Low IgM Levels in Adult IEI: Classification Challenges and Clinical Implications

Reduced serum IgM is increasingly encountered in adults evaluated for inborn errors of immunity (IEI), yet its clinical relevance and diagnostic utility remain uncertain. While some patients meet criteria for selective IgM deficiency (SIgMD), others remain unclassified, reflecting significant diagnostic heterogeneity. This study aimed to characterize adult IEI patients with low serum IgM and to assess the prognostic value of IgM levels for clinical outcomes. Among 378 adult IEI patients followed at a tertiary immunology center, 43 individuals (11.4%) with low serum IgM were included. Clinical, immunological, genetic, and treatment data were analyzed, and associations between IgM levels and clinical outcomes were assessed using regression and ROC analyses. The median age was 58 years, and most patients were referred following incidental detection of hypogammaglobulinemia. Recurrent infections and allergic diseases were the most common manifestations and generally preceded IEI diagnosis. While most clinical features occurred before diagnosis, hepatobiliary disease, lymphoproliferation, osteoporosis, and malignancies showed variable timing. Median IgM memory B-cell and naïve CD8⁺ T-cell percentages were below the reference range, with reductions observed in 52.9% and 53.8% of patients, respectively, and elevated IgE levels in 35.7%. Pathogenic or likely pathogenic variants were identified in a subset of patients, often without clear genotype–phenotype correlation. Serum IgM levels demonstrated limited predictive value for clinical outcomes. Low serum IgM in adults reflects a heterogeneous immunodysregulatory state rather than a uniform immunodeficiency. Serum IgM levels alone provide limited discriminatory and prognostic value, underscoring the need for integrated clinical and immunological assessment in diagnosis and management.