Increasing fibrinogen γ′ levels within the (patho)physiological range decrease fiber thickness and prolong lysis in static conditions. The effects of the percentage of fibrinogen γ′ on clot formation and breakdown were assessed in purified experiments in the presence (n = 3) or absence of FXIII (n = 3), as well as in patient plasma samples (low γ′, n = 41; normal γ′, n = 45; high γ′, n = 33) using turbidity and lysis assays. (A) Maximum optical density. (B) Average rate of clot formation. (C) Time to 50% lysis. (D) Average rate of clot lysis. Data are mean ± SD. *P < .05, **P < 01, ***P < .001, ****P < .0001, 1-way ANOVA followed by Tukey’s multiple-comparisons test.