What is the duration of action of Heparin (unfractionated heparin (UFH) and low molecular weight heparin (LMWH)) in the body?

Duration of Action of Heparin in the Body

Unfractionated heparin (UFH) has a short half-life of approximately 1.5 hours, while low molecular weight heparin (LMWH) has a longer half-life of 3-6 hours after subcutaneous injection. 1, 2

Unfractionated Heparin (UFH) Pharmacokinetics

• UFH is cleared through a combination of rapid saturable mechanisms (binding to endothelial cells and macrophages) and slower first-order mechanisms (primarily renal clearance) 2
• The half-life of UFH is approximately 1.5 hours when administered intravenously, but this is dose-dependent 1
• After entering the bloodstream, UFH binds to plasma proteins, endothelial cells, and macrophages, which affects its pharmacokinetics 2
• The anticoagulant effect of UFH is immediate when given intravenously but delayed by 1-2 hours when administered subcutaneously 2
• UFH's clearance is non-linear at therapeutic doses, with both intensity and duration of effect rising disproportionately with dose 2

Low Molecular Weight Heparin (LMWH) Pharmacokinetics

• LMWHs have a longer elimination half-life of 3-6 hours after subcutaneous injection 2
• The half-life of LMWH is dose-independent, unlike UFH 2
• Anti-Xa levels peak 3-5 hours after subcutaneous administration of LMWH 2
• LMWHs have higher bioavailability (approximately 90%) compared to UFH when administered subcutaneously 2
• LMWHs are predominantly cleared by the kidneys, which may prolong their biological half-life in patients with renal failure 2

Monitoring and Duration of Treatment

• For UFH, the activated partial thromboplastin time (aPTT) should be monitored approximately every 4 hours initially, then at appropriate intervals 3
• The therapeutic effect of UFH is considered adequate when the aPTT is 1.5 to 2 times normal 3
• For deep subcutaneous injections of UFH, tests for adequacy of dosage are best performed on samples drawn 4-6 hours after injection 3
• For LMWH, routine coagulation monitoring is generally not necessary except in specific populations (obese patients, renal insufficiency, pregnancy) 2
• When treating venous thromboembolism, a 5-day course of heparin has been shown to be as effective as a 10-day course 2

Clinical Implications

• When switching from heparin to oral anticoagulants like warfarin, full heparin therapy should be continued for several days until the INR reaches a stable therapeutic range 3
• For patients receiving intravenous heparin who are transitioning to non-warfarin oral anticoagulants, the IV infusion should be stopped immediately after administering the first dose of the oral agent 3
• Platelet counts should be monitored periodically during heparin therapy due to the risk of heparin-induced thrombocytopenia (HIT), which typically appears after 4-14 days of therapy 2
• Significant thrombocytopenia occurs in 1-5% of patients receiving heparin, while autoimmune HIT with thrombosis is a rare but dangerous complication (less than 0.2% incidence) 2

Understanding the duration of action of heparin is crucial for proper dosing, monitoring, and transitioning between anticoagulants to ensure optimal patient outcomes while minimizing bleeding risks.