Soluble BCMA as a biomarker reflecting tumor volume and treatment response in Waldenström macroglobulinemia

We evaluated serum soluble B-cell maturation antigen (sBCMA) levels in 67 Waldenström macroglobulinemia (WM) patients, healthy controls, and individuals with IgM-MGUS, IgM-multiple myeloma, other B-cell neoplasms, and reactive hypergammaglobulinemia. WM patients had higher sBCMA levels (median 100 ng/mL) compared to healthy controls (41.4 ng/mL, p < 0.001). Symptomatic WM patients had a higher level of sBCMA than asymptomatic patients (133 vs 73.8 ng/mL, p < 0.001). Asymptomatic WM did not progress to symptomatic disease when sBCMA was in MGUS level (<65 ng/mL). There was strong correlation between bone marrow CD20+ cell counts and sBCMA levels (ρ = 0.73), while serum IgM levels showed weaker correlation (ρ = 0.55). sBCMA levels demonstrated parallel changes with IgM levels when assessing treatment efficacy. Notably, IgM flares following rituximab administration or plasma exchanges had a minimal impact on sBCMA levels. These findings suggest that sBCMA is a decent biomarker of tumor burden, offering a reliable predictor of clinical outcomes in WM patients.