This project intends to conduct a single-center, prospective, observational cohort study to explore the impact of the timing of drug prophylaxis on the risk of postoperative bleeding and the preventive effect of VTE in the prevention and management of postoperative venous thromboembolism (VTE) after colorectal cancer (CRC) surgery in Chinese population, and to determine its application and promotion value. The research results of this project can provide useful reference for optimizing the prevention and management of VTE after CRC operation.

This study is a single-center, prospective, observational cohort study. The design and implementation of the whole study were carried out according to the CONSORT principle. The study was conducted at the Department of Gastrointestinal Surgery, Renji Hospital, Shanghai Jiao Tong University School of Medicine, and patients were followed up for 28 days.

Postoperative patients with colorectal cancer who met the criteria were included. All patients were initiated with low molecular weight heparin (nardraparin calcium injection) for drug prophylaxis after surgery. In clinical practice, patients who underwent surgery at different times on the same day received prophylactic administration of low molecular weight heparin at 24h after surgery, forming two cohorts of early initiation (within 24 hours after surgery) and late initiation (beyond 24 hours after surgery). According to the previous clinical data survey in the past year, since the number of operating tables and the end time of surgery are relatively fixed every day, the number of people in each group of early start and late start groups is about 300 per group per year according to the end time of surgery, which basically conforms to the ratio of 1:1.

The dose of low molecular weight heparin (Nardrheparin calcium injection) was 100AxaIU/kg, adjusted according to the patient's body weight, and was administered subcutaneously once a day throughout the patient's hospital stay. All patients underwent color Doppler ultrasound of lower limbs before discharge (7-10 days after surgery) and 28 days after surgery, including screening for thrombus in the proximal deep vein (femoral vein, popliteal vein), distal deep vein (posterior tibial vein, peroneal vein and muscular vein) and venous communication branch. Each enrolled patient will be followed up for 28 days, and all patients will be followed up 28 days after surgery. If the patient is unable to return to the hospital at the scheduled follow-up time point, the patient can be reminded by phone to be admitted for follow-up visit.

Inclusion Criteria:

• ① Age > 18 years old;

  ② The patient was diagnosed with colorectal cancer, and the cTMN stage was cT1-2, N0 or cT3, N0 or any cT, N1-2; (3) The patient was diagnosed with colorectal cancer and was able to receive radical surgery (laparoscopic or open surgery). The expected operation time was >45 minutes, and the expected postoperative survival time was >6 months.

  ④ The patient or guardian agrees to the study plan and signs the informed consent.

Exclusion Criteria:

• ① Renal insufficiency (CrCl<30 mL/min) or hepatic insufficiency (ALT> 3 times the upper limit of normal);

  • The patient was diagnosed with colorectal cancer with a cTMN stage of cT4 and/or local unresectable lesions. Unresectable local recurrent lesions included: (1) extensive lateral pelvic wall invasion, (2) external iliac vascular involvement, (3) tumor invasion into the great sciatic notch, sciatic nerve invasion, and (4) invasion of the second sacrum level and above.

Known allergy to low molecular weight heparin (LWMH), narcotic drugs or radiocontrast agents; The presence of systemic hemorrhagic disease or bleeding tendency, such as active peptic ulcer, uncontrolled hypertension, cerebral thrombosis, cerebral hemorrhage or neurosurgical history within 6 months;

• known brain metastases, endocarditis, or history of heparin-induced thrombocytopenia;

  • VTE occurred within 3 months before surgery;

    • Use heparin or oral anticoagulant therapy within 5 days before surgery; Women who are pregnant or breastfeeding;

      • Any situation in which the investigator determines that the subject is not suitable for anticoagulant therapy.

The patient was diagnosed with colorectal cancer and underwent radical surgery (laparoscopic or open surgery) with an estimated operation time of >45 minutes. Patients with expected survival >6 months after surgery