Heparin Does NOT Dissolve Existing Blood Clots
Heparin is an anticoagulant that prevents clot formation and extension but lacks fibrinolytic activity and therefore cannot dissolve existing clots. 1
Mechanism of Action
Heparin works by enhancing antithrombin III activity to inhibit the coagulation cascade, specifically targeting factors Xa and IIa (thrombin), thereby preventing new clot formation 2:
- Heparin binds to antithrombin (AT) through a specific pentasaccharide sequence, inducing a conformational change that converts AT from a slow to a rapid inhibitor of serine proteases 2
- The heparin-AT complex inactivates activated clotting factors including thrombin (Factor IIa), Factor Xa, IXa, XIa, and XIIa 2
- After AT binds covalently to the active center of coagulation enzymes, heparin dissociates and can be reused 2
Why Heparin Cannot Dissolve Clots
The FDA drug label explicitly states: "Heparin does not have fibrinolytic activity; therefore, it will not lyse existing clots." 1
This limitation exists because:
- Heparin prevents fibrin formation by inhibiting thrombin but cannot break down fibrin that has already formed 2
- The heparin-AT complex is unable to inactivate fibrin-bound thrombin effectively 2
- Fibrin actually protects bound thrombin from inhibition by the heparin-AT complex 2
- Heparin paradoxically increases thrombin's affinity for fibrin, further protecting fibrin-bound thrombin from inactivation 2
Clinical Implications
Heparin is effective for prevention and treatment of thrombosis by stopping clot propagation, not by dissolving existing clots 2:
- Used for prevention and treatment of venous thromboembolism and pulmonary embolism 2
- Effective for preventing mural thrombosis after myocardial infarction 2
- Used in unstable angina and acute MI to prevent thrombosis extension 2
Common Pitfall to Avoid
Do not confuse heparin's role in "treating" thrombosis with clot dissolution. When guidelines state heparin is used for "treatment" of deep vein thrombosis or pulmonary embolism, this means preventing clot extension and recurrent thrombosis while the body's natural fibrinolytic system gradually dissolves the clot 2. Heparin itself contributes nothing to the dissolution process 1.
For Clot Dissolution
If fibrinolysis is required, thrombolytic agents (not heparin) must be used, such as tissue plasminogen activator, streptokinase, or urokinase, which actively break down fibrin clots 2.