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●●●●●●●●●●●●●●492Guidelines1) and 30-50% in the Japanese prevention guidelines2). According to the 7th ACCP Guidelines, postoperative symptomatic PE and fatal PE occurred in 1-30% and 0.1-7.5% of cases, respec-tively1), compared to approximately 1% and 0.1% of cases, in the Japanese prevention guidelines2).The early detection of DVT is important for the prevention of PE because PE can lead to fatal complications. However, the methods and timing of examinations for DVT detection differ among the facilities. This study aimed to investigate and clarify the time, site, and frequency of DVT occur-rences after THA.Materials and MethodsWritten informed consent was obtained from all participants, and the study protocol was approved by the appropriate ethics committee (approval number: 17-37). Background characteristics such as age, sex, body mass index (BMI), diagnosis, type of operation, operation time, and type of anes-thesia were investigated in 383 patients who under-went THA at the Juntendo Nerima Hospital between April 2017 and April 2019. Patients with rheuma-tism, use of preoperative anticoagulants, female hormones or steroids, and a history of VTE were subsequently excluded.Table 1 shows the inspection flowchart. Blood tests of the coagulation system, such as D-dimer and fibrinogen degradation products (FDP), and vascular ultrasonography (US) of the lower limbs were performed preoperatively and on postopera-tive days (POD) 0, 1, 3, 7, 14, and 21 to investigate the time and site of thrombus occurrence before and after surgery.The US of the lower extremities was performed using a whole-leg US method, and we were able to observe the femoral and popliteal veins continu-ously using a compression method. Subsequently, PerioperativePatient backgroundBlood TestContrast enhanced CT or pulmonary perfusion scintigraphyUltrasonographyPOD, post-operative day Table 1　Flow chart of patient background surveys and blood sampling/ultrasound examinationsPOD 14Pre●●the posterior tibial, peroneal, soleal, and sural veins were carefully observed. Visualizing the veins with a probe, we defined cases in which there was no obstruction of the lumen of the vein by compres-sion of the probe and the blood flow signal was missing in the color Doppler as a positive finding. Furthermore, we classified patients as DVT-posi-tive if they had at least one DVT positive finding by US between preoperative and POD 21, and DVT-negative if they had no DVT positive find-ings. The presence of PE was examined using contrast-enhanced computed tomography (CT) or pulmonary perfusion scintigraphy on POD 7 for all patients.Continuous variables are summarized as means (standard deviation). Normally distributed contin-uous data sets were analyzed using Student’s two-sample t-test. Categorical data was analyzed using Fisher’s exact test. All the tests were two-sided, and a p-value < 0.05 was considered statistically significant. Statistical analysis was performed using GraphPad PRISM Version 7.03 (GraphPad Software, 2365 Northside Dr. Suite 560, San Diego, CA 92108, USA). We examined the occurrence of DVT using whole-leg US after THA in 383 patients from April 2017 to April 2019. For the present study, 35 patients were excluded because of rheumatics (n = 4), use of preoperative anticoagulants (n = 23), use of steroids (n = 3), a history of thrombosis (n = 5), and none of the patients used female hormones.The remaining patients comprised 64 men with a mean age of 53.5 ± 10.4 years (range, 39-73 years) and 284 women with a mean age of 67.5 years (range, 48-90 years). The diagnoses were osteoarthritis in 330 cases and osteonecrosis (non-steroid) in 18 cases. The mean operation time POD 1POD 0POD 7POD 3POD 21Results