Unfractionated heparin (otherwise known as just heparin) inactivates both factor Xa and thrombin. Because of its quick onset and offset you may most commonly see it being used perioperatively in hospitals. However, because of its short half life, it is typically given as a continuous infusion.
Low molecular weight heparin inactivates Xa. In comparison to UFH, it has better bioavailability, and because its effects are more predictable (better correlation between dose and response), it is typically not monitored in the lab. Because of its longer duration of action compared to UFH, it is more suitable for outpatient use than UFH.
Warfarin is an anticoagulant that works by decreasing the potassium dependent clotting factors (i.g. 10, 9, 7, 2). However, unlike heparin, it mandates routine laboratory monitoring, which patients often find burdensome.
Scenarios in which warfarin may be preferred over newer agents include
- CKD (as it is not renally cleared)
- Inexpensive
- Heart valve replacement patients
- Anti phospholipid syndrome
- Easily reversible (simply provide vitamin K, or frozen prothrombin complex concentrate – blood product which contains factors 10, 9, 7 and 2, protein C and protein S)
When should we avoid warfarin?
- Pregnancy (crosses the placenta)
- Requires bridging with heparin in order to first use as initially it pushes the patient into a pro-coagulant state by decreasing the levels of the body’s endogenous anticoagulants, protein C and S (which is also why we don’t use it in acute situations)
- Cirrhosis
- Cancer venous thromboembolism
How to monitor heparin, LMWH and warfarin?
- Simply put heparin and LMWH is not monitored. Sometimes in obese patients where physicians are not sure if they are dosing appropriately, they may order an anti-factor Xa level, but this is usually of little use
- Warfarin on the other hand must be monitored with routine INR measurements.
- The therapeutic range of INR for warfarin generally falls between 2-3 (INR normally should be 0.9 – 1.2), however for certain conditions like having a mechanical heart valve, a higher INR may be required