Evaluation of a modified thrombelastography assay initiated with recombinant human tissue factor in clinically healthy horses

Background: Thrombelastography (TEG) is used to evaluate the viscoelastic properties of blood during clotting and provides a global assessment of hemostasis and clot lysis. TEG analysis initiated with recombinant human tissue factor (TF) has not been evaluated in clinically healthy horses. Objectives: The purpose of this study was to determine whether TEG results are affected by the time elapsed between sampling and analysis (storage time) of equine blood samples and to establish a preliminary equine reference interval for a modified TEG assay, using recombinant human TF to initiate coagulation. Methods: Citrated blood samples were obtained from 20 clinically healthy adult horses. Thirteen samples were stored for 30, 60, and 120 minutes at room temperature before TEG analysis. Coagulation was initiated by adding 20 μL of CaCl₂ to 330 μL of blood and 10 μL of diluted recombinant TF for a final dilution of 1:3600. Reaction (R) and clotting (K) times, angle (α), and maximum amplitude (MA) were compared between time points. A preliminary reference interval (minimum–maximum values) was determined using data from all 20 horses after 30 minutes of sample storage. Results: There was a significant effect of storage time on R, K, and α but not MA. Reference intervals were: R, 3.65–6.4 minutes; K, 1.8–5.45 minutes; α, 33.4–66.2°; MA, 41.2–64.1 mm; lysis at 30 minutes post‐MA (LY30), <2.75%; and lysis at 60 minutes post‐MA (LY60), 1.55–9.5%. Conclusions: TEG can be performed on equine citrated blood samples using recombinant human TF to activate clot formation. TEG parameters were significantly affected by storage time, suggesting an incomplete inhibition of coagulation in citrated blood.     

Viscoelastic coagulation testing: technology, applications, and limitations

Use of viscoelastic point-of-care (POC) coagulation instrumentation is relatively new to veterinary medicine. In human medicine, this technology has recently undergone resurgence owing to its capacity to detect hypercoagulability. The lack of sensitive tests for detecting hypercoagulable states, along with our current understanding of in vivo coagulation, highlights the deficiencies of standard coagulation tests, such as prothrombin and partial thromboplastin times, which are performed on platelet-poor plasma. Viscoelastic coagulation analyzers can provide an assessment of global coagulation, from the beginning of clot formation to fibrinolysis, utilizing whole blood. In people, use of this technology has been reported to improve management of hemostasis during surgery and decrease usage of blood products and is being used as a rapid screen for hypercoagulability. In veterinary medicine, clinical use of viscoelastic technology has been reported in dogs, cats, foals, and adult horses. This article will provide an overview of the technology, reagents and assays, applications in human and veterinary medicine, and limitations of the 3 viscoelastic POC analyzers in clinical use.     

Thromboelastography in equine medicine: Technique and use in clinical research

Thromboelastography (TEG) is a viscoelastic, whole blood‐based assay that integrates information from both the cellular and soluble components of coagulation, providing a global evaluation of the haemostatic system. This contrasts with the conventional coagulation assays (i.e. platelet count, prothrombin time [PT], activated partial thromboplastin time [aPTT] and fibrinogen concentration [FIB]), which only provide information about one component (e.g. clotting factors in the case of PT and aPTT) of the haemostatic process, requiring the combination of several assays for a complete evaluation of haemostasis. Thromboelastography is an old technology that has been used in human medicine for over 50 years. However, it is relatively new in veterinary medicine and has only been applied to horses in the last 5 years. Clinical applications in human medicine include diagnosis and monitoring of coagulopathies. Currently, extensive research is being carried out to expand the use of TEG in dogs and cats. Therefore, it is expected that the use of this technique will also further expand in horses in the near future. To date, the available studies in the equine species have evaluated TEG in healthy horses, horses with gastrointestinal disease, septic foals, horses with exercise‐induced pulmonary haemorrhage (EIPH) and a filly with Glanzmann's thrombasthenia. The main objective of this review is to introduce the TEG technique to equine clinicians, providing information on how the TEG functions, blood sample collection and processing, variables measured and their interpretations, normal reference values and areas of potential clinical application.     

Thromboelastography in healthy horses and horses with inflammatory gastrointestinal disorders and suspected coagulopathies

Objectives – To evaluate the use of citrated recalcified (nonactivated) thromboelastography (TEG) in healthy horses and horses with colitis and suspected coagulopathies. Design – Prospective, observational study conducted between October 2007 and June 2009. Setting – Veterinary Teaching Hospital. Animals – Forty‐five healthy adult horses and 12 sick adult horses with colitis and prolonged prothrombin time (PT) or activated partial thromboplastin time (aPTT). Interventions – None. Measurements and Main Results – Whole blood was collected on admission. Coagulation profile (PT, aPTT, platelet count, and fibrinogen concentration) and citrated recalcified whole blood TEG analysis (R‐time [R], K‐time [K], angle [α], maximum amplitude [MA], G value [G], lysis at 60 min [LY60]) were evaluated. Mean values (SD) for TEG parameters in healthy horses were: R=10.4 (3.1) minutes; K=3.5 (1.2) minutes; α=46.3 (11.0)°; MA=55.6 (5.1) mm; G=6,429 (1,341) dyn/cm², and LY60=5.1 (2.4)%. Mean coefficients of variation for intra‐assay/interindividual variability in healthy horses were: R=4.7%/30.7%, K=4.8%/35.3%, α=4.4%/23.8%, MA=1.4%/9.3%, G=3.4%/20.8%, and LY60=13.1%/47.7%, respectively. Horses with colitis and prolonged PT and/or aPTT had longer mean values for R (P<0.001) and K (P<0.001), narrower mean α (P<0.001), decreased mean MA (P=0.001), and smaller mean G (P=0.02); changes consistent with hypocoagulability. Conclusions – Citrated recalcified (nonactivated) TEG demonstrated changes consistent with hypocoagulability in horses with colitis that had preidentified coagulation abnormalities. This technique has high interindividual variability and low intra‐assay variability. TEG may be useful for detecting hypocoagulable states in horses with colitis and suspected coagulopathies.