The clinical distinction between phototoxic and photoallergic contact dermatitis is difficult. Even a biopsy specimen from such a dermatitis rarely provides conclusive evidence. However, the histologic examination of biopsy specimens from photopatch tests in these cases is diagnostic, providing the tests are biopsied at the appropriate time.
Epstein S. Chlorpromazine PhotosensitivityPhototoxic and Photoallergic Reactions. Arch Dermatol. 1968;98(4):354-363. doi:10.1001/archderm.1968.01610160028006