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-, rheumatoid 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 selecting 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.