Investigation of IgG4‐positive cells in idiopathic multicentric Castleman disease and validation of the 2020 exclusion criteria for IgG4‐related disease

Patients with plasma cell type idiopathic multicentric Castleman disease (PC‐iMCD) often show elevated serum IgG4 levels and IgG4‐positive cell infiltration in tissues due to overproduction of interleukin‐6, and may meet the diagnostic criteria for IgG4‐related disease (IgG4‐RD). Although PC‐iMCD has been listed as a major exclusion disease for IgG4‐RD, distinguishing between these diseases is challenging due to a lack of highly specific diagnostic biomarkers. In 2020, we proposed exclusion criteria of IgG4‐RD mimickers. In this paper, we validated the accuracy of the criteria in excluding one of the mimickers, PC‐iMCD, from IgG4‐RD. Validation was performed on 57 PC‐iMCD patients (39 presenting lymph node lesions and 19 with lung lesions) and 29 IgG4‐RD patients (22 presenting lymph node lesions and seven with lung lesions). According to our results, 20.5% of the PC‐iMCD patients with lymph node lesions and 42.1% of those with lung lesions met the diagnostic criteria for IgG4‐RD. All these patients with PC‐iMCD were excluded from a diagnosis of IgG4‐RD by the proposed criteria. Additionally, 6.9% of IgG4‐RD patients met the exclusion criteria. Thus, if the exclusion criteria are met, diagnosis should be made based on a combination of findings including organ distribution of disease, response to steroid therapy, and other pathological findings.