In Vitro Blood Clot Mechanical Properties are Associated with Fibrinogen and White Blood Cell Subtypes in Addition to Hematocrit and Sex

Abstract Background Understanding the role of blood composition in clot mechanics may provide critical cues toward their diagnosis and treatment. To this end, we previously showed that sex and standard blood composition measures explain some variability in clot mechanics, but much remains unaccounted for. Ours and others’ studies ignored the roles of fibrinogen and white blood cell (WBC) subtypes, which is surprising given their physiological importance. Objective To develop a more complete understanding of what determines clots’ mechanical properties, we now study the role of previously untested factors, namely fibrinogen levels and WBC subtypes, in addition to sex and standard blood composition measures. Methods We drew blood from healthy young adults and prepared in vitro clot samples. Using pure shear and mode-I fracture tests, we measured clots’ stiffness, fracture toughness, strength, and work to rupture. We then used linear regressions to quantify how blood composition influences clot mechanical properties, reporting R 2 to assess explanatory power. Results We found that fibrinogen is the strongest predictor of clot mechanical properties, positively correlating with all four metrics and achieving the highest average R 2 . For example, fibrinogen accounts for 70% of the variation in stiffness and 78% in fracture toughness. Among WBC subtypes, neutrophils positively correlate with strength and work to rupture, whereas eosinophils negatively correlate with strength. Conclusion Our work shows that fibrinogen levels and WBC subtypes are previously untested but major determinants of in vitro clot mechanical properties. Future studies should connect these findings to in vivo clot behavior and clinical outcomes.