The fibrinopeptides, fibrinopeptide A (FpA) and fibrinopeptide B (FpB), are peptides which are located in the central region of the fibrous glycoprotein fibrinogen (factor I) and are cleaved by the enzyme thrombin (factor IIa) to convert fibrinogen into covalently-linked fibrin (factor IA) monomers.

[1][2] The N-terminal FpA is cleaved from the Aα chains of fibrinogen and FpB from the Bβ chains of fibrinogen, with FpA released before FpB.

[2] Hence, the fibrinopeptides are sensitive markers of fibrinogenesis (fibrin generation), thrombin activity, and coagulation.

[5][8] Hence, FpA levels provide a relatively transient measure of coagulation activation.

[9][10] Likewise, FpA levels have been reported to increase with estrogen therapy, including with combined birth control pills and menopausal hormone therapy, although research on FpA levels with these therapies appears to be relatively limited.