Clinical characteristics of adult patients with autoimmune rheumatic diseases coexisting with persistent isolated low IgM: a retrospective study in China.

OBJECTIVE

Selective IgM deficiency (sIgMD) is an uncommon immunological phenotype characterized by persistently reduced serum IgM, but its characteristics within autoimmune rheumatic diseases (ARDs) remain poorly defined, particularly in Chinese populations. This study aimed to characterize a persistent isolated low IgM phenotype compatible with sIgMD from an ARD-centered perspective, focusing on its clinical features, and infection spectrum in a cohort of 257 patients.

METHODS

A retrospective study of adult inpatients diagnosed with both ARDs and persistent isolated low IgM was conducted. Data on demographics, ARD types, infections, treatments, and laboratory parameters were collected and analyzed.

RESULTS

A total of 257 adult patients with ARDs and coexisting persistent isolated low IgM were included. The median age was 48 (35-60) years, with a female predominance (203/257, 79.0%). Systemic lupus erythematosus was the most common underlying ARD (138/257, 53.7%), followed by primary Sjögren's syndrome (51/257, 19.8%) and rheumatoid arthritis (16/257, 6.2%). Pneumonia was the most frequently observed infection type (125/257, 48.6%). Among the 257 patients, 39 (15.2%) had persistent isolated low IgM identified before exposure to immunosuppressive therapy (treatment-naïve patients), whereas 218 (84.8%) had received immunosuppressive treatment before low IgM was first documented. Overall, 40 of the 257 patients (15.6%) had documented use of intravenous immunoglobulin. Compared with treated patients, treatment-naïve patients had shorter disease duration (72 months vs. 108 months, P = 0.006) and higher IgG (15.0 vs. 12.0 g/L) and IgA (3.0 g/L vs. 2.1 g/L) levels (both P < 0.001). Multivariate analysis demonstrated that older age was associated with a higher likelihood of documented infection (OR = 1.049, P = 0.002).

CONCLUSION

This study provided a comprehensive characterization of persistent isolated low IgM in Chinese adults with ARDs. Our findings suggest that this phenotype may not be solely attributable to immunosuppressive treatment and highlight the need for further studies to clarify its underlying mechanisms and clinical significance, particularly in relation to infection. Key Points • This is the first ARD-centered study to systematically characterize persistent isolated low IgM and represents the largest clinical cohort reported to date. • Persistent isolated low IgM was observed across multiple autoimmune rheumatic diseases and was identified before immunosuppressive therapy in a subset of patients. • Older age was independently associated with documented infection in patients with ARDs and persistent isolated low IgM.