The test is useful to assess ongoing coagulation activation as may occur in thrombosis or DIC.

When prothrombin is converted to thrombin, the prothrombin fragment 1.2 is released from prothrombin.

The conversion of prothrombin to thrombin is a key event in the coagulation of blood. Prothrombin fragment 1.2 is an activation peptide released from prothrombin during thrombin formation. Activation of prothrombin takes place in the presence of factor Xa, factor Va, calcium ions and a phospholipid suface (platelets). Prothrombin fragment F1+2 may be generated during thrombin formation in two ways. Either prothrombin is cleaved to prethrombin 2 and prothrombin fragment F1+2. Subsequently prethrombin 2 is further cleaved at one position to yield active thrombin. The alternative way of activation is that prothrombin is cleaved to meizothrombin still containing the prothrombin fragment F1+2, and is then processed to thrombin, liberating the prothrombin fragment F1+2. The prothrombin fragment F1+2 may further be cleaved by thrombin to fragments F1 and F2.

Prothrombin Fragment 1+2 [F1+2] is an activation peptide that is generated when Prothrombin [Factor II] is converted to Thrombin. Prothrombin is activated to Thrombin [Factor IIa] by a membrane-bound enzyme complex - Prothrombinase and which is assembled through reversible interactions between Factor Xa and Factor Va on membranes containing Phosphatidylserine – a Phospholipid.

References:

Ota S, Wada H, Abe Y, Yamada E, Sakaguchi A, Nishioka J, Hatada T, Ishikura K, Yamada N, Sudo A, Uchida A, Nobori T. Elevated levels of prothrombin fragment 1 + 2 indicate high risk of thrombosis. Clin Appl Thromb Hemost. 2008 Jul;14(3):279-85. doi: 10.1177/1076029607309176. Epub 2007 Dec 26. PMID: 18160575.

Haeberli, A. (1999). Prothrombin fragment F1+2. In: Jespersen, J., Bertina, R.M., Haverkate, F. (eds) Laboratory Techniques in Thrombosis — a Manual. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-4722-4_23