Danville, Pa. - Effective office-based patch testing requires physicians to go beyond the limits of a popular commercially available test, when possible, and take second readings on every patient, an expert says.
"Many private practitioners don't have access to all the expanded allergens that we and many patch testing clinics have in our offices," says Christen M. Mowad, M.D., associate professor of dermatology, Geisinger Medical Center, Danville, Pa.
Many busy private practices rely on Allerderm's Thin-Layer Rapid Use (TRUE) Test, "which is a good starting point for patch testing and detecting allergic contact dermatitis (ACD). However, when limited allergens are used there is a risk of missing relevant allergens," says Dr. Mowad, who also is secretary-treasurer of the American Contact Dermatitis Society.
"Individuals who are patch testing must be aware that if they're only patching to the TRUE Test, they may miss allergens, and expanded allergy testing should be pursued," she adds.
Allergy or irritant?
"Patient histories and physicals are not enough," although they represent important first steps toward establishing a suspicion of ACD and directing allergen selection, Dr. Mowad says.
Patch testing represents the gold standard for diagnosing ACD, which makes up 20 percent of contact dermatitis cases. The other 80 percent of cases consist of irritant contact dermatitis (ICD).
"ICD is often confused with ACD because they can look similar - they often have a sharp demarcation and are itchy and scaly," Dr. Mowad says. "Patch testing can help differentiate the two."
More specifically, ICD refers to a local toxic effect from exposure to items such as soaps, solvents and other irritants, she says. Clinically, Dr. Mowad adds, it presents with a well-demarcated, eczematous eruption that can appear vesicobullous or as a patchy dermatitis.
While ACD eruptions often are sharply demarcated and eczematous like those of ICD, ACD's clinical picture can include urticaria, pruritus and erythroderma, Dr. Mowad says.
"More than 3,000 chemicals are known to cause ACD," she adds. "Allergens are typically lipid-soluble and low-molecular-weight chemicals."
And once an individual is sensitized, Dr. Mowad says, "One only needs exposure to a small concentration of that allergen to elicit a reaction."
Dr. Mowad says patients often believe that since they've been using a product for years, it can't be causing a problem.
"But as dermatologists," she says, "we are aware and must educate our patients that although one can use something safely for years, one can still develop an allergy to it."
Just as a patient's sensitivity may change, she explains, "The chemical ingredients may change without any significant change in the label" to alert users.
Similarly, Dr. Mowad says that while many patients assume allergic reactions immediately follow exposure, the example of poison ivy illustrates that reactions can take several days to develop. And more expensive products are no less likely than cheaper counterparts to provoke reactions, she notes.
"Just because one paid more for it doesn't mean a product is incapable of causing ACD," Dr. Mowad tells Dermatology Times.
While taking patient histories, she says dermatologists must break through such misconceptions to get accurate information.