You may have allergies or a cold. Practice Parameters for Allergy Diagnostic Testing. When should you get an allergy test? If you’re allergic to dust, for example, you can help reduce symptoms by installing proper air filters in your home, getting your air ducts professionally cleaned, and dusting your home regularly. Blood testing and skin prick testing are the most common tests used today to help determine your likelihood of when to retest for allergies an allergy. Even if you think you know what’s in a food, check the label.