Patients undergoing major spine surgery may acquire a perioperative coagulopathy from dilution of coagulation factors and/or platelets or fibrinolysis. The mechanisms of the coagulopathy and role of coagulation testing during these procedures are poorly defined. Theoretically, coagulation tests could be used perioperatively to determine which patients are at risk for significant bleeding and guide transfusion therapy. We retrospectively evaluated the sensitivity, specificity, and accuracy of coagulation tests in predicting excessive surgical bleeding in 244 consecutive patients undergoing thoracic, lumbar, or sacral spinal fusion with or without instrumentation. Excessive bleeding was reported by the surgeon in 39 of the patients and was defined as recurrent microvascular bleeding despite adequate use of electrocautery and suture or decreased clot formation of blood pooled within the surgical field. Patients with excessive clinical bleeding sustained larger estimated blood losses than those with normal hemostasis. The total number of allogeneic red blood cells, platelets, and fresh frozen plasma units were also larger in patients with excessive bleeding noted during surgery. The intraoperative coagulation tests with the most sensitivity and specificity were the international normalized ratio (INR), prothrombin time (PT), and activated partial thromboplastin time (aPTT). The INR had a sensitivity of 94%, a specificity of 88%, and an accuracy of 0.9 at a value of 1.4 (normal, 0.8–1.2). The PT had a sensitivity of 90%, a specificity of 64%, and an accuracy of 0.73 at a value of 13.5 s (normal, 8.4–12.0 s). The aPTT had a sensitivity of 85%, a specificity of 64%, and an accuracy of 0.71 at a value of 30.9 s (normal, 23–37 s). The thromboelastogram values were of marginal use. We conclude that the INR, PT, and aPTT may be helpful in guiding transfusion therapy in patients undergoing major spine surgery.