Abstract
IgG4 related disease (IgG4-RD) represents a recently characterized multi-systemic condition which can manifest a spectrum of skin findings (IgG4-related skin disease; IgG4-RSD), details of which are emerging, and for which dermatopathologic criteria have been proposed. The specificity of these dermatopathologic criteria, however, merit scrutiny given the potential histopathologic overlap of IgG4-RSD and both neoplastic and inflammatory skin conditions featuring lymphoplasmacytic infiltrates (IgG4-RSD mimics). This study sought to assess the specificity of histopathologic and immunophenotypic thresholds for IgG4-RSD by quantifying the frequency by which these IgG4-RSD mimics meet these thresholds. Following IRB approval, a total of 68 cases of IgG4-RSD mimics, representing thirteen different diagnoses boasting plasma cells (including necrobiosis lipoidica, morphea, necrobiotic xanthogranuloma, among others) were reviewed and analyzed for both histopathologic findings and immunohistochemical features, specifically whether three thresholds were met: (1) Maximum IgG4 count/hpf > 200; (2) IgG4/IgG ratio > 0.4 averaged over 3 hpf; (3) IgG4>10/hpf averaged over 3 hpf. Analysis revealed that no cases exceeded 200 IgG4+ cells; 18% (12/68) of cases, representing 7 of 13 diagnoses, demonstrated an IgG4/IgG ratio greater than 0.4 averaged over three high-powered fields (hpfs); and 28% (19 of 68) specimens, representing 8 of 13 diagnoses, demonstrated an IgG4+ cell count greater than 10 averaged over three hpfs. These results suggest that both an IgG4+ cell count of >10 and an IgG4/IgG ratio > 0.4 per hpf are not specific to cutaneous IgG4-RD, and that diagnosis of IgG4-RSD requires careful clinical, histopathologic, immunophenotypic, and serologic integration.Financial Disclosure: No current or relevant financial relationships exist.