Title: CLASSIFICATION OF ORTHOGNATHIC SURGERY PATIENTS INTO LOW AND HIGH BLEEDING RISK GROUPS USING THROMBELASTOGRAPHY Objectives: Orthognathic surgery involves surgical manipulation of jaw and face skeletal structure. A subgroup of patients undergoing orthognathic surgery suffers from excessive intraoperative blood loss. Classification of patients according to their bleeding risk will improve the surgical procedure with regard to staff composition, blood transfusion and patient safety. Thrombelastography is a global coagulation assay measuring the viscoelastic properties of whole blood samples, taking into account the complex interplay between coagulation factors, blood platelets and components of the fibrinolytic system. Patients undergoing orthognathic surgery were included in this prospective study, and their preoperative thrombelastographic data were collected and compared to their intraoperative blood loss. Methods: 41 consecutive patients undergoing simultaneous mandibular and maxillary osteotomy were included in the study, and whole blood samples were collected preoperatively from each patient. The sample were subjected to thrombelastographic analysis, and the intraoperative blood loss was carefully measured. Results: Patients were separated into two groups with intraoperative bleeding ≤ 400 mL, and intraoperative bleeding > 400 mL. The thrombelastographic data for each group were compared, and parameters showing significant differences, were analyzed with regard to negative and positive predictive values. An α angleex above 67o did with 95% certainty predict a blood loss below 400 mL, and a receiver-operating characteristic (ROC) curve showed an area under the curve (AUC) of 0.8. Conclusion: By means of the α angleex it is possible to separate orthognathic surgery patients according to their bleeding risk. This valuable knowledge will be useful with regard to optimization of patient safety, staff composition and transfusion preparations. This pilot study included only 41 patients, and further studies are needed to consolidate the observations done.