What’s a dermatologist to do when patch testing is negative?

Christen M. Mowad, MD: “Studies have been done that show a lot of people do not perform second readings, and the patch test world believes that that is necessary to avoid false negatives.”

Christen M. Mowad, MD: “Studies have been done that show a lot of people do not perform second readings, and the patch test world believes that that is necessary to avoid false negatives.”

negative patch test result can be a good news/bad news situation for patient and dermatologist. Good news: The patient is not allergic to a host of allergens. Bad news: You don’t know what’s causing his contact dermatitis. This sort of confounding problem was confronted head-on Friday at “What To Do When Patch Testing Is Negative,” presented by Christen M. Mowad, MD, director of the Contact and Occupational Dermatitis Clinic, Geisinger Medical Center, Danville, Pennsylvania, and immediate past-president of the American Contact Dermatitis Society.

Dr. Mowad is working on a formal study to assess how patch test experts manage patients with negative patch tests because much of the current information is anecdotal and not evidence-based. For the session, she polled some of her contact dermatitis colleagues. “We’ve had some fun bantering what we do with these patients,” she said.

First on the list is preventing a false-negative result by avoiding testing technique pitfalls. Sunburn, topical steroids applied to the back, systemic steroids, and other immunosuppressants can affect results. “Were the patches adhered properly, did the patient have a hairy back and you actually didn’t get chemicals in contact with the skin?” asked Dr. Mowad.

Follow-up is important. Did you leave the patches on for 48 hours? Was a second reading performed? “Studies have been done that show a lot of people do not perform second readings, and the patch test world believes that that is necessary to avoid false negatives,” she said. “Several allergens have delayed reactions, and if you don’t do a delayed reading you’re going to miss those.”

Next, the dermatologist should consider expanded patch testing based on a thorough history to reveal allergen exposures. “Unless you do that detailed history — including job and hobby exposures — you might not choose to put the right patches on the patient to actually identify the causative allergen,” Dr. Mowad said.

When expanded testing comes up negative, look for other causes of disease and consider doing a biopsy, which can at least rule out some possibilities. “With chronic diffuse eczema you need to make sure you’re not dealing with the development of CTCL,” she said. “Patients can have atopic dermatitis that was either present all along and was exacerbated by a contact allergen or many of us believe in later-onset endogenous eczema. So do they have other signs and symptoms of atopic dermatitis? Do they have keratosis pilaris? Do they have elevated IgE levels that might indicate latent onset of atopic dermatitis?”

Dr. Mowad has seen patients referred for patch testing who have actually had psoriasis, diffuse scabies, or extensive tinea rather than dermatitis.

In the pursuit of figuring out the negative patch test, Dr. Mowad also checks laboratory work and makes sure age-appropriate screenings are current. And, based on her experience, she believes in controlling the itch. “It makes the patient more comfortable, which makes them less frustrated, it makes them less tired, and perhaps allows them to manage their daily life better and perhaps do what you’re asking them to do better.”

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