What is a white clot (thrombus)?

White Clot (Thrombus): Composition and Clinical Significance

A white clot (thrombus) is primarily composed of platelet aggregates bound together by thin fibrin strands, forming under conditions of high blood flow in arterial vessels, and is distinctly different from red clots that form in venous circulation. 1

Composition and Formation

• Arterial (White) Thrombi:

  • Predominantly composed of platelet aggregates (55% of overall composition) 2
  • Contain thin fibrin strands (approximately 31%) 2
  • Minimal red blood cell content (approximately 6%) 2
  • Form under high-flow conditions typically in arteries 1
  • Often develop at sites of atherosclerotic plaque rupture 1

• Venous (Red) Thrombi:

  • Predominantly composed of red blood cells
  • Contain large amounts of interspersed fibrin
  • Relatively few platelets
  • Form in areas of blood stasis or slow flow 1

Clinical Significance

Pathophysiology

White clots play a critical role in arterial thrombotic events by:

• Forming at sites of vascular injury or atherosclerotic plaque rupture 1
• Potentially causing complete vessel occlusion when flow is slow, stenosis is severe, or thrombogenic stimulus is intense 1
• Leading to tissue ischemia through obstruction or distal embolization 1

Clinical Presentations

White clots are associated with:

• Acute arterial occlusions 3
• Acute ischemic stroke 2
• Heparin-induced thrombocytopenia and thrombosis (HITT), also known as "white clot syndrome" 4, 3

Imaging Characteristics

Compared to red clots, white clots are:

• Significantly smaller in size
• Less hyperdense on CT imaging
• More likely to cause distal vessel occlusions 2

Special Consideration: White Clot Syndrome

White clot syndrome (Heparin-associated thrombocytopenia and thrombosis) is a serious iatrogenic complication characterized by:

• Paradoxical thrombosis during heparin therapy
• Concomitant thrombocytopenia (typically <100,000/μL)
• Multiple, recurrent arterial and/or venous thromboembolic events
• Potentially fatal outcomes if not promptly recognized and treated 4, 3

Management Implications

The composition of thrombi has important treatment implications:

• Arterial (white) thrombi respond better to antiplatelet therapy due to their platelet-rich composition 1
• White clots are less successfully removed by aspiration alone and may require combination mechanical thrombectomy approaches 2
• In cases of white clot syndrome, immediate discontinuation of heparin is mandatory 4, 3

Clinical Pearl

Regular monitoring of platelet counts in patients receiving heparin therapy is essential for early detection of white clot syndrome. If thrombocytopenia develops with new thrombotic events, heparin should be discontinued immediately and alternative anticoagulation initiated 4, 3.