Fibrin Monomer in Thrombosis and Haemostasis: A Clinical Biomarker and Beyond.

Fibrin monomer (FM) is a transient intermediate of blood coagulation that functions as both an active regulator of haemostasis and a sensitive biomarker for prothrombotic states. Clinically, FM is measured indirectly as its derivative, soluble fibrin monomer complexes (SFMC), which is also often referred to as FM throughout the clinical literature. FM participates in a complex regulatory network modulating thrombin generation and fibrinolysis, interacting with platelet receptors, including integrin αIIbβ3 and GPVI, and engaging GPIb-vWF interactions. This comprehensive review examines FM's molecular mechanisms in haemostatic regulation and evaluates clinical evidence for FM as a biomarker. Particular focus is placed on FM's utility for risk stratification across thrombotic conditions, including disseminated intravascular coagulation, venous thromboembolism, ischemic stroke, myocardial infarction, and COVID-19-associated coagulopathy. Current challenges, including assay standardization and universal cut-off values, are discussed. By synthesizing mechanistic insights with clinical data, this integrated perspective may accelerate the translation of FM biology into improved risk assessment tools and novel therapeutic strategies.