Background Since the initial recognition of autoimmune pancreatitis (AIP), elevated serum IgG4 levels and increased numbers of IgG4-positive plasma cells in tissues have been regarded as diagnostic hallmarks of the disorder. However, the role of the IgG4 level in other non-AIP IgG4-RD is unclear.

Objectives To investigate the clinical features and factors associated with IgG4 test positivity over an 8-year period in 166 patients with IgG4-related diseases (IgG4-RD).

Methods The medical records of 2,968 adult subjects who underwent serum IgG4 level tests conducted between December 2007 and August 2014 were reviewed.

Results Among 2,968 patients, 166 (5.6%) were diagnosed with IgG4-RD and 2,802 (94.4%) were determined to have other diseases (non-IgG4-RD group). The mean IgG4 level in subjects with IgG4-RD was 1087.5 mg/dL (median: 597.0, range: 15–6790 mg/dL), which was significantly higher than that of subjects with non-IgG4-RD (mean: 104.1, median: 75.0, range: 1–4400 mg/dL; p<0.0001). Among the 166 IgG4-RD patients, the mean age was 58.5 ± 15.9 years (range: 21–87). Of 166 patients with IgG4-RD, 87 (52.4%) had an IgG4-RD other than autoimmune pancreatitis (non-AIP IgG4-RD). Twenty-seven out of 166 IgG4-RD (16.3%) patients had other organ involvement (OOI) outside of the pancreatobiliary system. IgG4 levels were elevated (>135 mg/dL) in 143 (86.1%) of 166 patients with IgG4-RD and in 652 (23.3%) of 2,802 patients with non-IgG4-RD (p<0.001). The present study showed that the best IgG4 cutoff was 222 mg/dL for the diagnosis of IgG4-RD.

Conclusions In the present study, we demonstrated that more than half of the diseases associated with IgG4-RD were non-AIP IgG4-RD. Among the 166 IgG4-RD patients, the manifestations covered the full range of organ system involvement. Moreover, the best IgG4 cutoff (measured with nephelometry using a Siemens BN ProSpec instrument and Siemens reagent) was 222 mg/dL for IgG4-RD.

Disclosure of Interest None declared