ASCP Coagulation 2 — Questions and Answers
Question 1: Which factor is the first to decrease to critically low levels during warfarin therapy due to its shortest half-life?
- Factor VII (Correct answer)
- Factor IX
- Factor X
- Factor II
Correct answer: Factor VII
Factor VII has the shortest half-life (approximately 4–6 hours) among vitamin K-dependent factors II, VII, IX, and X. It decreases first during warfarin initiation, explaining the early PT prolongation before full anticoagulation is achieved.
Warfarin inhibits vitamin K epoxide reductase (VKOR), blocking regeneration of vitamin K and thus carboxylation of glutamate residues on Factors II, VII, IX, X, and proteins C and S. Half-lives: VII approximately 4–6 hours, IX approximately 24 hours, X approximately 36–40 hours, II approximately 60–72 hours. Because Factor VII is in the extrinsic pathway measured by PT, PT rises quickly. Full antithrombotic effect (Factor II reduction) takes 3–4 days.
Question 2: The Bethesda assay is used to quantify:
- Heparin concentration
- Factor VIII inhibitor titer (Correct answer)
- Lupus anticoagulant
- Fibrinogen level
Correct answer: Factor VIII inhibitor titer
The Bethesda assay measures Factor VIII inhibitor (neutralizing antibody) titer in Bethesda units (BU). One BU is the amount of inhibitor that neutralizes 50% of Factor VIII activity in a normal plasma mixture.
In hemophilia A, approximately 30% of patients develop alloantibodies (inhibitors) against infused Factor VIII. The Nijmegen-modified Bethesda assay improves specificity by buffering the normal pooled plasma. Serial dilutions of patient plasma are incubated with normal plasma; residual Factor VIII is measured. Titer greater than 5 BU is a high titer inhibitor, requiring bypassing agents such as activated prothrombin complex concentrate or recombinant Factor VIIa. Low titer below 5 BU may be manageable with high-dose Factor VIII.
Question 3: Lupus anticoagulant (LA) causes what pattern on mixing studies?
- Shortens the aPTT
- Prolongs the aPTT that corrects on mixing study
- Prolongs the aPTT that does NOT correct on mixing study (Correct answer)
- Prolongs the PT only
Correct answer: Prolongs the aPTT that does NOT correct on mixing study
Lupus anticoagulant prolongs phospholipid-dependent clotting tests (aPTT, dRVVT) in vitro, but does NOT correct when mixed with normal plasma because the antibody is still present in the diluted mixture — an inhibitor pattern.
LA is an antiphospholipid antibody that binds phospholipids used in the aPTT reagent, prolonging clotting time in vitro. Paradoxically, it causes thrombosis in vivo. Mixing study: patient plus normal plasma 1:1 — if aPTT remains prolonged, an inhibitor is present. Confirmatory testing uses phospholipid dependence via hexagonal phase phospholipid or dilute Russell's viper venom time (dRVVT) correction with excess phospholipid. Persistent positive on two occasions at least 12 weeks apart confirms antiphospholipid syndrome.
Question 4: Protein C deficiency leads to thrombosis because protein C normally:
- Activates plasminogen
- Inactivates Factors Va and VIIIa (Correct answer)
- Inhibits thrombin directly
- Activates antithrombin III
Correct answer: Inactivates Factors Va and VIIIa
Activated protein C (APC), with its cofactor protein S, proteolytically inactivates Factors Va and VIIIa, downregulating thrombin generation. Protein C deficiency removes this brake, causing hypercoagulability.
Protein C is a vitamin K-dependent serine protease activated by the thrombin-thrombomodulin complex on endothelium. APC, with protein S as cofactor, cleaves and inactivates Factor Va and Factor VIIIa. This limits amplification of the coagulation cascade. Heterozygous protein C deficiency presents with VTE in young adults; homozygous or compound heterozygous deficiency causes neonatal purpura fulminans.
Question 5: A prolonged thrombin time (TT) with a normal reptilase time most likely indicates:
- Fibrinogen deficiency
- Heparin contamination (Correct answer)
- Dysfibrinogenemia
- Factor XIII deficiency
Correct answer: Heparin contamination
Thrombin is inhibited by heparin via antithrombin, prolonging TT. Reptilase (ancrod) is not inhibited by heparin or antithrombin, so reptilase time is normal with heparin. Both TT and reptilase time are prolonged in fibrinogen deficiency and dysfibrinogenemia.
Thrombin time measures the final step of coagulation: thrombin converting fibrinogen to fibrin. Causes of prolonged TT include heparin (antithrombin-mediated), direct thrombin inhibitors like hirudin or argatroban, hypofibrinogenemia, dysfibrinogenemia, and elevated FDPs. Reptilase from Bothrops atrox venom also cleaves fibrinogen to fibrin but is not inhibited by antithrombin or heparin. Result interpretation: TT prolonged plus reptilase normal equals heparin effect; both prolonged equals fibrinogen problem.
Question 6: A patient with hemophilia B (Christmas disease) has a deficiency of which factor?
- Factor VIII
- Factor IX (Correct answer)
- Factor XI
- Factor XII
Correct answer: Factor IX
Hemophilia B (Christmas disease) is caused by deficiency of Factor IX. It is X-linked recessive, clinically identical to hemophilia A, and is treated with Factor IX concentrates or recombinant Factor IX.
Factor IX is a vitamin K-dependent serine protease activated by Factor XIa (intrinsic pathway) or the Factor VIIa-tissue factor complex (extrinsic pathway). Factor IXa complexes with Factor VIIIa (tenase complex) on phospholipid surfaces to activate Factor X. Deficiency leads to aPTT prolongation with normal PT. Incidence is approximately 1 in 30,000 males. Severity correlates with Factor IX activity: severe below 1%, moderate 1–5%, mild 5–40%.
ASCP Coagulation 2
Which factor is the first to decrease to critically low levels during warfarin therapy due to its shortest half-life?