Allergy Skin Testing-On the day of testing the back and upper arms should be clean and free of lotion, perfumes, and powders. Allow at least 1 1/2 to 2 hours for the time with the provider, skin testing, and summary. The skin (usually your back) is pricked with a small amount of each item you may be allergic to (allergen) when skin testing is done. If you are allergic, a bump and redness appears where the skin was pricked. The skin test is measured for swelling and redness. A large enough skin reaction is a positive test. If the scratch test is negative, a higher strength may be given in the upper arm. Small amounts of certain allergens are injected under the skin similar to a TB test. The skin is measured for swelling and redness. A large enough skin reaction is a positive test which will be reviewed with you by the provider.