Monoclonal Gammopathy of Thrombotic Significance.

Monoclonal gammopathy of undetermined significance (MGUS) is an asymptomatic premalignant precursor to multiple myeloma (MM). While development of end-organ damage in MM largely reflects increasing clonal burden, emerging evidence indicates that qualitative properties of the monoclonal immunoglobulins in MGUS can be directly pathogenic, leading to serious organ injury independent of disease burden. This has led to the recognition of a broader spectrum of disorders collectively termed monoclonal gammopathy of clinical significance (MGCS). MGCS can be further subclassified based on the organ system involved; for example, monoclonal gammopathy of renal significance specifically refers to monoclonal immunoglobulin-driven renal injury. Along these lines, we propose monoclonal gammopathy of thrombotic significance (MGTS) as encompassing thrombotic disorders in which there is definitive, mechanistically supported evidence that a monoclonal (M)-protein associated with a clonal plasma or B-cell disorder directly contributes to thrombosis. Based on current evidence, monoclonal protein-induced immune thrombocytopenia and thrombosis is the only disorder that meets our criteria for MGTS. Other thrombotic disorders, such as thrombotic microangiopathy and antiphospholipid syndrome, may occur in the setting of a monoclonal protein and have biologically plausible M-protein-mediated mechanisms; however, direct evidence implicating the M-protein in thrombosis is currently lacking. Accordingly, we provisionally classify these disorders as thrombotic syndromes associated with M-proteins, pending causal validation. We also highlight thrombotic disease associations with multifactorial pathogenesis that should not be classified as MGTS. Broader recognition of MGTS is essential to advance consensus definitions, establish diagnostic criteria, and develop evidence-based management strategies for this clinically important group of disorders.