Mind the Gap: Influence of Monoclonal Immunoglobulin on Anion Gap in Multiple Myeloma

Abstract Background Multiple myeloma (MM) is a plasma cell disorder in which anion gap and sodium measurement are critically influenced by the presence of abnormal protein concentrations. This study investigates the relationship between anion gap, pseudohyponatremia, and monoclonal immunoglobulin levels (IgG, IgA, IgM), particularly the common IgG. Focusing on the clinical implications of these laboratory results. Methods We retrospectively analyzed data from 136 patients diagnosed with MM, focusing on anion gap values, monoclonal immunoglobulin levels, and sodium levels. The patients were categorized based on their anion gap values (<8) into five groups (anion gap=3, 4, 5, 6, and 7). Monoclonal immunoglobulin levels were standardized to the same units as the anion gap for direct comparison. Additionally, we investigated the pseudohyponatremia by examining sodium levels in these patients. Results Among the 136 MM patients, 70 patients’ anion gap were lower than 8. A negative correlation between MM monoclonal immunoglobulins and anion gap values were observed. The anion gap decreased from 7 to 3 as the IgG average levels ranged increasing from 1755 mg/dL to 2655 mg/dL (RI 610-1616 mg/dL). Similarly, the anion gap decreased from 6 to 3 as the IgM average levels ranged increasing from 1170 mg/dL to 2880 mg/dL (RI 35-242 mg/dL). Additionally, a number of low sodium levels (<136 mmol/L) was observed in each group. Conclusion Our study highlights the negative impact of MM monoclonal immunoglobulins, particularly IgG and IgM, on the anion gap. Both laboratory and clinical personnel should be aware of these phenomena, might prevent unnecessary diagnosis, streamlining patient management and avoiding potential mismanagement.