A sudden onset of urticaria, which lasts a short time, is quite common and is called acute urticaria. Often the rash is sudden in onset and may resolve within hours or days. Usually there is a clear cause or association with food, insect sting, medication, contact with a pet, or some other specific cause. However chronic urticaria or persistent hives is different. It is defined as hives that occur at least 3 times a week for more than six weeks in a row. Unfortunately, more than 75% of cases of chronic urticaria cannot be linked to any specific allergy trigger, even after extensive investigation and the involvement of qualified specialists. Therefore chronic urticaria can occur without clearly being caused by food, medicine, animal allergy, pollen, dust, chemicals, or any other identified factor. Testing for a possible cause can be performed by an allergist for individuals with chronic urticaria in order to correctly diagnose allergy in people who do have allergic urticaria. If you are diagnosed with chronic urticaria that appears to be non allergic in origin, it is usually not necessary to begin diet restriction or an extensive home clean up. Fortunately even urticaria without a clear cause often can be treated successfully with proper medication.