A 6-year-old girl has had nosebleeds and easy bruising since a viral illness 2 weeks ago. She takes no medicines. Results: PT 19.8 s (reference 11.0-13.5 s), APTT 66 s (reference 25-35 s), thrombin time and fibrinogen normal, platelets 285 × 10^3/µL (285 × 10^9/L). Immediate 1:1 mix with normal plasma: PT 12.9 s, APTT 54 s. Factor activities (reference 50-150%): II 12%, V 96%, VII 88%, X 92%. The dRVVT screen-to-confirm ratio is 1.62 (positive above 1.20). The most likely diagnosis is:

Lupus anticoagulant-hypoprothrombinemia syndrome

Vitamin K deficiency from poor intake

An acquired inhibitor of factor VIII

Lupus anticoagulant with no factor deficiency

In a 1:1 mix, a prolonged APTT caused by a single factor deficiency corrects because:

The mix contains at least 50% of the factor

Normal plasma adds extra phospholipid

Mixing dilutes the excess citrate

Normal plasma neutralizes the patient's inhibitor

APTT results: patient 80 s, normal pooled plasma 30 s, 1:1 mix 35 s. The Rosner index is (mix − normal pool) ÷ patient × 100, and the laboratory reads an index above 15 as non-correction. What is the index and its meaning?

6; correction, as in a deficiency

6; non-correction, as with an inhibitor

44; non-correction, as with an inhibitor

17; non-correction, as with an inhibitor

Lab: mixing studies and inhibitor assays

11 ASCP MLS practice questions on Lab: mixing studies and inhibitor assays, part of Hematology. The first 11 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
A 6-year-old girl has had nosebleeds and easy bruising since a viral illness 2 weeks ago. She takes no medicines. Results: PT 19.8 s (reference 11.0-13.5 s), APTT 66 s (reference 25-35 s), thrombin time and fibrinogen normal, platelets 285 × 10^3/µL (285 × 10^9/L). Immediate 1:1 mix with normal plasma: PT 12.9 s, APTT 54 s. Factor activities (reference 50-150%): II 12%, V 96%, VII 88%, X 92%. The dRVVT screen-to-confirm ratio is 1.62 (positive above 1.20). The most likely diagnosis is:
1. ALupus anticoagulant-hypoprothrombinemia syndrome
2. BVitamin K deficiency from poor intake
3. CAn acquired inhibitor of factor VIII
4. DLupus anticoagulant with no factor deficiency
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Answer: A · Lupus anticoagulant-hypoprothrombinemia syndrome
The APTT fails to correct in the mix and the dRVVT is positive, so a lupus anticoagulant is present. A lupus anticoagulant alone does not cause bleeding or a low factor II, but here prothrombin is 12% and the PT corrects on mixing, which shows a true factor II deficiency. In this syndrome, seen mainly in children after viral infections and in lupus, antiprothrombin antibodies bind prothrombin without neutralizing it, and the complexes are cleared quickly from the blood. Vitamin K deficiency would also lower factors VII and X, and a factor VIII inhibitor would not prolong the PT. The bleeding usually resolves as the antibody fades.
- B: Vitamin K deficiency would lower factors VII and X as well, and both mixes would correct.
- C: A factor VIII inhibitor leaves the PT normal and does not lower factor II.
- D: A lupus anticoagulant alone does not lower factor II to 12% or cause bleeding.
Question 2 · Medium
A 72-year-old woman has new, extensive bruising and no previous bleeding history. Her PT is normal. Refer to the APTT mixing study shown. What do the results indicate?
Figure: MedExams
1. ALupus anticoagulant
2. BAcquired factor VIII inhibitor
3. CHeparin contamination
4. DFactor VIII deficiency
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Answer: B · Acquired factor VIII inhibitor
The patient's APTT (74.2 s) corrects to 34.1 s in the 1:1 mix tested at once but prolongs to 58.6 s after the mix is incubated for 2 hours at 37 °C. Patient and normal plasma incubated separately and then mixed give 35.0 s, so the change is not just loss of labile factors during incubation. A time- and temperature-dependent inhibitor in an older adult with new bleeding is typical of an acquired factor VIII autoantibody (acquired hemophilia A). The next steps are a factor VIII assay and a Bethesda titer.
- A: A lupus anticoagulant usually fails to correct even in the immediate mix and rarely causes bleeding.
- C: Heparin acts at once, so the immediate mix would not correct.
- D: A deficiency corrects in both the immediate and the incubated mix.
Question 3 · Hard
A man with severe hemophilia A no longer responds to factor VIII infusions. In a Bethesda assay, dilutions of his plasma were incubated with normal pooled plasma for 2 hours at 37 °C. From the factor VIII results shown, his inhibitor titer is:
Figure: MedExams
1. A8 BU/mL
2. B16 BU/mL
3. C64 BU/mL
4. D32 BU/mL
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Answer: B · 16 BU/mL
One Bethesda unit is the amount of inhibitor that destroys half the factor VIII in an equal volume of normal plasma in 2 hours at 37 °C. Residual activity is judged against the control mix: 50% of 48 IU/dL is 24 IU/dL, which the patient's plasma gives at the 1:16 dilution, so the titer is 16 BU/mL. Results are most reliable from a dilution whose residual activity lies between 25% and 75%. A titer of 5 BU/mL or more marks a high responder, who needs bypassing agents or emicizumab rather than factor VIII.
- A: At 1:8 only 25% of the control activity remains.
- C: 1:64 is the dilution nearest 50 IU/dL, not 50% of the control.
- D: At 1:32 about 71% remains, so 1:32 is not the 50% point.
Question 4 · Easy
In a 1:1 mix, a prolonged APTT caused by a single factor deficiency corrects because:
1. AThe mix contains at least 50% of the factor
2. BNormal plasma adds extra phospholipid
3. CMixing dilutes the excess citrate
4. DNormal plasma neutralizes the patient's inhibitor
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Answer: A · The mix contains at least 50% of the factor
A factor level of about 50% is usually enough to give a normal PT or APTT. Even if the patient has no factor at all, a 1:1 mix with normal plasma (about 100%) contains about 50%, so the clotting time returns to normal. An inhibitor, by contrast, also acts on the factors in the normal plasma, so the mix stays prolonged. Specific factor assays then show which factor is low.
- B: Phospholipid is supplied by the reagent, not by the normal plasma.
- C: Citrate is not the cause of a prolonged clotting time in a deficiency.
- D: A deficiency has no inhibitor to neutralize.
Question 5 · Hard
APTT results: patient 80 s, normal pooled plasma 30 s, 1:1 mix 35 s. The Rosner index is (mix − normal pool) ÷ patient × 100, and the laboratory reads an index above 15 as non-correction. What is the index and its meaning?
1. A6; correction, as in a deficiency
2. B6; non-correction, as with an inhibitor
3. C44; non-correction, as with an inhibitor
4. D17; non-correction, as with an inhibitor
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Answer: A · 6; correction, as in a deficiency
The index is (35 − 30) ÷ 80 × 100 = 6.25, about 6, which is below the cutoff of 15, so the mix corrects and points to a factor deficiency. Dividing by the normal pool instead of the patient gives 17, and using mix ÷ patient gives 44; both would wrongly suggest an inhibitor. Laboratories validate their own cutoffs and criteria, such as a mix within the reference range. An incubated mix is still needed to exclude a time-dependent inhibitor.
- B: An index of 6 is below 15, so it shows correction, not an inhibitor.
- C: 44 is mix ÷ patient × 100 without subtracting the normal pool.
- D: 17 divides by the normal pool clotting time instead of the patient's.
Question 6 · Medium
A patient's prolonged PT and APTT both correct in a 1:1 mix with normal plasma. This points to:
1. AA lupus anticoagulant
2. BAn acquired factor V inhibitor
3. CLow levels of one or more factors
4. DUnfractionated heparin in the sample
Answer and explanation in LearnFree account
Question 7 · Easy
What ratio of patient plasma to normal pooled plasma is used in a routine mixing study?
1. A1:1
2. B1:4
3. C1:9
4. D4:1
Answer and explanation in LearnFree account
Question 8 · Medium
Two patients with no bleeding history have isolated prolonged APTTs. Patient 1's APTT corrects in a 1:1 mix; patient 2's does not. The most likely causes are:
1. A1: lupus anticoagulant; 2: factor XII deficiency
2. B1: factor VIII deficiency; 2: factor XI deficiency
3. C1: factor XI deficiency; 2: factor VIII inhibitor
4. D1: factor XII deficiency; 2: lupus anticoagulant
Answer and explanation in LearnFree account
Question 9 · Medium
Why is a 1:1 APTT mix tested again after 1 to 2 hours at 37 °C, as well as immediately?
1. ATo let lupus anticoagulants bind phospholipid
2. BTo allow labile factors in the mix to decay
3. CTo detect time-dependent factor VIII inhibitors
4. DTo confirm that heparin is in the sample
Answer and explanation in LearnFree account
Question 10 · Hard
An ICU patient with no anticoagulant ordered has an APTT of 98 s (reference 25-35) that does not correct in a 1:1 mix, and a normal PT. The thrombin time is over 120 s (14-19) and the reptilase time is normal. What is the most likely cause?
1. AA lupus anticoagulant
2. BDysfibrinogenemia
3. CA factor VIII inhibitor
4. DHeparin in the sample
Answer and explanation in LearnFree account
Question 11 · Hard
A bleeding patient has a prolonged aPTT and a normal PT, and the aPTT corrects in a 1:1 mix with normal plasma. Which deficiency is most likely?
1. AFactor VIII, IX or XI
2. BFactor XIII
3. CFactor VII
4. DA lupus anticoagulant
Answer and explanation in LearnFree account

