Distribution characteristics of elevated serum immunoglobulin G₄ (IgG₄) and its relationship with IgG₄-related...

Objective: To elucidate the distribution characteristics of elevated serum immunoglobulin G₄ (IgG₄) and its relationship with immunoglobulin G₄-related disease (IgG₄-RD).Methods: In total, 14 522 unique patients who were tested for serum IgG₄ over a 4.5 year period were enrolled. Demographic and serological characteristics of all patients, and the range of diseases and serological characteristics of patients with elevated serum IgG₄, were retrospectively analysed. Organ involvement in IgG₄-RD patients and the utility of serum immunoglobulin for distinguishing IgG₄-RD from non-IgG₄-RD were also investigated.Results: Elevated serum IgG₄ (> 1.4 g/L) was observed in 1793/14 522 patients (12.3%) and primarily appeared in middle-aged and elderly men. Only 7.3% of patients with elevated serum IgG₄ met the diagnostic criteria for IgG₄-RD. Elevated serum IgG₄ was also found in over 30 types of non-IgG₄-RD. Serum IgG₄ levels were higher in IgG₄-RD patients with multiple-organ involvement and were positively correlated with the number of affected organs. The diagnostic utility of serum IgG₄ for IgG₄-RD was higher than that of the IgG₄/IgG ratio and serum IgG. The optimal serum IgG₄ cut-off value for diagnosing IgG₄-RD was 2.06 g/L, with a sensitivity of 95.6% and specificity of 94.7%.Conclusions: Elevated serum IgG₄ may be present in multiple non-IgG₄-RD conditions. However, a high serum IgG₄ concentration is of great value for IgG₄-RD diagnosis, and there appears to be a positive relationship between serum IgG₄ levels and organ involvement in patients with IgG₄-RD. Examining the IgG₄/IgG ratio and serum IgG does not improve the ability of IgG₄-RD diagnosis.