Free oscillation rheometry in the assessment of platelet quality

Platelets play an important role in the haemostatic process in order to seal damaged blood vessels. The platelets form a platelet plug at the damaged area and prevent blood loss. Once the damage to the vessel wall has been covered, the platelets retract the coagulum, to allow the blood to flow freely in the vessel. Free oscillation rheometry (FOR) can be used for analysis of coagulation as measured by clotting time and changes in clot elasticity (G'). Clot G' provides information about the fibrin network in the coagulum and the platelets’ ability to retract the coagulum. FOR analysis is performed using the ReoRox 4 instrument. The blood sample is added to a cylindrical sample cup, which is set into free oscillation. The frequency and damping of the oscillation is recorded over time as the blood coagulates. The change in G' is calculated from the frequency and damping measured. Patients with malignant haematological diseases are often thrombocytopaenic and require platelet transfusions to prevent or stop bleeding. To ensure good haemostatic function in the recipient it is important that the quality of the platelets used for transfusion is well preserved. The aim of this thesis was to determine the quality of platelet concentrates (PCs), during storage, using various in vitro methods, including FOR, and to investigate how various preparation processes affect the quality. We also investigated whether FOR can be used to evaluate the haemostatic status in subjects at risk for thrombosis or bleeding as well as how the haemostatic status was affected by a platelet transfusion. We show that FOR can provide information about the coagulation properties in subjects at risk of thrombosis (pregnant women) or bleeding (thrombocytopaenic patients). We also show that the coagulation as measured by FOR is influenced by red blood cells and the fibrinogen concentration. However, the presence of functional platelets accounted for 90% of the G'. Furthermore we present data that FOR can provide information on the haemostatic effect of platelet transfusions and on the function of the transfused platelets. PCs produced by two different cell separators showed similar quality during storage for 7 days as assessed by FOR analysis. Leukocytes in the PCs can cause transfusion-associated graft-versus-host disease which can be prevented by gamma irradiation of the PCs. Gamma irradiation did not affect the quality of PCs during 7 days of storage as analysed by FOR. The clotting time was unchanged during the storage period. The capacity of platelets to retract the coagulum was reduced from days 1 to 5 of storage as seen by a prolonged time to reach maximum G' and the reduced mean change in G' per minute. However, if sufficient time is allowed for the platelets to regain their function, the clot will be fully retracted (as seen by a well maintained maximum G'). The FOR parameters were similar for 5and 7-day old PCs, which, combined with other in vitro tests (e.g. hypotonic shock response, changes in pH, swirling, lactate and glucose), support the prolongation of the platelet storage period to 7 days. Intercept treatment of PCs can be performed to inhibit replication of contaminating bacteria in PCs. Intercept treatment of PCs did not diminish the clot-promoting capacity of the platelets as assessed by FOR clotting time. In conclusion, FOR is a promising method for assessing hyperand hypocoagulability. It can provide information on the haemostatic effect of platelet transfusions and the function of the transfused platelets. FOR was also shown to be useful for analysing PC quality during different preparation and storage conditions.