How do they do allergen tests for eczema?

Eczema & Allergy. There are 2 types of dermatitis or eczema caused by allergy - contact dermatitis and atopic dermatitis. Contact dermatitis is like the rash you get with poison ivy and testing consists of a number of patches that are worn for 48 - 72 hours. Atopic dermatitis can be caused by food allergy and testing is done by skin prick with a drop of each suspected food and left on for 20 minutes.