Thrombin-triggered clotting. (A) Thrombin was added to platelet-poor plasma. Clotting times were 31.4% shorter in STAT5-deficient mice (○) compared with control (●; n = 8,6; *P < .0001; Student t test). (B) Thrombin-triggered fibrin clot formation and firmness were measured over time in whole blood isolated from STAT5-deficient (- - -) and control mice (—) using a thromboelastometer. This experiment was performed 3 times, and a representative curve is shown. The clotting times were shorter (P = .0002; n = 3; Student t test) and maximal clot firmness was greater (P = .0316; n = 3; Student t test) in STAT5-deficient plasma compared with control. (C) STAT5-deficient (○) and control (●) defibrinated plasma were supplemented with human fibrinogen to achieve a final concentration of 3.0 mg/mL. Thrombin-triggered clot times were 25.4% shorter in STAT5-deficient versus control samples (n = 3; *P = .0289; Student t test). (D) Thrombin-triggered fibrin clot formation and firmness were measured as in panel B using defibrinated pooled (n = 3) STAT5-deficient (- - -), defibrinated pooled control (n = 3; —), and defibrinated normal human plasma (gray —) mixed 1:1 with normal human plasma. This experiment was performed 2 times, and a representative curve is shown. The addition of STAT5-deficient plasma prolonged thrombin-triggered clot times by 185% compared with human, while control plasma prolonged clot times by 370% compared with human. Maximum clot firmness was increased by 20% with addition of control plasma and 60% with STAT5-deficient plasma.