PT & aPTT, Coagulation Testing — Vocabulary Flashcards

Thirty vocabulary flashcards covering key terms, factors, assays, pathways, and clinical concepts related to PT, aPTT, inhibitor testing, and direct thrombin inhibitor monitoring.

Prothrombin Time (PT)

Laboratory test measuring the extrinsic and common coagulation pathways by timing clot formation after adding tissue factor and calcium.

Tissue Factor (Thromboplastin)

Phospholipid-rich reagent that activates the extrinsic pathway in the PT assay.

Extrinsic Pathway

Coagulation pathway triggered by tissue factor; involves Factor VII and feeds into the common pathway.

Common Pathway

Final part of the coagulation cascade where Factors X, V, II, and I generate thrombin and fibrin.

Clotting Factor I (Fibrinogen)

Soluble plasma protein converted by thrombin into insoluble fibrin to form a stable clot.

Clotting Factor II (Prothrombin)

Vitamin K–dependent zymogen converted to thrombin in the common pathway.

Activated Partial Thromboplastin Time (aPTT)

Test that evaluates the intrinsic and common pathways by timing clot formation after adding a contact activator, phospholipid, and calcium.

Intrinsic Pathway

Coagulation pathway activated by contact with negatively charged surfaces; involves Factors XII, XI, IX, and VIII.

Contact Activator (Kaolin)

Negatively charged substance used in aPTT to initiate the intrinsic pathway.

International Normalized Ratio (INR)

Standardized PT result allowing comparison across laboratories by correcting for reagent variability.

International Sensitivity Index (ISI)

Calibration factor provided with thromboplastin reagents used to calculate the INR.

Warfarin

Oral vitamin K antagonist anticoagulant monitored primarily with PT/INR.

Heparin (Unfractionated)

Parenteral anticoagulant that potentiates antithrombin; dosing is monitored with aPTT.

Lupus Anticoagulant

Autoantibody that prolongs phospholipid-dependent clotting assays yet paradoxically increases thrombosis risk.

Mixing Study

Test where patient plasma is mixed with normal plasma to distinguish factor deficiency from inhibitor presence.

Circulating Anticoagulant

Autoantibody that interferes with overall coagulation, such as lupus anticoagulant or nonspecific inhibitors.

Specific Factor Inhibitor

Antibody directed against a single clotting factor (e.g., Factor VIII), causing targeted functional loss.

Factor VIII Inhibitor

Autoantibody that neutralizes Factor VIII activity, common in hemophilia A or acquired hemophilia.

Bethesda Assay

Quantitative test that measures the titer of factor inhibitors by assessing residual clotting activity after incubation.

Bethesda Unit (BU)

Amount of inhibitor that lowers Factor VIII activity by 50 % in the Bethesda assay.

Nijmegen-Bethesda Modification

Improved Bethesda method using imidazole-buffered plasma and stabilizers to reduce assay variability.

Ecarin Clotting Time (ECT)

Assay that measures the anticoagulant effect of direct thrombin inhibitors via ecarin-generated meizothrombin.

Direct Thrombin Inhibitor (DTI)

Drug class (e.g., hirudin, dabigatran) that binds thrombin directly; monitored with ECT or ECA.

Meizothrombin

Intermediate formed when ecarin converts prothrombin; inhibited by DTIs in the ECT.

Ecarin Chromogenic Assay (ECA)

Variant of ECT using a chromogenic substrate to quantify meizothrombin generation, unaffected by heparin or vitamin K antagonists.

Kaolin

Fine clay mineral used as contact activator in aPTT to initiate the intrinsic pathway.

Prolonged PT

Extended clotting time indicating possible deficiencies of Factors I, II, V, VII, X; liver disease; vitamin K deficiency; or warfarin effect.

Prolonged aPTT

Extended clotting time suggesting intrinsic pathway defects, presence of inhibitors, lupus anticoagulant, or unfractionated heparin therapy.

Factor V and VIII Degradation

Heat-induced loss of labile clotting factors that can falsely prolong PT and aPTT if plasma remains >5 min at 37 °C.

3.2 % Sodium Citrate

Anticoagulant used in coagulation tubes that chelates calcium while preserving clotting factors for PT/aPTT testing.