Thrombophilia and antiphospholipid syndrome

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Thrombophilia and antiphospholipid syndrome, part of Coagulation and Hemostasis. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A 38-year-old man had an unprovoked deep vein thrombosis. Three months after stopping anticoagulation, his thrombophilia screen shows: protein S activity (clot-based APC-cofactor assay) 41% (reference 65-140%), free protein S antigen 92% (65-140%), total protein S antigen 101% (70-140%), protein C activity 104% (70-140%), antithrombin 96% (80-120%) and a modified APC-resistance ratio (patient plasma diluted in factor V-deficient plasma) of 1.55 (normal above 2.0). His liver tests are normal and he takes no regular medicines. What is the most likely interpretation?

   1. AType III protein S deficiency (low free form)
   2. BFactor V Leiden falsely lowering the protein S activity
   3. CType II (qualitative) protein S deficiency
   4. DFactor V Leiden with type I protein S deficiency

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   Answer: B · Factor V Leiden falsely lowering the protein S activity

   Free and total protein S antigen are both normal, so there is no quantitative deficiency: type I lowers both, and type III lowers the free form. A low clot-based activity with a normal antigen would suggest type II, but the modified APC-resistance ratio of 1.55 points to factor V Leiden. The clot-based assay measures how much added APC, with the patient's protein S, prolongs clotting; factor Va Leiden resists APC, so clotting is prolonged less and protein S activity reads falsely low. Factor V Leiden should be confirmed by genotyping, and type II protein S deficiency diagnosed only after APC resistance is excluded. Many laboratories screen with free protein S antigen for this reason.

   • A: Type III has a low free protein S antigen; here it is normal.
   • C: Type II is diagnosed only after APC resistance, which lowers clot-based results, is excluded.
   • D: Type I lowers total and free antigen; both are normal here.

2. Question 2 · Hard

   dRVVT results: screen, patient 58 s and normal pool 36 s; confirm, patient 40 s and normal pool 34 s. The laboratory calculates the normalized ratio as (patient screen ÷ pool screen) ÷ (patient confirm ÷ pool confirm), with a cutoff of 1.20. What is the result?

   1. A0.73, lupus anticoagulant not detected
   2. B1.37, lupus anticoagulant detected
   3. C1.18, lupus anticoagulant not detected
   4. D1.61, lupus anticoagulant detected

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   Answer: B · 1.37, lupus anticoagulant detected

   The screen ratio is 58 ÷ 36 = 1.61 and the confirm ratio is 40 ÷ 34 = 1.18. The normalized ratio is 1.61 ÷ 1.18 = 1.37, which is above the 1.20 cutoff, so a lupus anticoagulant is detected. The confirm reagent contains excess phospholipid that overwhelms the antibody, so a clotting time that shortens with extra phospholipid shows phospholipid dependence rather than a factor deficiency. A positive result must persist on a repeat sample at least 12 weeks later before it counts toward antiphospholipid syndrome.

   • A: 0.73 divides the confirm ratio by the screen ratio, the wrong way round.
   • C: 1.18 is the confirm ratio alone, which does not express phospholipid dependence.
   • D: 1.61 is the screen ratio alone, not corrected by the confirm ratio.

3. Question 3 · Medium

   During warfarin therapy for an unprovoked DVT, which thrombophilia test still gives a reliable result?

   1. AProtein C activity
   2. BFree protein S antigen
   3. CDilute Russell viper venom time
   4. DFactor V Leiden genotyping

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   Answer: D · Factor V Leiden genotyping

   Genotyping for factor V Leiden and prothrombin G20210A tests DNA, so anticoagulants, acute thrombosis and liver function do not affect it. Warfarin lowers the vitamin K-dependent anticoagulants protein C and protein S, so low levels during therapy cannot be interpreted. Warfarin also prolongs the dRVVT, which can give a false-positive lupus anticoagulant result. Protein C and protein S are usually tested after warfarin has been stopped for several weeks.

   • A: Warfarin lowers protein C, so a low level on therapy cannot be interpreted.
   • B: Warfarin lowers protein S, so a low level on therapy cannot be interpreted.
   • C: Warfarin prolongs the dRVVT and can give a false-positive lupus anticoagulant result.

