Venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism, is a common cardiovascular disease associated with significant morbidity ranging from painful leg swelling, chest pain, shortness of breath, and even death. About 50% of all VTE events occur as a result of a current or recent hospital admission for surgery or acute medical illness. Hospital-acquired VTE is preventable, with interventions including anticoagulants and mechanical measures, including compression stockings and intermittent pneumatic compression. Prevented hospital acquired VTE is the focus of health services and the strongest hospital strategy to improve patient health in the world.

Low molecular weight heparins (LMWH) are commonly used injectable anticoagulants for venous thromboembolism (VTE) prophylaxis and treatment. LMWH forms an inhibitory complex with antithrombin to inactivate activated factor X (Xa). Due to the predictable pharmacokinetics and pharmacodynamics of LMWH, it is not necessary to routinely monitor anti-Xa levels. However, LMWH pharmacokinetics and pharmacodynamics may be less predictable in certain patient populations including renal impairment, obesity, malignancy, or pregnancy .

Both increased risk of bleeding and suboptimal efficacy are possible in obese patients. LMWHs distribute into lean body mass, therefore, obese patients with a lower proportion of lean body mass to adipose tissue receiving LMWH dosed according to actual body weight may achieve supratherapeutic drug concentrations which could increase bleeding risk . On the other hand, fixed-dose VTE prophylaxis regimens do not account for higher body weight associated with obesity potentially resulting in subtherapeutic drug concentrations increasing the risk for therapeutic failure.

As LMWH are primarily renally eliminated, impaired renal function can contribute to drug accumulation and increased risk of major bleeding.The prolonged LMWH monotherapy used in cancer-associated VTE treatment also raises concerns about drug accumulation and increased bleeding, especially in those with fluctuating renal function. In addition, pregnancy can potentially increase the clearance and volume of distribution of LMWH, increasing the potential for subtherapeutic anti-Xa levels. Thus, anti-Xa level assays are often performed for these specific patient populations in an attempt to provide optimal LMWH therapy.

Critically ill patients are higher risk populations of VTE and bleeding with complex conditions, for example sedation, mechanical ventilation, central venous catheter, and have severe infection, renal insufficiency/failure. So, the purpose of this RCT is to explore the effect of anti Xa monitoring LMWH in preventing VTE in critically ill patients and the optimal time of anti Xa monitoring, reduce mortality and serious adverse events.

Inclusion Criteria:

• Aged 18 years or older
• No gender limited
• Prospectively screened for risk and included if they received LMWH
• The patient or his / her legal representative is able and willing to sign the informed consent

Exclusion Criteria:

• History of hemorrhage or high risk of hemorrhage, including subarachnoid hemorrhage, cerebral hemorrhage, traumatic brain injury, blood system diseases, etc
• Severe renal insufficiency before randomization (creatinine clearance rate (CCr) < 30mL/min)
• Expected length of ICU stay less than 3 days
• Known to be allergic to LMWH
• Pregnancy
• History of heparin induced thrombocytopenia
• Patients with iliac vein compression syndrome
• Receive non LMWH for prevention VTE according to the physician