OBJECTIVE To evaluate the degree of activation of the contact pathway in citrated equine whole blood over holding times _ 30 minutes and assess effects of contact activation on recalcification-initiated thromboelastometry. ANIMALS 11 healthy adult mixed-breed horses. PROCEDURES Blood was collected by atraumatic jugular venipuncture into prewarmed evacuated siliconized glass tubes containing citrate anticoagulant and held at 37°C for _ 30 minutes. Thromboelastometry was performed with an in vitro viscoelasticity (thromboelastometry) monitoring system. Factor XII and factor XI procoagulant activities were determined in contemporaneously collected platelet-poor plasma samples by assessing changes in turbidity for 1 hour at approximately 25°C, with clotting times calculated by fitting a line to the steepest segment of the absorbance curve and determining its intersection with baseline. Effect of holding time on thromboelastometry parameters and plasma enzyme activity was evaluated by repeated-measures ANOVA on ranks. Association of procoagulant activities with coagulation time was determined by Spearman rank-order correlation analysis. RESULTS Thromboelastometry parameters (coagulation time, clot formation time, _ angle, and maximum clot firmness) reflected significant increases in coagulability during the holding period. Factor XII and factor XI procoagulant activities were significantly increased at 30 minutes, compared with 2 or 10 minutes (indicating contact activation of samples), and had significant negative correlation with coagulation time. CONCLUSIONS AND CLINICAL RELEVANCE Ex vivo activation of the contact system in equine whole blood was evident, suggesting that recalcification of blood in the absence of a trigger is not an acceptable method of assessing the hemostatic system in horses.
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