COMP regulates thrombin activity. Serum fibrinogen (FIB, A), activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT) (B) of the platelet-free plasma isolated from WT and COMP^−/− mice (n = 6, *P < .05; NS, not significant). (C) APTT, PT, and TT of WT platelet-free plasma in the absence or presence of purified COMP. Platelet-free plasma that had no coagulation at 600 seconds is shown as >600 seconds (n = 4, *P < .05 vs 0 ng/mL COMP). (D) Representative platelet aggregation tracings following thrombin (0.1 U/mL), collagen (1 μg/mL), or ADP (10 μM) treatment with or without purified COMP (top). Statistical results show the maximal percentage of platelet aggregation (n = 4, *P < .05 vs 0 ng/mL COMP) (bottom). (E) Measurement of ATP released in the supernatant of platelets treated with thrombin (0.1 U/mL), collagen (1 μg/mL), or ADP (10 μM) in the absence or presence of COMP for 5 minutes (n = 4, *P < .05 vs 0 ng/mL COMP). (F) Representative flow cytometry analysis of phosphatidylserine on the surface of platelets treated with thrombin in the presence or absence of COMP (300 ng/mL) for 15 minutes. Bar graph shows statistical results of the mean fluorescence intensity (MFI). n = 4, *P < .05. (G) Representative images of the clots in thrombin-induced mouse platelet-rich plasma with or without purified COMP (300 ng/mL) at different time points (left). Statistic bar graph shows the percentage of nonclot size to initial clot size (n = 4, *P < .05) (right).