Lab: mixing studies and inhibitor assays

A 72-year-old woman has new, extensive bruising and no previous bleeding history. Her PT is normal. Refer to the APTT mixing study shown. What do the results indicate?

A man with severe hemophilia A no longer responds to factor VIII infusions. In a Bethesda assay, dilutions of his plasma were incubated with normal pooled plasma for 2 hours at 37 °C. From the factor VIII results shown, his inhibitor titer is:

In a 1:1 mix, a prolonged APTT caused by a single factor deficiency corrects because:

APTT results: patient 80 s, normal pooled plasma 30 s, 1:1 mix 35 s. The Rosner index is (mix − normal pool) ÷ patient × 100, and the laboratory reads an index above 15 as non-correction. What is the index and its meaning?

A patient's prolonged PT and APTT both correct in a 1:1 mix with normal plasma. This points to:

What ratio of patient plasma to normal pooled plasma is used in a routine mixing study?

Two patients with no bleeding history have isolated prolonged APTTs. Patient 1's APTT corrects in a 1:1 mix; patient 2's does not. The most likely causes are:

Why is a 1:1 APTT mix tested again after 1 to 2 hours at 37 °C, as well as immediately?

An ICU patient with no anticoagulant ordered has an APTT of 98 s (reference 25-35) that does not correct in a 1:1 mix, and a normal PT. The thrombin time is over 120 s (14-19) and the reptilase time is normal. What is the most likely cause?

A bleeding patient has a prolonged aPTT and a normal PT, and the aPTT corrects in a 1:1 mix with normal plasma. Which deficiency is most likely?