Mixing studies and factor assays

16 DOH Abu Dhabi (HAAD) lab licensing practice questions on Mixing studies and factor assays, part of Coagulation and Hemostasis. The first 16 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 72-year-old man has had large spontaneous bruises for 2 weeks and no earlier bleeding problems. PT is normal and APTT is 86 s (reference 25-35 s). A 1:1 mix with normal plasma gives 38 s immediately and 69 s after 2 hours at 37 °C. The dRVVT normalized ratio is 1.04 (positive above 1.20). One-stage factor assays at 1:10 and 1:40 plasma dilutions give: factor VIII 2% and 2%; factor IX 34% and 81%; factor XI 30% and 84%; factor XII 38% and 92% (reference 50-150%). What is the best interpretation?

   1. ACombined intrinsic factor deficiency
   2. BAcquired factor VIII inhibitor
   3. CNonspecific lupus anticoagulant
   4. DUndiagnosed congenital hemophilia A

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   Answer: B · Acquired factor VIII inhibitor

   The 1:1 mix nearly corrects at first but prolongs again after incubation, the typical time- and temperature-dependent behavior of an autoantibody against factor VIII. Factor VIII stays at 2% at every dilution, while factors IX, XI and XII rise into the normal range as the plasma is diluted: they were falsely low because the inhibitor in the patient's plasma neutralized the factor VIII present in each factor-deficient substrate plasma. A lupus anticoagulant would make all the intrinsic factors, including factor VIII, rise on dilution, and the dRVVT here is negative. True deficiencies of IX, XI and XII would not rise on dilution, and congenital hemophilia does not first appear at 72 with an inhibitor pattern in the mix. The next step is a Nijmegen-Bethesda assay to measure the titer.

   • A: True deficiencies of IX, XI and XII would not rise on dilution.
   • C: A lupus anticoagulant would give a positive dRVVT, and factor VIII would rise on dilution too.
   • D: Congenital hemophilia does not first bleed at 72 or show a time-dependent inhibitor.

2. Question 2 · Medium

   A man whose maternal uncle has hemophilia A bleeds after a tooth extraction. His one-stage factor VIII is 62 IU/dL (62%), but a chromogenic factor VIII on the same sample is 15 IU/dL (15%). VWF antigen, VWF activity and VWF factor VIII binding are normal. These results most likely show:

   1. AA normal factor VIII by the routine method
   2. BMild hemophilia A with assay discrepancy
   3. CType 2N von Willebrand disease
   4. DA lupus anticoagulant lowering the chromogenic result

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   Answer: B · Mild hemophilia A with assay discrepancy

   Up to about a third of people with mild hemophilia A have a discrepancy between the one-stage and chromogenic factor VIII assays, caused by particular F8 variants. In one form, the one-stage result is clearly higher than the chromogenic result and may even be normal, so the diagnosis is missed if only the one-stage assay is used. The chromogenic result here better fits his bleeding and family history. When mild hemophilia A is suspected and the one-stage level is normal, a chromogenic assay should be done.

   • A: The one-stage value is misleading here; the history and the chromogenic result point to hemophilia.
   • C: Normal VWF binding of factor VIII excludes type 2N von Willebrand disease.
   • D: A lupus anticoagulant mainly lowers APTT-based one-stage results, not chromogenic ones.

3. Question 3 · Hard

   In a Nijmegen-Bethesda assay, residual factor VIII at three patient-plasma dilutions is: 1:10, 12%; 1:40, 50%; 1:80, 71%. The laboratory uses only dilutions with 25-75% residual activity. What inhibitor titer should be reported?

   1. A40 BU/mL
   2. B20 BU/mL
   3. C1 BU/mL
   4. D31 BU/mL

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   Answer: A · 40 BU/mL

   The 1:10 dilution leaves only 12% residual factor VIII, which is outside the valid range, so it is not used. At 1:40 the residual is 50%, which equals 1 BU in the mixture; multiplied by the dilution this gives 1 x 40 = 40 BU/mL. The 1:80 result agrees: log2(100 / 71) = about 0.49 BU, x 80 = about 40 BU/mL. A titer this high is a high-titer inhibitor.

   • B: 20 BU/mL halves the answer, as if 50% residual meant 0.5 BU.
   • C: 1 BU/mL is the value in the test mixture before multiplying by the dilution of 40.
   • D: 31 BU/mL comes from the 1:10 dilution, whose 12% residual is outside the valid range.

4. Question 4 · Medium

   In acquired hemophilia A, a patient can have a factor VIII of 8 IU/dL (8%) and a Bethesda titer of 20 BU/mL at the same time. This combination reflects:

   1. AType 1, complete inhibitor kinetics
   2. BType 2, non-linear inhibitor kinetics
   3. CA lupus anticoagulant mimicking an inhibitor
   4. DFalsely raised factor VIII from heparin

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   Answer: B · Type 2, non-linear inhibitor kinetics

   Alloantibodies in severe hemophilia A usually show type 1 kinetics: they inactivate factor VIII completely, in proportion to the antibody concentration. Autoantibodies in acquired hemophilia usually show type 2 kinetics: inactivation is fast at first and then levels off, so some factor VIII activity remains even with a high titer. Neither the residual factor VIII nor the titer predicts bleeding well, and these patients can bleed severely. Treatment uses bypassing agents or recombinant porcine factor VIII, with immunosuppression to remove the antibody.

   • A: Type 1 kinetics inactivate factor VIII completely and are typical of alloantibodies.
   • C: A lupus anticoagulant lowers several intrinsic factors at once and rarely causes bleeding.
   • D: Heparin prolongs clotting times and would lower, not raise, apparent factor VIII.

