Microfluidic assay of hemophilic blood clotting: distinct deficits in platelet and fibrin deposition at low factor levels

Summary Background Coagulation factor deficiencies create a range of bleeding phenotypes. Microfluidic devices offer controlled hemodynamics and defined procoagulant triggers for measurement of clotting under flow. Objectives We tested a flow assay of contact pathway‐triggered clotting to quantify platelet and fibrin deposition distal of dysfunctional thrombin production. Microfluidic metrics were then compared with PTT or % factor activity assays. Methods Whole blood ( WB ) treated with low level corn trypsin inhibitor (4 μg  mL −1 ) from nine healthy donors and 27 patients (deficient in factor [F] VIII , 19 patients; FIX , one patient; FXI , one patient; VWF , six patients) was perfused over fibrillar collagen at wall shear rate = 100 s −1 . Results Using healthy WB , platelets deposited within 30 s, while fibrin appeared within 6 min. Compared with healthy controls, WB from patients displayed a 50% reduction in platelet deposition only at < 1% factor activity. In contrast, striking defects in fibrin deposition occurred for patients with < 13% factor activity (or PTT > 40 s). Full occlusion of the 60‐μm high channel was completely absent over the 15‐min test in patients with < 1% factor activity, while an intermediate defect was present in patients with > 1% factor. Conclusion Spontaneous bleeding in patients with < 1% factor activity may be linked to deficits in both platelet and fibrin deposition, a risk known to be mitigated when factor levels are raised to > 1% activity ( PTT of ~40–60 s), a level that does not necessarily rescue fibrin formation under flow.