- with a positive prick skin test (diameter of wheal > 3 mm) to at least one perennial allergen (e.g. dust mite, animal dander, cockroaches), within the past 5 years or at Visit 1, to which the patient will be exposed on a regular basis (most days) for the duration of the study. A RAST test may be performed for patients with a borderline skin prick test result.