A Cell based Model
Recognising the role of the platelet and specific cellular receptors
"Normal haemostasis is controlled activation of clot formation and lysis to prevent hemorrhage without [inappropriate] thrombosis" according to Laposata.5 haemostasis is a balancing act - and when the system is in balance, when the myriad components of haemostasis are interacting at appropriate levels and in appropriate ways, there is neither bleeding nor inappropriate clotting. When the hemostatic system is out of balance, the result can be disastrous hemorrhage or thrombosis.
The traditional way of viewing haemostasis has been as separate intrinsic and extrinsic coagulation systems, with a sequential waterfall effect of coagulation mechanisms and control mechanisms of feedback amplification and inhibition.
- These views were based on a plasma milieu, and are not applicable in vivo in moving blood
- Routine tests such as PT and aPTT are based on this model and measure coagulation factor interaction in plasma, without the participation of platelets
These outdated views are being augmented with a better understanding of cell-based haemostasis that recognizes the role of the platelet and specific cellular receptors for coagulation proteins. Hoffman and Monroe6 have proposed that the cell-based model occurs in three overlapping stages:
- Initiation: occurs on a tissue factor bearing cell
- Amplification: platelets and cofactors are activated
- Propagation: large amounts of thrombin are generated on the platelet surface
The TEGŪ 5000 system reflects the cell-based model of haemostasis, using a whole blood sample to measure:
- The net effect of the interaction of all haemostatic components, plasmatic and cellular elements - especially platelets
- All phases of haemostasis from clot initiation through propagation through lysis
