IL-13 Inhibitor Response Can Guide Patch Testing Decisions in Eczematous Dermatitis

Correctly diagnosing eczematous and contact dermatitis often comes down to asking the right questions in the right order, according to Matthew Zirwas, MD, of Bexley Dermatology, who presented a case-based session at Maui Derm NP+PA Summer 2026 in Colorado Springs.

In an interview with HCPLive, Zirwas described how a decade of clinical experience has substantially changed both his diagnostic framework and his use of patch testing and why IL-13 inhibitors now play a central role in guiding it.

Starting With History in Eczematous Dermatitis

When a challenging eczematous case presents, Zirwas said the starting point is always history, specifically looking for product or exposure changes that correlate temporally with symptom onset.

"If it's [on] your face, did you change your face wash? Did you change your shampoo [or] your conditioner? Are you getting anything new done with your nails?" he said. "If it's hands, any changes with soap, gloves, moisturizer, [or] what you're doing at work?"

For widespread dermatitis, he probes for changes in laundry detergent and body wash. The history-first approach reflects a substantial recalibration in his clinical thinking over the past decade. For much of his career, he treated contact dermatitis as something that could not reliably be diagnosed through history alone. He has since reversed that position.

"What I learned over time was that those relevant positives on testing rarely ever got anybody better," Zirwas said. "And then, whenever I put them on a drug like dupilumab or lebrikizumab, they would suddenly get better, and I realized that I was massively overdiagnosing contact dermatitis and massively underdiagnosing atopic dermatitis."

Nickel and Ammonium Persulfate: The 2 Most Missed Allergens

Zirwas identified nickel and ammonium persulfate as the allergens most frequently overlooked in routine eczematous disease workups for distinct reasons.

Nickel allergy is familiar to most dermatologists, but the dietary dimension of nickel sensitization is often overlooked. Systemic reactions, including hand dermatitis and generalized eczematous eruptions, can occur due to dietary nickel ingestion in sensitized individuals.² Zirwas estimated that between 1% and 10% of nickel-allergic patients may be ingesting enough nickel through food to sustain a systemic contact dermatitis. The presentation can include hand eczema, foot eczema, or nonspecific rashes on the elbows, patterns that may not immediately suggest a contact allergen.

"It's not that people are missing the allergen—it's that they are not even thinking about contact dermatitis, and especially not thinking about nickel, whenever they see these widespread rashes,” Zirwas said. “Even if they get a positive test in nickel, they're not putting it together.”

The relationship between dietary nickel and systemic contact dermatitis remains somewhat controversial, though evidence supports that dietary nickel can be the chief trigger for symptom persistence in a subset of patients with chronic allergic-like dermatitis and nickel sensitization.³

Ammonium persulfate, an oxidizing agent used primarily in hair bleaching products and as a pool and hot tub shock treatment, presents a different diagnostic challenge. Allergy to potassium peroxymonopersulfate used as a hot tub shock chemical appears to be more common than previously appreciated and has been underdiagnosed in part because neither the compound nor ammonium persulfate is typically included when patch testing patients with widespread dermatitis. In sensitized patients, exposure can trigger a widespread rash that closely mimics nonspecific or atopic dermatitis.

A North American Contact Dermatitis Group analysis of > 10,500 patients found that 1.8% had positive patch test reactions to ammonium persulfate, with swimming pool and spa chemicals identified as important sources of exposure.⁴

Zirwas said the clinical screen for this allergen is straightforward: ask the patient whether they have a hot tub.

Repositioning Patch Testing as a Later Step

The most significant shift in Zirwas's practice involves when he reaches for patch testing. He now describes it as a high-cost, high-burden procedure that should follow other diagnostic steps rather than lead them.

His current approach sequences as follows: take a thorough history, implement any relevant empirical avoidance measures, then trial the patient on an IL-13 inhibitor, either dupilumab or lebrikizumab, across 6 weeks and 3 doses. If the patient responds meaningfully, he continues on that treatment path. If IL-13 inhibition produces no improvement or worsens the dermatitis, he takes that as a signal favoring contact dermatitis, at which point patch testing becomes warranted.

"If those first 3 doses of IL-13 inhibition didn't do anything or made them worse, that then makes me say this is really likely contact dermatitis, and now it's worth putting you through patch testing,” Zirwas said.

Editor’s note: Reported disclosures for Zirwas include GENZYME CORPORATION, Regeneron Healthcare Solutions, Dermavant Sciences, and more.

References

1. Zirwas M. Eczematous and Contact Dermatitis: Cases That Will Leave You Scratching Your Head. Session presented at Maui Derm NP+PA Summer 2026 in Colorado Springs, on June 24.

2. Zirwas M. Dietary Nickel as a Cause of Systemic Contact Dermatitis. JCAD - The Journal of Clinical and Aesthetic Dermatology. Published June 5, 2009. Accessed June 24, 2026. https://jcadonline.com/dietary-nickel-as-a-cause-of-systemic-contact-dermatitis/

3. Antico A, Soana R. Nickel sensitization and dietary nickel are a substantial cause of symptoms provocation in patients with chronic allergic-like dermatitis syndromes. Allergy Rhinol (Providence). 2015;6(1):56-63. doi:10.2500/ar.2015.6.0109

4. Warshaw EM, Ruggiero JL, DeKoven JG, et al. Patch testing with ammonium persulfate: The North American Contact Dermatitis Group Experience, 2015-2018. J Am Acad Dermatol. 2022;87(5):1014-1023. doi:10.1016/j.jaad.2021.08.005