Lab: factor assays and von Willebrand assays

7 ASCP MLS practice questions on Lab: factor assays and von Willebrand assays, part of Hematology. The first 7 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 16-year-old boy bleeds for 2 days after a tooth extraction. His maternal uncle has mild hemophilia A. Results: PT normal, APTT 33 s (reference 25-35 s), platelets normal, VWF antigen 96% and VWF activity 91% (reference 50-150%). One-stage (clot-based) factor VIII is 58% (reference 50-150%), with parallel results at three plasma dilutions. A chromogenic factor VIII assay on the same sample gives 14%. A lupus anticoagulant screen is negative. The best interpretation is:

   1. ALupus anticoagulant lowering the chromogenic result
   2. BMild hemophilia A missed by the one-stage assay
   3. CType 2N von Willebrand disease
   4. DNormal factor VIII; the chromogenic result is wrong

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   Answer: B · Mild hemophilia A missed by the one-stage assay

   Some F8 variants found in mild hemophilia A give one-stage results that are normal or nearly normal, while the chromogenic assay, which in these patients better reflects the bleeding tendency, is clearly low. With an X-linked family history, delayed bleeding and a chromogenic factor VIII of 14%, he has mild hemophilia A. For this reason guidelines advise measuring factor VIII by both methods when mild hemophilia A is suspected, since a normal APTT and one-stage result can miss it. Parallel dilutions and a negative screen exclude a lupus anticoagulant, which in any case affects clot-based, not chromogenic, assays. Type 2N VWD lowers factor VIII by both methods, so the two assays would agree.

   • A: A lupus anticoagulant affects clot-based assays, not chromogenic ones, and the screen is negative.
   • C: Type 2N lowers factor VIII by both methods, so the two assays would agree.
   • D: A low chromogenic result with bleeding and an X-linked family history should not be dismissed as an error.

2. Question 2 · Medium

   Three dilutions of a patient's plasma were tested and their clotting times plotted beside the reference line. Which report is correct?

   

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   1. AFactor VIII 25 IU/dL
   2. BNo value; an inhibitor is present
   3. CFactor VIII 50 IU/dL
   4. DFactor VIII 12.5 IU/dL

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   Answer: A · Factor VIII 25 IU/dL

   The patient's three points form a line parallel to the reference line, so the dilutions behave alike and there is no sign of an inhibitor or lupus anticoagulant. The patient's 1:10 clotting time (60 s) equals the reference at 1:40, and each patient dilution matches the reference two doubling steps further on, so the patient has a quarter of the reference activity. With the reference at 100 IU/dL, factor VIII is 100 × 10 ÷ 40 = 25 IU/dL (0.25 IU/mL). A non-parallel line, with apparent activity rising as the plasma is diluted, would point to an inhibitor.

   • B: Parallel lines mean the dilutions agree; an inhibitor gives a non-parallel line.
   • C: 50 IU/dL would put the patient line one dilution step from the reference, not two.
   • D: 12.5 IU/dL would need a shift of three dilution steps.

3. Question 3 · Hard

   A factor VIII assay is run at three plasma dilutions. Results corrected for dilution are 22 IU/dL at 1:10, 38 IU/dL at 1:20 and 61 IU/dL at 1:40. These results most likely show:

   1. AA true factor VIII deficiency
   2. BA clotted, activated specimen
   3. CNormal, parallel dilution behavior
   4. DAn inhibitor that is diluted out

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   Answer: D · An inhibitor that is diluted out

   In a valid one-stage assay, results corrected for dilution agree closely at each dilution, meaning the patient's dose-response line is parallel to the calibration curve. Here the activity rises steadily with dilution, so something in the plasma, such as a lupus anticoagulant, a specific factor inhibitor or an anticoagulant drug, is being diluted out. The 1:10 result underestimates the true factor level. The lab should test further dilutions until results agree, or report the result as nonparallel and consider a chromogenic assay.

   • A: A true deficiency gives the same corrected result at each dilution.
   • B: Activated factors raise results most at the lowest dilution, the opposite of this pattern.
   • C: Parallel results agree after correction; these rise nearly threefold.

4. Question 4 · Easy

   In a one-stage factor VIII assay, which substrate plasma is added to the diluted patient plasma?

   1. AFactor VIII-deficient plasma
   2. BNormal pooled plasma
   3. CBarium-adsorbed plasma
   4. DFactor IX-deficient plasma

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   Answer: A · Factor VIII-deficient plasma

   The substrate plasma supplies every factor except factor VIII, so the APTT of the mixture depends only on the factor VIII in the patient's diluted plasma. The clotting time is compared with a calibration curve made from dilutions of a reference plasma with known factor VIII activity. Results are reported in IU/dL or as percent of normal. The same design, with factor IX-deficient plasma, measures factor IX.

   • B: Normal pooled plasma would supply factor VIII and mask a deficiency.
   • C: Barium-adsorbed plasma lacks the vitamin K-dependent factors, not factor VIII.
   • D: Factor IX-deficient plasma is the substrate for a factor IX assay.

5. Question 5 · Medium

   Which type of von Willebrand disease shows platelet aggregation with low-dose ristocetin (about 0.5 mg/mL) and often mild thrombocytopenia?

   1. AType 3
   2. BType 2B
   3. CType 2N
   4. DType 2A

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   Answer: B · Type 2B

   Type 2B VWF has a gain-of-function change that makes it bind platelet GPIb too readily, so the patient's platelet-rich plasma aggregates even with low-dose ristocetin. The bound VWF and platelets are cleared, which removes the largest multimers from plasma and often lowers the platelet count. Platelet-type (pseudo) VWD, caused by a GPIb defect, looks the same and is separated by mixing patient platelets with normal plasma or by genetic testing. Desmopressin can worsen the thrombocytopenia in type 2B.

   • A: Type 3 has almost no VWF, so ristocetin aggregation is absent.
   • C: Type 2N affects factor VIII binding, and ristocetin aggregation is normal.
   • D: Type 2A has reduced, not enhanced, ristocetin-induced aggregation.

6. Question 6 · Medium

   A woman with heavy menstrual bleeding has factor VIII 14 IU/dL (0.14 IU/mL) with normal VWF antigen and VWF activity. Her VWF binds factor VIII poorly. The diagnosis is:

   1. AHemophilia A carrier with low factor VIII
   2. BType 2N von Willebrand disease
   3. CType 2M von Willebrand disease
   4. DType 1 von Willebrand disease

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7. Question 7 · Hard

   VWF antigen 48 IU/dL (0.48 IU/mL; reference 50-150), VWF activity (GPIb binding) 13 IU/dL (0.13 IU/mL; reference 50-150), factor VIII 52 IU/dL, a normal multimer pattern and no aggregation with low-dose ristocetin. Which von Willebrand disease type fits best?

   1. AType 2B
   2. BType 2M
   3. CType 2A
   4. DType 1

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