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Preanalytic Variables in Special Coagulation Testing

Clinical Implications

PT and aPTT Diagram

Slide 7

October 2008

This next slide provides a framework to understand the relationship between the coagulation cascade and screening tests such as the PT and the aPTT. The coagulation cascade consists of extrinsic and intrinsic pathways which converge on a final common pathway.

Performance of the PT consists of adding either recombinant tissue factor or other sources of the tissue from the plastin to patient’s plasma along with calcium and phospholipids. This results in activation of factor 7 in the extrinsic pathway, which leads to activation of components of the final common pathway, and ultimately clot formation. The prothrombin time is the time that it takes for the clot to form.

Likewise, for the aPTT test, a contact activator, calcium and phospholipid, are added to patient plasma and the time to clot represents the aPTT. So, a prolonged AT or aPTT may be due to coagulation facet deficiencies or inhibitors of one or more of the coagulation pathways. The aPTT is typically prolonged by anticoagulants like heparin and direct thrombin inhibitors.

PT and aPTT Diagram


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