High detection rate for perivascular deposits of immunoglobulins in immune complex vasculitis from biopsies of early macular lesions

Background: In immune complex vasculitis the detection of perivascular immunoglobulins by direct immunofluorescence (DIF) not only helps to confirm the diagnosis, but also to define the type of vasculitis (e.g., IgA-, IgG/IgM-, rheuma- toid or cryoglobulinemic vasculitis). The value of DIF, though, has been questioned due to the heterogeneous yield of positive reactions in various studies. One major reason for a negative DIF is a biopsy of older lesions. To ensure selection of fresh lesions, we consistently apply morphological criteria: partially blanchable macules with only a minor petechial and papular component and in proximity to palpable or retiform purpura. This study aimed to evaluate retrospectively the detection rate attainable by this procedure. Patients and Methods: In our department, we identified 56 patients from 2017– 2024 with histologically and clinically confirmed immune complex vasculitis from whom a corresponding biopsy had been obtained. Results: 92.9% of these patients showed perivascular deposition of at least one immunoglobulin (mostly IgA (85,7%), with or without IgG or IgM, 7,1% showed no IgA, but IgG or IgM). Biopsies positive only for C3 were considered negative. Of the IgA-positive patients 15% had a systemic, 83% a skin-limited IgA-vasculitis and 2% recurrent macular vasculitis. Conclusions: When using defined morphological or clinical criteria for select- ing appropriate biopsy sites, DIF demonstrates high sensitivity in identifying the nature of perivascular immunoglobulins in immune complex vasculitis and may serve as a valid criterion in diagnostic algorithms.