C3 and acquired polycythemia vera: Direct immunofluorescence examination of biopsy samples taken in perilesional areas is considered the “gold standard” to confirm PV diagnosis, revealing intercellular deposits of IgG and/or C3 on the surface of keratinocytes in the epidermis or mucosa (Harman et al., 2017; Joly et al., 2020), but with the disadvantage of requiring invasive sampling.