Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1110 page)

   Inhibitors

   Therapy with unfractionated heparin

   Therapy with warfarin (variable response)

   Therapy with antithrombin agents such as hirudin and its derivatives, argatroban, and newer antithrombin (e.g., dabigatran), as well as anti-Xa agents (e.g., rivaroxaban)

   High-titer LA

   Moderate to severe von Willebrand disease

Decreased (< 22 seconds) In

   Excessive generation of thrombin. No clinical correlation with a predisposition to thromboembolism has been definitely demonstrated.

Normal In

   Thrombocytopenias and thrombocytopathies without associated clotting defects

   Majority of cases of mild von Willebrand disease

   Isolated defects of factor VII or XIII

   Limitations

Preanalytic Pitfalls

   Partial clotting of sample due to insufficient mixture with anticoagulant

   Overfilling or underfilling the test tube, thereby changing the 9 (blood)-to-1 (anticoagulant) ratio