4. Question 4 · Hard

   A 30-year-old man with a DVT is tested 3 months after stopping anticoagulation. Results: total protein S antigen 94% (reference 70-140%), free protein S antigen 31% (65-140%), protein S activity 29% (65-140%). Liver tests are normal. Which protein S deficiency type fits these results?

   1. AType III: low free and normal total antigen
   2. BAcquired: a lingering warfarin effect
   3. CType I: low total and free antigen
   4. DType II: normal antigens with low activity

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   Answer: A · Type III: low free and normal total antigen

   Protein S circulates about 60% bound to C4b-binding protein and 40% free, and only free protein S acts as a cofactor for activated protein C. Type I deficiency lowers total antigen, free antigen and activity; type II lowers activity only, with normal antigen levels; and type III (also called type IIa) lowers free antigen and activity while total antigen stays normal. Here total antigen is normal but free antigen and activity are low, which fits type III. Testing 3 months after stopping warfarin, with normal liver tests, makes the common acquired causes unlikely.

   • B: Warfarin's effect has passed 3 months after stopping, and it would lower total protein S too.
   • C: Type I would lower total protein S antigen as well.
   • D: Type II would show a normal free antigen with low activity only.

5. Question 5 · Hard

   A 42-year-old woman with no bleeding history has an APTT of 61 s (reference 25-35 s) with a normal PT and thrombin time. One-stage assays at a 1:10 plasma dilution give factors VIII, IX, XI and XII of 18-25%; at 1:40 all four rise above 60% (reference 50-150%). The dRVVT normalized ratio is 1.6 (cutoff 1.20). The most likely explanation is:

   1. AA specific acquired inhibitor of factor VIII
   2. BA lupus anticoagulant affecting the factor assays
   3. CHeparin contamination of the sample
   4. DCombined deficiency of factors VIII, IX, XI and XII

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   Answer: B · A lupus anticoagulant affecting the factor assays

   A lupus anticoagulant prolongs the APTT-based clotting step of one-stage intrinsic factor assays, so several factors appear low at the usual dilution. As the plasma is diluted further the antibody is diluted out and the apparent levels rise (non-parallel results), which a true deficiency does not do. The positive dRVVT, the normal thrombin time and the absence of bleeding all support a lupus anticoagulant. Factor levels should be read from the higher dilutions or measured by methods less sensitive to the antibody, such as chromogenic assays.

   • A: A specific factor VIII inhibitor would leave factors IX, XI and XII normal.
   • C: Heparin would prolong the thrombin time, which is normal here.
   • D: A true multiple deficiency would give the same dilution-corrected level at every dilution.

6. Question 6 · Easy

   To meet the laboratory criteria for antiphospholipid syndrome, a positive lupus anticoagulant or antiphospholipid antibody result must be confirmed on a repeat sample taken at least:

   1. A12 weeks later
   2. B12 months later
   3. C6 weeks later
   4. D2 weeks later

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7. Question 7 · Medium

   A woman at 26 weeks' gestation is tested after her sister's DVT. Free protein S antigen is 38% (nonpregnant reference 60-130%); protein C and antithrombin are normal. The best interpretation is:

   1. AAn expected fall in pregnancy; retest after delivery
   2. BA sign of an acquired protein S antibody
   3. CAn artifact of collecting into EDTA
   4. DConfirmed inherited protein S deficiency

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8. Question 8 · Medium

   Why is plasma for lupus anticoagulant testing centrifuged twice before testing or freezing?

   1. ASpinning twice concentrates the antibodies in the plasma
   2. BSpinning twice removes fibrinogen that blocks the test
   3. CLeftover platelets supply phospholipid that hides the inhibitor
   4. DSpinning twice inactivates any heparin in the sample

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9. Question 9 · Hard

   A prolonged aPTT does not correct on a 1:1 mix with normal plasma and shortens when excess phospholipid is added. What is the most likely cause?

   1. AFactor VIII deficiency
   2. BA lupus anticoagulant
   3. CFactor VII deficiency
   4. Dvon Willebrand disease

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