PASADENA, CALIF. – Patch tests for allergy are biologic tests that don’t necessarily give clear positive or negative results.
When in doubt, retest, Dr. Patricia Engasser said at the annual meeting of the Pacific Dermatologic Association.
Or, if a patch test result is a weak positive – a 1+ with some induration or a palpable component – it’s important to repeat the test to see if the result is reproducible, emphasized Dr. Engasser, an allergy specialist in Menlo Park, Calif. Don’t dismiss a weak positive result, and don’t overplay it, she advised.
Patients have a range of allergies, she explained. Some are very allergic, and the slightest elicitation dose on patch testing will trigger a reaction. Others can tolerate an allergen under certain conditions. “When faced with a questionable reaction, we need to tell patients it’s not straightforward” and we need to investigate further, she said.
Dr. Engasser recommended that, under ideal conditions, dermatologists should wait a month after the first test to repeat patch testing and start counseling. But, for some patients with time constraints, there might only be time to repeat a patch on the arm and let the patient read it later on.
Be aware, too, of possible delayed reactions, especially to corticosteroids, formaldehyde, metals, and antibiotics, Dr. Engasser added. A recent international survey of patch test methodology used by expert groups of allergy specialists found that almost all place the patch test under occlusion for 2 days and do the first reading at 72 or 92 hours (Dermatitis 2009;20:257-60).
“Especially if you read at 72 hours, be prepared to [also] do a delayed reading” and ask the patient to come in again, “or at least tell them we would like to see them back if the site becomes positive,” she said.
Positive reactions to bacitracin, for example, often take 96 hours or longer to develop. The prevalence of bacitracin allergy among U.S. patch-tested patients is on the rise, she said. The allergy panel used in the thin-layer rapid use epicutaneous (TRUE) test does not include bacitracin.
Bacitracin also is the most common topical antibiotic to cause anaphylaxis. “There is no evidence it is indicated for clean surgical wounds, so why are we still using it so much?” Dr. Engasser asked.
Approximately 3% of patients patch tested to corticosteroids will have allergy, and 2.7% of these will have delayed-type allergy, recent studies suggest. “This is probably the most important allergen that we’re semi-aware of. We need to be aware of it, because we miss it” when looking for allergy, she said.
Screening with tixocortol pivalate, budesonide, and hydrocortisone butyrate will detect about 93% of patients allergic to corticosteroids. It’s also important to consider that patients might be allergic to other ingredients in topical corticosteroids such as sorbitan sesquioleate, an emulsifier in high-potency corticosteroids.
She often is asked whether patch testing for other allergies can be performed reliably in someone who has been treated with topical corticosteroids or other immunosuppressants. Older data suggested that 20 mg/day of oral prednisone wouldn’t interfere too much with patch testing, but a recent randomized, double-blind, multicenter cross-over study “really makes us doubt this,” Dr. Engasser said.
Patients who were allergic to nickel were patch tested while on 20 mg/day of oral prednisone or placebo. The prednisone decreased the number of positive reactions and increased the patch test dose needed to elicit a positive reaction (Cont. Derm. 2004;50:298-303).
“So 20 mg seems to indeed interfere with patch testing,” she said.
The data on patch testing patients who are on other biologic immunosuppressant agents are less clear. Use of biologics shouldn’t be considered an absolute contraindication to patch testing, Dr. Engasser suggested, “but it certainly is going to cause some difficulty” in interpreting results. ☐
Disclosures: Dr. Engasser said she had no relevant disclosures.