Background. True acantholytic pemphigus belongs to the group of rare bullous dermatoses; it is characterized by a severe course and requires long-term, usually lifelong use of systemic glucocorticosteroids. Diagnosis of pemphigus remains one of the most difficult issues of modern dermatovenereology, so the availability of sensitive and specific laboratory techniques seems to be particularly relevant.
Description of the clinical case. In a 70-year-old patient G., rapidly opening blisters with a thin cover were formed against the background of moderate itching of the skin within 3 months. Crusts appeared on the surface of erosions, under which spontaneous epithelialization with the formation of secondary hyperpigmentation occurred. A dual cytological examination did not reveal acantholytic cells; eosinophils 3–5 in the field of view and leukocytes 10–15 in the field of view were detected. A dual histological examination concluded that the pathomorphological picture corresponded to pyoderma gangrenosum or bacterial infection. An indirect immunofluorescent test revealed IgG autoantibodies to desmoglein 1, which made it possible to verify the diagnosis of seborrheic pemphigus. The prescribed systemic glucocorticosteroid therapy led to remission.
Conclusion. The described clinical case demonstrates the difficulties of differential diagnosis of bullous dermatoses and proves that compared to routine cytological and histological methods, which are estimated subjectively, more modern and objective laboratory tests (direct and indirect immunofluorescence), which allow to identify circulating or fixed in the epidermis IgG autoantibodies to surface antigens of epidermocytes have greatest diagnostic value today.

Keywords: immunofluorescent test, seborrheic pemphigus, bullous dermatoses, acantholytic pemphigus, glucocorticosteroids