in a variety of uncommon ways. Matthew Zirwas, MD, used a series of instructive cases to show how to use patch testing, patient interviews, observation, and medical fundamentals to uncover some well-concealed drivers of eczema and contact dermatitis in a presentation at Maui Derm NP+PA Fall 2021, held September 30 to October 2, 2021, live in Asheville, North Carolina, and virtually.1

Patch testing is an importnt tool in diagnosis, but it is more of a starting point than a final ruling, according to Zirwas, who is the director at the Clinical Trials and Dermatology Center in Bexley, Ohio. “My experience is that only between 30% and 50% of patients with positive patch tests will get better with avoidance,” he said. “So, what’s affecting the rest of the patients?”

He offered these tips for interpreting unrelated positive patch tests of the latter group of patients:

  • Look for signs of undiagnosed contact dermatitis. “It’s very likely,” he said.
  • Evaluate the patient for dermatitis not otherwise specified (NOS) (“aka atopic dermatitis (AD)”
  • Recognize that most cases of contact dermatitis are “obviously not nonspecific dermatitis."

Next Steps for a Positive Patch Test

Zirwas recommended tailoring protocols to reflect how the results and rash match up with established parameters for that particular allergen. While a positive patch test and rash that are “highly consistent” with the presentation of an allergy to the allergen probably can be solved by avoidance, less clear-cut cases require different management approaches.

“If a positive patch test and rash that ‘fits’ but isn’t obviously the allergen, I’d recommend avoidance with a follow-up at 3 months. If the patient still isn’t better, first rule out incorrect avoidance or hidden exposures. If there are none, I’d start to manage this as dermatitis NOS,” he advised. 

However, in cases where there is a positive patch test and “as rash that has contact dermatitis,” physicians may not be able to offer immediate answers. “Explain to your patient what you’re looking for and advise them they may have to keep back until you figure it out,” Zirwas said.

Focus on Every Factor of the Presentation

Zirwas offered case-based pointers on how to assess signs and symptoms that do not conform with textbook standards. 

In 1 case, a 12-year-old boy presented with vesicular dermatitis on both palms but not on the dorsal hands. “It was intensely pruritic,” he noted. “The patient failed all topicals over 1.5 years of treatment. His patch test was positive to thiuram mix. However, he said he does not wear rubber gloves—ever. And, too, kids almost never get endogenous chronic vesicular hand dermatitis.”

Other aspects of the presentation were also unusual. For example, palmar only presentations are usually atopic/dyshidrotic, he said. Zirwas talked with the patient and his parents about whether he encountered anything made of rubber that he touched only with his palms. The answer was a basketball, and switching from a rubber to a leather ball resulted in complete clearance.

For cases in which patients may not have an obvious exposure to an allergen that gets a positive result, he recommended taking a deeper dive into ingredient lists on the products they use and possible interactions. An 83-year-old patient with an infra-nasal rash who had a positive patch test for tea tree oil said she never used it, but later noted that she regularly used a product with eucalyptus oil, which can cross-react with tea tree oil. He advised checking ingredient lists against other references, including herbal or plant-based products.

Some allergens are also expanding their role as possible causes of skin rashes and flares. Ammonium persulfate is usually associated with hair bleaches. However, in the case of a patient positive to this allergen but who never bleached his own or anyone else’s hair, the problem centered on the shock treatment he used in his hot tub. It contained potassium peroxymonosulfate which cross-reacts with ammonium persulfate. 

“Since then, we have started to patch test routinely for this,” noted Zirwas. “It is a really common allergen. We found that 2-3% of patients test positive to ammonium persulfate. That is higher than the percentages for many well-known allergens. We’ve missed for years. An allergy to shock in hot tubs is one of the most overlooked generalized causes of unidentified dermatitis. If a patient presents with an itchy rash all over their body, ask if they have hot tub.” 

Treatment protocols call for staying out of the hot tub for 2 months. If the patient’s skin improves, Zirwas recommended telling them to shock the hot tub and get in as soon as it is safe to do so after the shock. “If the rash returns, you’ve confirmed the diagnosis,” he added.

Nickel is well-known as an allergen in the context of jewelry or other topical contact. But, said Zirwas, it is important to understand that nickel allergies may also be caused by nickel in food. Itchy papules on the extensor elbows are a clue. Zirwas treats this with oral chelation with calcium disodium EDTA.


Zirwas is on the speakers bureau, is a consultant, and receives grants from AbbVie, Arcutis, Asana, Avillion, AsepticMD, Bausch Health, Dermavant Sciences, Sanofi, Regeneron Pharmaceuticals, Edessa Biotech, Eli Lilly and Company, Fit Bit, Foamix, Galderma, Genentech, Incyte Corporation, Novartis, Janssen, L’Oréal, LEO Pharma, Ortho Dermatologics, Pfizer Inc, Menlo, and Sol-Gel Technologies.


1. Zirwas M. Zebras wearing pearls: Instructive cases with uncommon presentations of common allergens. Presented at: Maui Derm NP+PA Fall 2021, held September 30 to October 2 live in Asheville, North Carolina, and virtual.

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