Patch testing is essential in determining the substances responsible for allergic contact dermatitis but is also associated with significant challenges for both clinician and patient, according to Dr. Mark Davis and his colleagues.
"Although the results of patch testing can often be rewarding for the patient and physician, the process of patch testing can be expensive, challenging, time consuming, and frustrating," the researchers wrote. "Often, it raises more questions than answers."
Writing in the September issue of Seminars in Cutaneous Medicine and Surgery (Semin. Cutan. Med. Surg. 2013;32:158-68), Dr. Davis and his colleagues at the Mayo Clinic in Rochester, Minn., and the University of Minnesota, Minneapolis, examined key issues with patch testing for allergic contact dermatitis and provided practical guidance on how to deal with these issues.
Allergic contact dermatitis can occur at any age, and the authors recommended that in general, everyone with recurrent or persistent dermatitis suspected of having a component of allergic contact dermatitis should be patch tested.
However, a positive patch test may contradict other evidence, such as a patient showing a positive reaction to gold sodium thiosulfate despite having always tolerated a gold ring.
Patch tests may also deliver a positive result although the patient shows no signs of dermatitis, such as a positive response to nickel, the researchers noted.
They advised against patch testing to dust mites in particular, because most patients undergoing patch testing to dust mites have positive reactions regardless of whether they have atopic dermatitis.
"Research at our institution paradoxically showed that atopic patients had fewer reactions to the dust mites than patients who were nonatopic," they said.
Determining the relevance of a positive result can be difficult. In one researcher’s reported experience, most reactions were relevant, but predicting positive relevant allergens was often complicated by the occurrence of unexpected reactions.
One common question about patch testing is how to distinguish between an irritant versus an allergic patch test reaction, the researchers noted.
"Irritant reactions occur within minutes to hours after application of the chemical, whereas allergic reactions occur over days," they noted. "Irritant patch test reactions are usually very prominent initially and then fade with time (decrescendo), whereas an allergic patch test reaction usually increases with time (crescendo)."
Another area of controversy is allergic contact dermatitis to laundry detergent, which the authors said was so rare that it had been described as an urban legend. Despite this, both patients and physicians may conclude that laundry detergent is involved when allergic contact dermatitis is diagnosed.
In addition, not all patch test results are clear, and up to 50% of reactions may be very weak. In particular, some problematic patch test results are associated with allergens such as cocamidopropyl betaine 1% aqueous (aq), benzalkonium chloride 0.1% aq, and jasmine absolute 2% pet (Dermatitis 2010;21:91-7), the researchers wrote.
Although the management of weak reactions is controversial, the researchers noted that in their own practice at the Mayo Clinic, they err on the side of caution and regard even very weak reactions as potentially relevant.
The authors had no financial conflicts to disclose.