5. Question 5 · Hard

   A patient's APTT is 52 s (reference 25-35). Mixing results: immediate 1:1 mix 33 s; mix incubated 2 hours at 37 °C 41 s; patient and normal plasma incubated separately for 2 hours, then mixed, 40 s; normal pool incubated alone 34 s. The best interpretation is:

   1. AA time-dependent acquired factor VIII inhibitor
   2. BHeparin contamination from a venous line
   3. CFactor deficiency plus labile factor decay at 37 °C
   4. DA lupus anticoagulant acting immediately

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   Answer: C · Factor deficiency plus labile factor decay at 37 °C

   The immediate mix corrects, which points to a factor deficiency. The incubated mix is longer, but the control made by incubating the two plasmas separately and mixing them at the end is just as long. The extra prolongation is therefore caused by loss of the labile factors V and VIII at 37 °C, not by an antibody acting over time. A time-dependent factor VIII inhibitor would make the incubated mix clearly longer than the separately incubated control.

   • A: A time-dependent inhibitor would prolong the incubated mix well beyond the separately incubated control.
   • B: Heparin acts at once and would prevent correction of the immediate mix.
   • D: A lupus anticoagulant usually fails to correct even in the immediate mix.

6. Question 6 · Hard

   A 40 kg boy with severe hemophilia A (baseline factor VIII below 1 IU/dL) receives 2,000 IU of factor VIII. Each IU/kg normally raises plasma factor VIII by about 2 IU/dL. His 15-minute post-infusion level is 14 IU/dL (14%). This result most likely indicates:

   1. APoor recovery, suggesting a factor VIII inhibitor
   2. BExpected dilution of the dose in a small child
   3. CLoss of the dose through its short half-life
   4. DNormal recovery for the dose given

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7. Question 7 · Easy

   Normal plasma for mixing studies is pooled from many healthy donors rather than taken from one donor. Why?

   1. APooling makes the plasma stable at room temperature
   2. BPooling brings each factor close to 100% activity
   3. CPooling removes platelets from the plasma
   4. DPooling neutralizes any heparin in donor samples

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8. Question 8 · Hard

   A 4-year-old boy has an APTT of 68 s (reference 25-35) and a normal PT. His plasma is mixed 1:1 with normal plasma and with plasmas that each lack one factor. APTT results: with normal plasma 33 s; with factor VIII-deficient plasma 34 s; with factor IX-deficient plasma 66 s; with factor XI-deficient plasma 35 s. Which factor is he most likely missing?

   1. AFactor XI
   2. BFactor XII
   3. CFactor IX
   4. DFactor VIII

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9. Question 9 · Medium

   By definition, 1 Bethesda unit leaves 50% residual factor VIII in the test mixture. With type 1 kinetics, what residual factor VIII is expected when the mixture contains 3 BU?

   1. A25%
   2. B16.7%
   3. C12.5%
   4. D0%

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10. Question 10 · Medium

    A lupus anticoagulant can give a false low-titer result in a Bethesda assay that measures residual factor VIII with a one-stage APTT method. Which change best avoids this?

    1. AHeating the patient plasma at 56 °C before testing
    2. BExtending the incubation from 2 hours to 4 hours
    3. CMeasuring residual factor VIII chromogenically
    4. DUsing unbuffered rather than buffered normal plasma

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11. Question 11 · Hard

    A 2-year-old boy with severe hemophilia A, after about 15 exposure days to factor VIII, has a Nijmegen-Bethesda titer of 3 BU/mL. After a further dose of factor VIII for a joint bleed, the titer rises to 24 BU/mL. How should his inhibitor be classified?

    1. ATransient inhibitor
    2. BLow-responding inhibitor
    3. CNon-specific lupus inhibitor
    4. DHigh-responding inhibitor

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12. Question 12 · Medium

    Before a Bethesda assay, plasma that may contain infused factor VIII is heated at 56 °C for 30 minutes. What does this step achieve?

    1. AIt inactivates any lupus anticoagulant in the plasma
    2. BIt destroys factor VIII but spares the antibody
    3. CIt converts IgM antibodies into IgG antibodies
    4. DIt removes heparin left from the infusion line

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13. Question 13 · Easy

    A one-stage factor VII activity assay is based on the:

    1. AThrombin time (TT)
    2. BActivated partial thromboplastin time (APTT)
    3. CReptilase time
    4. DProthrombin time (PT)

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14. Question 14 · Medium

    A 34-year-old woman with no personal or family history of bleeding is screened before a laparoscopy. Her coagulation results are shown. They are most consistent with:

    

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    1. AFactor VIII deficiency
    2. BA lupus anticoagulant
    3. CAn acquired factor VIII inhibitor
    4. DHeparin contamination

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15. Question 15 · Medium

    A patient's APTT is prolonged. Refer to the clotting times of the reference plasma and of three dilutions of the patient's plasma in a one-stage factor VIII method. What is the correct interpretation?

    

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    1. AA valid assay: the 1:10 result can be reported
    2. BA true deficiency: the two lines are parallel
    3. CA calibration fault: the reference line is too steep
    4. DAn inhibitor: activity rises as the plasma is diluted

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16. Question 16 · Hard

    A 2-year-old boy whose maternal uncle has hemophilia B bruises easily. His APTT is prolonged and his PT normal. In the one-stage factor IX assay shown, each patient dilution has been read on the reference line, and the reference plasma is 100 IU/dL. His factor IX activity, and its severity by ISTH criteria, is:

    

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    1. A8 IU/dL: mild hemophilia B
    2. B4 IU/dL: moderate hemophilia B
    3. C16 IU/dL: mild hemophilia B
    4. D160 IU/dL: no hemophilia

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