Under flow, increasing fibrinogen γ′ levels within the (patho)physiological range increase platelet-dependent fibrin deposition at arterial and venous shear. Fibrinogen-deficient plasma repleted with increasing fibrinogen γ′ (3%, 10%, 30%) or selected LEADS plasma samples with high (n = 5, 30%) or low (n = 5, 3%) γ′ were flowed over preformed platelet aggregates at arterial (500 s⁻¹) and venous (150 s⁻¹) shear rates, and fibrin accumulation was quantified by fluorescence over time. (A-D,I-J,M-N) Arterial shear. (E-H,K-L,O-P) Venous shear. (A-B,E-F,I,K,M,O) Repleted plasma. (C-D,G-H,J,L,N,P) LEADS plasma. (A,C,E,G) Fibrin formation originating from platelet thrombi over time, measured by the accumulation of fluorescent fibrinogen. (B,D,F,H) Time to first fibrin deposition around platelet thrombi. (I-L) Maximum fibrin formation measured by maximum fluorescence. (M-P) Average rate of fibrin formation. Data are mean ± SD. *P < .05, **P < .01, ***P < .001, 1-way ANOVA followed by Tukey’s multiple-comparisons test (B,F,I,K,M,O); unpaired Student t test (D,H,J,L,N,P). AU, arbitrary unit; Fluor, fluorescence.