Patch test<unk>  can be difficult to diagnose, but its presence can be confirmed by the process of phototesting. There are several forms of these tests including photopatch tests, phototests, photoprovocation tests, and laboratory tests. All of these are necessary to determine the exact infliction that the patient is suffering from. Photopatch tests are patch tests conducted when it is believed that a patient is experiencing certain symptoms due to an allergy that will only occur when in contact with sunlight. After the procedure, the patient is given a low dosage of UVA radiation. Another test known as a phototest is the most useful in identifying<unk> . In this test, one centimeter segments of skin are subject to varying amounts of UVA and UVB radiation in order to determine the specific dosage of the certain form of radiation that causes the urticaria to form. When testing for its less intense form (fixed<unk> , 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 patient's 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<unk> , 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<unk>  is UV radiation or light. SU may be classified based on the wavelength of the radiative energy that causes the allergic reaction; known as Harber's classification, six types have been identified in this system. Type I<unk>  is caused by UVB (ultraviolet B) radiation, with wavelengths ranging from 290–320 nm. Type II is induced by UVA (ultraviolet A) radiation with wavelengths that can range from 320–400 nm. The wavelength range of type III and IV spans from 400 to 500 nm, while type V can be caused by UVB radiation to visible light (280–600 nm). Type VI has only been known to occur at 400 nm. Another classification distinguishes two types. The first is a hypersensitivity caused by a reaction to photoallergens located only in people with SU; while the second is caused by photoallergens that can be found in both people with SU and people without it. A subgroup of<unk> , fixed<unk> , has also been identified. It is a rare, less intense form of the disease with wheals (swollen areas of the skin) that affect certain, fixed areas of the body. Fixed<unk>  is induced by a broad spectrum of radiative energy with wavelengths ranging from 300–700 nm. Differential diagnosis Polymorphous light eruption (PMLE) is the easiest disease to mistake for<unk>  because the locations of the lesions are similar (the V of the neck and the arms). However, patients with SU are more likely to develop lesions on the face. Also, a reaction with PMLE will take a greater amount of time to appear than with<unk> . Lupus erythematosus has been mistaken for SU; however, lesions from lupus erythematosus will take a longer amount of time to go away. Furthermore, when being tested for the two diseases, patients with SU have a reaction immediately while patients with lupus erythematosus will have a delayed reaction. Patients who have experienced<unk> l 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<unk>  because the heat from the sun will cause a person with the disease to have a reaction.
Solar urticaria
Cold urticaria
Heat urticaria
Aquagenic urticaria
Pressure urticaria
Cholinergic urticaria
Delayed pressure urticaria
Physical urticaria