When working up patients with allergic contact dermatitis (ACD), the patch test used may depend on how frequently testing is performed in the practice, and the type of allergies that are being evaluated, according to Joseph Fowler Jr., MD, of the department of dermatology at the University of Louisville (Ky).
T.R.U.E. TEST is more convenient than standard patch testing is but misses allergic contact dermatitis up to 40% of the time, he pointed out. The benefit of the T.R.U.E. TEST is that it’s easy to use, allergens come preapplied to a gel-based tape so there’s very little prep time, and they are well standardized with the same quantity on each patch every time.
Dr. Joseph Fowler
T.R.U.E. TEST seems to work well for when testing for metal allergies, as well as allergies to topical antibiotics, steroids, and rubber, but not as well for dental implants, fragrances, newer preservatives, surfactants, acrylates, and some industrial and cosmetic allergens. It’s not so effective in many occupational settings, but even so, T.R.U.E. TEST is a good option when testing is performed infrequently, and “is much better than no patch testing at all,” according to Dr. Fowler, who spoke at the Annual Coastal Dermatology Symposium, jointly presented by the University of Louisville and Global Academy for Medical Education.
In a presentation on contact dermatitis and itch, he pointed out that what appears to be atopic dermatitis (AD) in a patient might actually be ACD and that ACD is common in patients with AD and complicates its treatment. Metals, fragrances, and topical components – namely lanolin and neomycin – are the most likely allergens to cause trouble in AD. Nickel allergy can be particularly problematic, causing severe lesions beyond the point of contact (Dermatitis. 2012 Nov-Dec;23[6]:275-80).
“Strongly consider patch testing any chronic, difficult to control atopic patient,” especially when AD is not affecting the typical areas – or spreads beyond them – and when it doesn’t respond to the usual treatments. The onset of AD beyond age 5 years is another clue that contact dermatitis might be at work. Patch testing atopic patients is “more likely to be helpful in disease management than scratch or RAST [radioallergosorbent] testing,” Dr. Fowler said.
It’s best if patch testing is done while patients are off immunosuppressants, but current immunosuppressive therapy should not be an absolute contraindication to testing, he said. Not all of them throw off the results. “You do not need to worry about patch testing a patient who is on antihistamines, tumor necrosis factor–alpha inhibitors, NSAIDs, or methotrexate.” However, when it comes to patch testing a patient on cyclosporine, tacrolimus, azathioprine, and mycophenolate mofetil, he said, “probably not” (Dermatitis. 2012 Nov-Dec;23[6]:301-3).
Pruritus might or might not be related to the skin issues. For itch caused by skin diseases such as scabies, dermatitis, or psoriasis, “treat the dermatosis to treat the itch,” Dr. Fowler said.
Several topicals can help while the skin problems are being tamed, including hypochlorous acid to stabilize mast cells; strontium 4% hydrogel; and compounded topical ketamine, amitriptyline, and lidocaine, which seems to be particularly helpful (J Am Acad Dermatol. 2017 Apr;76[4]:760-1). Other than for urticaria, antihistamines are of little use, except to provide sedation.
Renal disease, liver disease, lymphoma, and neurologic abnormalities are among the systemic problems that can cause itch; the giveaway is that there’s no primary skin disease, Dr. Fowler said. While systemic problems are being addressed, gabapentin, tricyclic antidepressants, and anxiolytics can help. For generalized pruritus, with no primary skin disease, a referral to a neurologist is essential, he said.
This publication and the Global Academy for Medical Education are owned by Frontline Medical News. Dr. Fowler is a consultant, speaker, and/or researcher for a number of companies, including AbbVie, Regeneron/Sanofi, Allergan, Galderma, and Merck.