Ten weeks after an uncomplicated first delivery, a 31-year-old woman has large spontaneous bruises and a painful thigh hematoma. She had no abnormal bleeding before, including at delivery. Results: PT 12.4 s (reference 11.0-13.5 s), APTT 71 s (reference 25-35 s), platelets 310 × 10^3/µL (310 × 10^9/L). APTT mixing studies: immediate 1:1 mix 36 s; 1:1 mix incubated 2 hours at 37 °C, 59 s; patient and normal plasma incubated separately for 2 hours and then mixed, 37 s. Factor VIII is 3% (reference 50-150%), and factors IX and XI are normal. What is the most likely diagnosis?

Symptomatic hemophilia A carrier

A boy with severe hemophilia A receives emicizumab prophylaxis, and his aPTT is now normal. Which method gives a valid factor VIII inhibitor titer?

Chromogenic Bethesda with bovine reagents

One-stage Bethesda after heparinase treatment

Chromogenic Bethesda with human reagents

Standard one-stage (aPTT-based) Bethesda

A 76-year-old woman with no previous bleeding problems develops large spontaneous bruises over one week. PT is normal, aPTT is 72 s (reference 25-35 s), factor VIII is 3% (0.03 IU/mL), and a Bethesda assay shows 14 BU/mL. The most likely diagnosis is:

Acquired von Willebrand syndrome

The Nijmegen modification of the Bethesda assay buffers the normal pooled plasma and uses factor VIII-deficient plasma in the control mixture. What is its main purpose?

