How is patch testing done?

Because ICD and ACD can be indistinguishable both clinically and histologically, patch testing is the only method available to diagnose ACD and differentiate it from ICD. Two patch test methods are currently in widespread use: the Finn chamber (Fig. 9-4A) and True Test systems (Fig. 9-4B). With the Finn chamber method, a small amount of the allergen, usually in a petrolatum vehicle, is placed into individual aluminum wells affixed to a strip of paper tape. With the True Test method, no advance preparation is necessary, as the allergens have already been commercially incorporated into the back of the paper tape strips. Only 28 “screening” allergens are currently available with the True Test, while hundreds are available with the Finn chamber method. These strips are applied to the patient’s upper back, which is the preferred testing site. After 48 hours, the patches are removed and the initial reading is recorded. Because these allergic reactions are delayed, a second interpretation must be performed at 72 hours, 96 hours, or even at 1 week after the initial test application. Additional readings beyond 48 hours increase the positive patch test yield by 34%. The classic positive allergic patch test reaction shows spreading erythema, edema, and closely set vesicles that persist after removal of the patch or that appear after 2 to 7 days. Irritant reactions may have a glazed, scalded, follicular, or pustular appearance that usually fades after the patch is removed.

Davis MD, Bhate K, Rholinger AI, et al: Delayed patch test reading after 5 days: the Mayo Clinic experience, J Am Acad Dermatol 59:225–233, 2008.

Rietschel RL, Adams RM, Mailbach HI, et al: The case for patch test readings beyond day 2, J Am Acad Dermatol 18:42–45, 1988.