Diagnostic performances of serum IgG4 concentration and IgG4/IgG ratio in IgG4-related disease.

Patients with IgG4-related disease (IgG4-RD) often have elevated serum IgG4 levels. Here, we aimed to evaluate the diagnostic performances of elevated serum IgG4 concentration and IgG4/IgG ratio for IgG4-RD. We retrospectively analyzed 1381 patients subjected to serum IgG subclass testing to differentiate IgG4-RD from other diseases at Peking University People's Hospital from 2012 to 2016. This sample included 133 IgG4-RD patients and 1248 non-IgG4-RD patients. Serum IgG subclass concentrations were measured using Siemens reagents. The median values (25th-75th percentile) for serum IgG4 concentration and IgG4/IgG ratio, respectively, were 8640 (3970-17750) mg/L and 0.339 (0.229-0.517) in IgG4-RD patients and 450 (220-920) mg/L and 0.032 (0.014-0.061) in non-IgG4-RD patients (p < 0.001). For distinguishing IgG4-RD from non-IgG4-RD, the optimal cut-off values of IgG4 and IgG4/IgG were 2100 mg/L and 0.114, respectively. The corresponding area under the curve (AUC) values were 0.964 and 0.970, respectively. Comparison of the receiver operating characteristic curves revealed a significant difference between these AUC values (p = 0.002). The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), respectively, were 94.7, 91.6, 54.5, and 99.4% for the IgG4 optimal cut-off value and 96.2, 92.1, 56.4, and 99.6% for the IgG4/IgG optimal cut-off value. Our results confirmed that elevated serum IgG4 concentration and IgG4/IgG ratio were of great value for IgG4-RD diagnosis.