Tissue factor-factor VIIa complex (extrinsic tenase)
Pathway · Factor VII PT / INR Vitamin K dep.
Tissue factor binds factor VII and accelerates its activation to VIIa; the TF-VIIa complex on a calcium-loaded phospholipid surface then activates factor X directly and also activates factor IX, which is how the extrinsic trigger recruits the intrinsic limb. This is the initiation step of the cell-based model of coagulation.
Traced from the start
- Tissue factor and the extrinsic triggerVitamin K cycle and gamma-carboxylation
- Tissue factor-factor VIIa complex (extrinsic tenase)
Detail
- Components
- Tissue factor + factor VIIa + Ca2+ on an anionic phospholipid surface
- Activates
- Factor X to Xa (the classical extrinsic route) and factor IX to IXa (the Josso loop, which links the two limbs)
- Measured by
- Prothrombin time; an isolated prolonged PT with a normal APTT points to factor VII deficiency, early warfarin or vitamin K deficiency
- Inhibited by
- Tissue factor pathway inhibitor, and by warfarin since factor VII is vitamin K dependent
- Exploited by
- Recombinant activated factor VII, which bypasses the tenase complexes in haemophilia with inhibitors
When it goes wrong
Congenital factor VII deficiency (autosomal recessive, rare)
Isolated prolonged PT with a normal APTT; bleeding severity correlates poorly with the measured factor level
Acute hepatocellular failure, for example paracetamol overdose
PT/INR rises within hours and is the most sensitive marker of synthetic function; an INR above 6.5 with creatinine above 300 micromol/L and grade III-IV encephalopathy meets King's College transplant criteria
Practise this structure
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