Frequently Asked Questions in Dermatology: Contact Dermatitis

Selected by Pietro Cazzola

Contact dematitisQuestion: Can patch testing be performed on prednisone?

Answer: Ideally patch testing is performed when a patient is completely off prednisone for at least 1-2 weeks. However, sometimes it is necessary to patch test while a patient is still on prednisone. If this is the case, the goal is to get the patient to the lowest dose possible that still allows for clear enough skin to apply the patches, this is often about 20 mg/day.  Patch testing on prednisone may result in a false negative test as the steroids may blunt a hypersensitivity response but if the patient is allergic to an allergen applied through patch testing and still reacting through the prednisone, hopefully they will mount an allergic reaction at the site of the allergen.
Topical steroids to the site of patch applications should also be avoided for at least a week before the allergens are applied.

Question: Can patch testing be done while on antihistamines?

Answer: Patch testing may be performed while a patient is on antihistamines as they do not effect the Type IV delayed type hypersensitivity reaction. This is different than prick testing when the patient must stop antihistamines as they do blunt a Type I reaction.  If the concern is contact urticaria then antihistamines should be stopped.

Question: Do you need to do a second reading?

Answer: Patch testing should only be conducted if the patient can commit to three visits, patch application, patch removal at 48 hours and a second follow up visit 3- 7 days later. If a second reading is not performed, delayed allergens may be missed and up to 30 % of allergens will not be detected.

Question: What allergens should I use for patch testing?

Answer: The FDA approved allergens are 36 in number and come as a preprepared series from TRUE Test. Many contact dermatitis specialists use individual allergens available from outside the country- Canada or Sweden. These allergens are hundreds in number and cover many different specialty series, for example acrylates, hair dressing, rubber series, etc. The North American Contact Dermatitis Group has a series of core allergens as does the American Contact Dermatitis Society.

Question: What do you tell patients after patch testing?

Answer: The education of patients after patch testing is very important to the success of the patch testing procedure. Once allergens are identified, patients need to be educated as to what the allergens are they tested positive to, where they are found and how they can avoid them. Physicians must instruct their patients on the names, synonyms of the allergens the patient is allergic to as well as how to label read and avoid the known allergens. The Contact Allergen Management Program Database is a very helpful resource that is available to members of the American Contact Dermatitis Society. It allows the physician to enter the allergens a patient is allergic to into a database and a list is produced that contains products free of the known allergens. It also eliminates cross reacting chemicals. The patient can then use the list to find products that are safe to use.

Question: How do you find products free of the allergens that a patient tests positive to?

Answer: The CAMP database allows physicians to provide patients with a list of products free of the allergens the patient is allergic to. Patients should also be instructed how to label read so they can check labels of their personal care products and make sure they are free of their known allergens.

Christen Maria Mowad