Extrinsic pathway triggered by tissue factor on cells outside blood vessels.

Intrinsic pathway triggered by subendothelial surfaces activating factor XII, then XI, then IX.  IX and VIII combination with calcium and platelet membrane phospholipids activates X.

Common pathway then continues, with X combining with V, platelet membrane phospholipids and calcium to convert prothrombin to thrombin.

Thrombin converts fibrinogen to fibrin, to form thrombus.  Factor XIII stabilises clot.


  • Vitamin K deficiency: 1:1200 breast fed (low levels), 1: 8500 formula fed. Preventable with single IM dose of Vitamin K. Can present up to 6 months later though typically 3 months.
  • Vitamin K may be affected in babies by maternal medicines eg anti-epileptics, or by liver disease.
  • Disseminated intravascular coagulation especially sepsis. Platelets fall too.
  • Haemophilia
  • Haemophagocytic syndrome (anaemia and other cell lines affected)
  • Thrombophilia