High serum IgA levels in patients with IgG4-related disease (IgG4-RD) are associated with mild inflammation, sufficient disease-specific features, and favorable responses to treatments

Although elevated serum IgA levels are thought to exclude a diagnosis of IgG4-related disease (IgG4-RD), IgG4-RD has been definitively diagnosed in some patients despite elevated serum IgA levels. This study aimed to clarify the prevalence of elevated IgA levels in patients with IgG4-RD and to compare the clinical features of IgG4-RD patients with and without elevated IgA levels.The clinical features of 169 IgG4-RD patients were retrospectively compared among those with and without elevated serum IgA levels.Of the 169 patients with IgG4-RD, 17 (10.1%) had elevated serum IgA levels. Those with elevated serum IgA levels showed higher serum CRP levels and lower prevalence of relapse than those without. Other clinical features did not differ significantly, including inclusion scores of the ACR/EULAR classification criteria. Cox regression analysis showed that elevated serum IgA levels were associated with a lower incidence of relapse. Moreover, patients with elevated serum IgA levels showed prompt improvement in response to glucocorticoids in the IgG4-RD responder index.Some patients diagnosed with IgG4-RD have high serum IgA levels. These patients may form a subgroup, characterized by good response to glucocorticoids, less frequent relapse, mildly elevated serum CRP levels, and possible complications of autoimmune diseases.