To prevent false-positive low-titer results

To detect factor IX inhibitors at the same time

To remove lupus anticoagulant interference

To shorten the incubation to 30 minutes

Disease states: inhibitors

6 ASCP MLS practice questions on Disease states: inhibitors, part of Hematology. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
Ten weeks after an uncomplicated first delivery, a 31-year-old woman has large spontaneous bruises and a painful thigh hematoma. She had no abnormal bleeding before, including at delivery. Results: PT 12.4 s (reference 11.0-13.5 s), APTT 71 s (reference 25-35 s), platelets 310 × 10^3/µL (310 × 10^9/L). APTT mixing studies: immediate 1:1 mix 36 s; 1:1 mix incubated 2 hours at 37 °C, 59 s; patient and normal plasma incubated separately for 2 hours and then mixed, 37 s. Factor VIII is 3% (reference 50-150%), and factors IX and XI are normal. What is the most likely diagnosis?
1. AAcquired hemophilia A
2. BType 2N von Willebrand disease
3. CSymptomatic hemophilia A carrier
4. DLupus anticoagulant
Show answerHide answer
Answer: A · Acquired hemophilia A
The immediate mix nearly corrects, but the incubated mix prolongs again while the separately incubated control stays short; this is the signature of a time- and temperature-dependent inhibitor rather than loss of labile factors. With factor VIII at 3% and the other intrinsic factors normal, the inhibitor is directed against factor VIII. New bleeding within months of pregnancy fits postpartum acquired hemophilia A, an autoantibody that usually appears within a year of delivery. A lupus anticoagulant acts immediately and does not cause soft-tissue bleeding, while a carrier or type 2N VWD has a true deficiency that corrects in both mixes and would have bled before. The next step is a Nijmegen-Bethesda assay to measure the inhibitor titer.
- B: Type 2N is an inherited deficiency that corrects on mixing and would have caused earlier bleeding.
- C: A carrier has a true deficiency that corrects in both mixes, and she had no earlier bleeding.
- D: A lupus anticoagulant prolongs the immediate mix and causes thrombosis rather than bleeding.
Question 2 · Hard
A boy with severe hemophilia A receives emicizumab prophylaxis, and his aPTT is now normal. Which method gives a valid factor VIII inhibitor titer?
1. AChromogenic Bethesda with bovine reagents
2. BOne-stage Bethesda after heparinase treatment
3. CChromogenic Bethesda with human reagents
4. DStandard one-stage (aPTT-based) Bethesda
Show answerHide answer
Answer: A · Chromogenic Bethesda with bovine reagents
Emicizumab is a bispecific antibody that bridges human factor IXa and factor X, doing the job of factor VIIIa. It shortens the aPTT and makes one-stage and human-reagent chromogenic factor VIII results falsely high, which hides any inhibitor. It does not react with bovine factor IXa and X, so a chromogenic Bethesda assay with bovine reagents measures factor VIII and its inhibitor accurately. The laboratory must know that a patient is on emicizumab before testing.
- B: Heparinase removes heparin, not emicizumab, which is an antibody.
- C: Emicizumab works with human factors IXa and X, so human-reagent chromogenic assays read falsely high.
- D: Emicizumab interferes with any aPTT-based assay, so the one-stage method is invalid.
Question 3 · Medium
A 76-year-old woman with no previous bleeding problems develops large spontaneous bruises over one week. PT is normal, aPTT is 72 s (reference 25-35 s), factor VIII is 3% (0.03 IU/mL), and a Bethesda assay shows 14 BU/mL. The most likely diagnosis is:
1. AAcquired hemophilia A
2. BMild hemophilia A
3. CAcquired von Willebrand syndrome
4. DLupus anticoagulant
Show answerHide answer
Answer: A · Acquired hemophilia A
Acquired hemophilia A is caused by an autoantibody against factor VIII in a person with no earlier bleeding history. It occurs mostly in older adults and after childbirth, and is linked to autoimmune disease and cancer, though about half of cases have no clear cause. Bleeding is typically into skin and muscle rather than joints, and can be severe even at modest titers. A low factor VIII with a positive Bethesda titer confirms it.
- B: Mild hemophilia A is inherited, gives lifelong bleeding and has no inhibitor.
- C: Acquired von Willebrand syndrome lowers VWF and does not give a Bethesda titer.
- D: A lupus anticoagulant is linked to thrombosis, not to sudden severe bruising.
Question 4 · Medium
The Nijmegen modification of the Bethesda assay buffers the normal pooled plasma and uses factor VIII-deficient plasma in the control mixture. What is its main purpose?
1. ATo detect factor IX inhibitors at the same time
2. BTo remove lupus anticoagulant interference
3. CTo prevent false-positive low-titer results
4. DTo shorten the incubation to 30 minutes
Show answerHide answer
Answer: C · To prevent false-positive low-titer results
In the original Bethesda assay, the pH of the unbuffered normal plasma rises during the 2-hour incubation, and factor VIII decays unevenly between the test and control tubes. This can produce apparent inhibitors of less than 1 BU in patients who have none. Buffering with imidazole and matching the protein content of the control with factor VIII-deficient plasma keep factor VIII stable, so low titers are more reliable. The incubation stays at 2 hours at 37 °C.
- A: The assay remains specific for factor VIII inhibitors.
- B: A lupus anticoagulant can still interfere with clot-based factor VIII readings.
- D: The 2-hour incubation at 37 °C is kept.
Question 5 · Easy
One Bethesda unit is the amount of inhibitor that neutralizes how much of the factor VIII in an equal volume of normal plasma during 2 hours at 37 °C?
1. A50% of the factor VIII
2. BAll of the factor VIII
3. C75% of the factor VIII
4. D25% of the factor VIII
Show answerHide answer
Answer: A · 50% of the factor VIII
By definition, 1 Bethesda unit (BU) leaves 50% residual factor VIII activity after normal pooled plasma is mixed 1:1 with patient plasma and incubated for 2 hours at 37 °C. The incubation is needed because factor VIII inhibitors act slowly and depend on temperature. Residual activity follows a log scale, so 25% residual activity equals 2 BU. Only dilutions giving 25% to 75% residual activity are used to calculate the titer.
- B: Neutralizing all the factor VIII would represent a far higher titer.
- C: Neutralizing 75% (25% residual activity) equals 2 BU.
- D: Neutralizing 25% (75% residual activity) is about 0.4 BU.
Question 6 · Hard
In a Nijmegen-Bethesda assay, patient plasma diluted 1:10 is mixed with buffered normal pooled plasma and incubated for 2 hours at 37 °C. Factor VIII is 25% in the test mixture and 50% in the control mixture. What is the inhibitor titer?
1. A10 BU/mL
2. B20 BU/mL
3. C1 BU/mL
4. D0.1 BU/mL
Answer and explanation in LearnFree account

Disease states: inhibitors

A boy with severe hemophilia A receives emicizumab prophylaxis, and his aPTT is now normal. Which method gives a valid factor VIII inhibitor titer?

A 76-year-old woman with no previous bleeding problems develops large spontaneous bruises over one week. PT is normal, aPTT is 72 s (reference 25-35 s), factor VIII is 3% (0.03 IU/mL), and a Bethesda assay shows 14 BU/mL. The most likely diagnosis is:

The Nijmegen modification of the Bethesda assay buffers the normal pooled plasma and uses factor VIII-deficient plasma in the control mixture. What is its main purpose?

One Bethesda unit is the amount of inhibitor that neutralizes how much of the factor VIII in an equal volume of normal plasma during 2 hours at 37 °C?

In a Nijmegen-Bethesda assay, patient plasma diluted 1:10 is mixed with buffered normal pooled plasma and incubated for 2 hours at 37 °C. Factor VIII is 25% in the test mixture and 50% in the control mixture. What is the inhibitor titer?