The diagnostic value of serum IgG4 for the diagnosis of IgG4-related disease: and is that so great?

IgG4-related disease (IgG4-RD) is a systemic immune mediated condition that is characterized by the formation of tumor-like fibroinflamma-tory foci in different organs and by the elevation of serum and tissue IgG4 levels in the majority of patients. The pathogenesis of the disease, including the role of IgG4, has not been established exactly. Serum and tissue IgG4 hypersecretion is a nonspecific sign and occurs in many rheumatic, infectious, and malignant diseases.

Objective: to determine the range of nosological entities associated with the increase in serum IgG4 levels, as well as the frequency and nature of this increase in patients with IgG4-RD.

Patients and methods. The results of all serum IgG4 measurements carried out in the Laboratory of Immunology and Molecular Biology of Rheumatic Diseases, V.A. Nasonova Research Institute of Rheumatology, in 2017—2018 were analyzed. Serum IgG4 parameters were separately estimated in 52 patients with verified IgG4-RD according to the universal diagnostic criteria proposed by H. Umehara et al (2011).

Results and discussion. In 2017—2018, a total of247patients were tested for serum IgG4 levels. The latter were elevated in 76 (30.8%) patients, but only 28 (36.8%) were diagnosed as having IgG4-RD. Along with IgG4-RD, antineutrophilic cytoplasmic antibody (ANCA)-associated vasculitis, rheumatoid arthritis (RA), and systemic lupus erythematosus (SLE) were characterized by increased serum IgG4 levels. The highest median IgG4 level was found in patients with a verified diagnosis of IgG4-RD: 6.31 vs 3.2, 3.22, and 2.69 g/L in ANCA-associated vasculitis, RA, and SLE, respectively.

In 52 patients with IgG4-RD, the IgG4 level >1.35 g/L was found in 88% of cases. The median serum IgG4 level was 3.45 g/L (2.1; 11.4). The highest level was observed in patients with generalized lymphadenopathy and in those with IgG4-related sialoadenitis and dacryoadenitis (Mikulicz disease). The serum IgG4 level was positively correlated with the number of affected organs (Spearman's correlation coefficient, 0.39; p=0.0056, Student's t-test). All the patients showed a tendency towards decreasing serum IgG4 levels during treatment regardless of its clinical response; however, the levels returned to normal only in 73% after 12 months of treatment.

Conclusion. The increased serum and tissue IgG4 concentration is not specific, but at the moment it is the only disease marker available in clinical practice. To correctly interpret the diagnosis, it is necessary to assess the entire set of clinical manifestations, imaging data, and morpholopathological findings.

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