The diagnosis is usually made on the basis of the clinical history. The following investigations are sometimes indicated:
• Lupus serology when cutaneous lupus erythematosus is considered in the differential diagnosis, particularly if treatment with prophylactic phototherapy is considered, antinuclear antibody and anti-Ro and La antibodies should be requested.12
• Histopathology when a superficial and deep, perivascular, dermal inflammatory infiltrate is seen. Histopathology and direct immunofluorescence can help differentiate PLE and lupus erythematosus.13,14
• Phototesting Monochromator phototesting is usually normal in PLE, but can be useful in excluding solar urticaria or chronic actinic dermatitis if these are considered possible alternative, or concomitant, diagnoses. Repeated irradiation provocation testing to 4 x 4 cm or larger areas is positive in a proportion (<50% in some series) of patients, but can be helpful in cases of diagnostic uncertainty.
• Patch testing and photopatch testing to sunscreens. These are useful when sunscreen photoallergy or contact allergy is suspected as a co-existent diagnosis.15-18
• Porphyrin plasma spectrofluorimetry Cutaneous porphyrias occasionally feature as differential diagnoses, and can be excluded if this simple test is negative.
• HLA class II typing This can help to distinguish actinic prurigo (see below).19,20
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