Related Experiment Video
Updated: Dec 11, 2025

Measurement of Factor V Activity in Human Plasma Using a Microplate Coagulation Assay
Published on: September 9, 2012
C-reactive protein-induced activated partial thromboplastin time prolongation in heparinized samples is attenuated by
Vadim Kostousov1, Sridevi Devaraj1, Karen Bruzdoski1
1Departments of Pathology & Immunology, Texas Children's Hospital, Baylor College of Medicine, Houston, TX, USA.
Introduction:
Activated partial thromboplastin time (aPTT) and antifactor Xa (anti-Xa) activity are used to monitor unfractionated heparin therapy in children on extracorporeal membrane oxygenation (ECMO). Elevated C-reactive protein (CRP) can prolong aPTT and cause discrepancy between these two assays. We aimed to evaluate CRP effect on aPTT and anti-Xa assays in the presence of heparin and to determine whether elevated CRP affects laboratory monitoring in pediatric ECMO patients.
Materials And Methods:
Citrated normal specimens were spiked with CRP, heparin, and recombinant factor VIII (FVIII) and followed by measurement of aPTT and anti-Xa activity. Additionally, aPTT, anti-Xa activity, FVIII, fibrinogen, and CRP were measured in 18 ECMO specimens.
Results:
Elevated CRP prolonged aPTT in normal specimens with or without heparin, but did not affect anti-Xa assay. In contrast, ECMO specimens showed similar aPTT and anti-Xa values regardless of CRP level. Elevated CRP in specimens was accompanied by increased fibrinogen and FVIII activity. Additional in vitro experiments confirmed that FVIII spiked simultaneously with CRP attenuated CRP-induced aPTT prolongation in heparinized specimens.
Conclusion:
In vitro CRP-induced aPTT prolongation is not observed in pediatric ECMO samples due to concomitant FVIII increase. Discordant changes of CRP and FVIII in plasma could contribute to aPTT/anti-Xa discrepancies observed during heparin therapy in the pediatric population. The anti-Xa assay is preferable for heparin monitoring in pediatric ECMO settings.
Related Concept Videos
Anticoagulant Drugs: Low-Molecular-Weight Heparins
Venous Thrombosis III: Interprofessional Care
Extrinsic and Intrinsic Pathways of Hemostasis
The Extrinsic Pathway
The extrinsic pathway of coagulation is typically initiated by tissue damage that exposes blood to tissue factor (TF), a protein released by the damaged tissue cells outside the blood vessels—this interaction with TF triggers biochemical reactions involving specific clotting factors. The key player here is Factor VII, which...
Anticoagulant Drugs: Vitamin K Antagonists and Direct Oral Anticoagulants
Warfarin, a prominent vitamin K antagonist family member, exerts its effect by inhibiting the enzyme VKORC1 (vitamin K epoxide reductase complex 1). By hindering this enzyme, warfarin...
Formation of the Platelet Plug
As the injured blood vessel contracts, endothelial cells undergo contraction, revealing collagen fibers in the basement membrane and underlying connective tissue. Furthermore, the plasma membrane of endothelial cells becomes adhesive, preparing the site for platelet adhesion. Platelets...
Clot Retraction and Fibrinolysis

