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In the DVT-positive group, thrombus formation was observed in 66.6% of cases by POD 1, indi-cating that the thrombus appeared very early after surgery. This finding may be reflected by the high D-dimer value observed during the early postoper-ative period. Furthermore, body posture during surgery and bed rest until POD 1 may facilitate thrombus formation. However, there was no signif-icant difference in the operation time between the DVT-positive and DVT-negative groups. In addi-tion, there were no significant differences between the operated and non-operated sides. The affected femur and lower leg were always grasped by the assistant, and the limb position was continuously moved according to the surgical situation. There-fore, it is possible that the massage effect was equivalent to that on the healthy side to which the intermittent pneumatic compression device was attached. It is also possible that postoperative reha-bilitation may be affected by the early start of range of motion training on the affected side. Regarding the operation, the duration of vein occlusion in the affected hip joint with flexion and the internal rota-tion position at the time of implant insertion may be intraoperative factors. However, we did not examine these factors in this study. Nevertheless, we can conclude that a distal thrombus formed very early after the operation. Moreover, we can clarify that many thrombus cases after THA were found by POD 1, and almost all thrombus formation was completed by POD 7. Attention should be paid to PE because its results are serious. Nakamura et al.31) investigated 108 acute PE patients and reported that 57% devel-oped symptoms while standing or walking and 22% developed symptoms after defecation or urination. Therefore, we need to pay careful attention to the physical condition of patients when they get out of bed, depending on the tendency for increased thrombus formation. In high-risk patients, it is necessary to check for thrombus formation using US to prevent symptomatic PE preoperatively and when they get out of bed. Therefore, as we did, it is better to perform blood sampling and a lower limb US test during the perioperative period and to perform contrast-enhanced CT for positive individ-uals. In this study, the previously mentioned prevention did not cause symptomatic PE in all subjects except the high-risk group. 1) Geets WH, Pineo GF, Heit JA, et al: Prevention of venous thromboembolism: the Seventh ACCP Confer-ence on Antithrombotic and Thrombolytic Therapy. Chest, 2004; 126: 338s-400s. 2) Japanese Society of Phlebology: Guidelines for Diag-nosis, Treatment and Prevention of Pulmonary Throm-boembolism and Deep Vein Thrombosis, Japanese Society of Phlebology（JCS 2017）[Japanese]. 3) Bergqvist D, Efsing HO, Hallböök T, Hedlund T: Thromboembolism after elective and post-traumatic hip surgery - a controlled prophylactic trial with dextran 70 and low-dose heparin. Acta Chir Scand, 1979; 145: 213-218. 4) Evarts CM, Feil EJ: Prevention of thromboembolic disease after elective surgery of the hip. J Bone Joint Surg Am, 1971; 53: 1271-1280. 5) Harris WH, Salzman EW, Desanctis RW: The preven-tion of thromboembolic disease by prophylactic antico-agulation. A controlled study in elective hip surgery. J Bone Joint Surg Am, 1967; 49: 81-89. 6) Harris WH, Salzman EW, Athanasoulis CA, Waltmam A limitation of this study is that US was performed by one person, and hence, the inter-rater reliability cannot be calculated accurately.In conclusion, the majority of thrombus cases were found on POD 1, and thrombus formation was mostly completed by POD 7. The thrombus formed very early after the surgery, indicating that US should be performed as soon as possible during the perioperative period. In patients who have a tendency for thrombus formation postoperatively, it may be necessary to examine thrombus formation.The authors thank Alison Sherwin, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/) and Editage (www.editage.com) for editing a draft of this manuscript.No funding was received.MN, SK, YS, HO, SK obtained informed consent from the patients and performed total hip arthro-plasty and various tests for research. MI super-vised this study and provided advice and guidance.All authors read and approved the final manuscript.Authors declare that there are no conflicts of interest.497AcknowledgementsFundingAuthor contributionsConflicts of interest statementReferences