INTRODUCTION: Maintaining optimal coagulation is vital for successful microvascular flap transfer. Hypercoagulate states are risk factors for pedicle thrombosis and flap loss.1 Therefore, identifying patients who are at risk for such events is paramount. The viscoelastic Thromboelastography (TEG) is a modern method to assess a patient’s coagulation status and in the past, it has predominantly been used in critical care, cardiac or trauma surgery.2 The aim of our study was to evaluate its diagnostic capabilities, its role compared to classic coagulation tests, and the effect of low-dose heparin in reconstructive breast microsurgery. METHODS: After approval from the institutional review board, clinic charts of our senior author were analyzed for all patients between 2012 and 2016 who underwent autologous free flap breast reconstruction and received perioperative TEG. Patient demographics, their medical history, clinical and operative details were documented. All coagulation studies, such as thrombocyte count, prothrombin time (PT), activated partial thromboplastin time (aPTT), and the numerous TEG parameters were gathered for baseline, the intraoperative, and the first two postoperative days (POD1, POD2). Statistical calculations sought to determine any risk factors associated with adverse outcomes of reconstructive microsurgery. RESULTS: 100 patients were subsequently identified who underwent 172 abdominal-based free flaps for breast reconstruction. Intraoperatively, 91 patients received unfractioned heparin (UFH) as their TEG-G values were significantly more hypercoagulable at baseline and again on POD1 and POD2. Intraoperative TEG-G was indifferent and borderline hypocoagulate for both the 91 heparin and the 9 non-heparin patients. PT/aPTT showed no changes perioperatively. Thrombocytes slightly decreased due to blood dilution. Within a mean follow-up of 17.4 ± 11.1 months, 3 bleeding, 6 wound/infection, and 6 thrombotic complications occurred. Of the latter group, 2 resulted in flap loss (1.2%). Intraoperative bleeding was related to high aPTT (0.029). Wound complications were related to high BMI (p = 0.022) and diabetes (0.028). The 6 thrombotic events had much steeper increases of TEG-G between surgery and POD2 (p = 0.003). Both flap losses occurred in 2 patients with a history of abdominal surgery despite intraoperative UFH. CONCLUSION: TEG is more dynamic and accurate to show the effects of intraoperative anticoagulation than conventional coagulation tests. Our applied heparin regimen was successful to avoid significant detrimental outcomes. The 2 flap losses were associated with previous abdominal surgery and scarring. Reference Citations: 1. Wang TY, Serletti JM, Cuker A, et al. Free tissue transfer in the hypercoagulable patient: a review of 58 flaps. Plast Reconstr Surg. 2012;129:443–453. 2. Bolliger D, Seeberger MD, Tanaka KA. Principles and practice of thromboelastography in clinical coagulation management and transfusion practice. Transfus Med Rev. 2012;26:1–13.