When testing for its less intense form (fixed solar urticaria), phototesting should be conducted only in the areas where the hives have appeared to avoid the possibility of getting false-negative results.A third form of testing is the photoprovocation test which is used to identify disorders instigated by sun burns. The process of this test involves exposing one area of a patients arm to certain dosage of UVB radiation and one area on the other arm to a certain dosage of UVA radiation. The amount of radiation that the patient is exposed to is equal to that "received in an hour of midday summer sun." If the procedure produces a rash, then the patient will undergo a biopsy. Finally, there are laboratory tests which generally involve procedures such as blood, urine, and fecal biochemical tests. In some situations, a skin biopsy may be performed. Classification
Solar urticaria, due to its particular features, is considered to be a type of physical urticaria or light sensitivity. Physical urticaria arises from physical  factors in the environment,  which in the  case of solar urticaria is   UV radiation or light. SU may be classified based on the wavelength of the radiative energy that causes the allergic reaction; known as Harbers classification, six types have been identified in this system. Type I solar urticaria is caused by UVB (ultraviolet B) radiation, with wavelengths ranging from 290 to 320 nm. Type II is induced by UVA (ultraviolet A) radiation with wavelengths that can range from 320 to 400 nm.
Patients who have experienced solar urticarial symptoms from a young age could mistakenly be thought to have erythropoietic protoporphyria. However, the main symptom for this disease is pain and patients with have been found to have abnormal levels of protoporphyrin in their blood while these levels are normal in SU patients. Finally, cholinergic urticaria, or urticaria induced by heat, can occasionally appear to be solar urticaria because the heat from the sun will cause a person with the disease to have a reaction. Management
Antihistamines
Histamines are proteins associated with many allergic reactions. When the UV radiation or light comes in contact with a person with solar urticaria, histamine is released from mast cells. When this occurs, the permeability of vessels near the area of histamine release is increased. This allows blood fluid to enter the vessels and cause inflammation. Antihistamines suppress the activity of the histamine.Diphenhydramine, a first-generation H1 receptor antagonist or medicine that combats the H1 receptor that is associated with many allergic reactions, has been found to be the most potent antihistamine for this particular disease. Patients prescribed 50 milligrams four times per day have been able to sustain normal exposure to the sun without developing a reaction. Patients with less potent forms of solar urticaria such as fixed solar urticaria can be treated with the medication fexofenadine, which may also be used prophylactically to prevent recurrence.