An 8-year-old boy is listed for dental extractions. His maternal uncle has mild hemophilia A with a baseline factor VIII of 32 IU/dL, and the boy bruises easily after football. Results: platelets 260 × 10^9/L, PT 12.1 s (reference 11.0-13.5 s) and APTT 36 s (reference 27-38 s). The laboratory's own verification data show that its APTT reagent first rises above the upper reference limit when factor VIII falls below about 30 IU/dL. The dentist asks whether the boy can be cleared for surgery. What is the best next step?

Measure factor VIII activity directly

Repeat the APTT with longer activation

Report that hemophilia A is excluded

A grossly lipemic sample gives an optical error flag for the PT and no result. What is the best next step?

Test it on a mechanical clot-detection analyzer

Add extra calcium chloride to the test

Dilute the plasma 1:2 with saline and retest

Report the PT with a comment on lipemia

After a new thromboplastin lot is introduced, the laboratory forgets to update the MNPT. The old MNPT was 11.0 s; the correct MNPT for the new lot is 12.5 s. What happens to patients' reported INRs?

Only INRs below 2.0 are affected

They are unaffected, as the ISI corrects for it

PT/INR and APTT

19 DOH Abu Dhabi (HAAD) lab licensing practice questions on PT/INR and APTT, part of Coagulation and Hemostasis. The first 19 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 58-year-old woman with a mechanical mitral valve takes warfarin, with a target INR of 2.5-3.5. Today her PT is 32.0 s. For the current thromboplastin lot on this analyzer, the ISI is 1.50, the PT reference interval is 11.2-13.8 s, and the mean normal PT (geometric mean of fresh plasmas from 30 healthy adults) is 12.5 s. She reports no bleeding and no change in her diet or medicines. To one decimal place, what INR should be reported, and how does it compare with her target?
1. A3.8, above her target range
2. B3.5, within her target range
3. C2.6, within her target range
4. D4.1, above her target range
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Answer: D · 4.1, above her target range
INR = (patient PT ÷ MNPT)^ISI = (32.0 ÷ 12.5)^1.50 = 2.56^1.50 = 4.1, which is above her target of 2.5-3.5. The MNPT must be the geometric mean of normal PTs for the same lot and analyzer, not the upper limit of the reference interval; dividing by 13.8 s gives 3.5, which would wrongly look acceptable. The ISI is an exponent, not a multiplier: 2.56 × 1.50 gives 3.8. Reporting the PT ratio of 2.6 leaves out the ISI correction, which is what makes INRs comparable between thromboplastins. The clinic should review her warfarin dose.
- A: This multiplies the ratio by the ISI instead of raising it to the power of the ISI.
- B: This divides by the upper reference limit (13.8 s) instead of the MNPT.
- C: This is the PT ratio without the ISI correction.
Question 2 · Hard
An 8-year-old boy is listed for dental extractions. His maternal uncle has mild hemophilia A with a baseline factor VIII of 32 IU/dL, and the boy bruises easily after football. Results: platelets 260 × 10^9/L, PT 12.1 s (reference 11.0-13.5 s) and APTT 36 s (reference 27-38 s). The laboratory's own verification data show that its APTT reagent first rises above the upper reference limit when factor VIII falls below about 30 IU/dL. The dentist asks whether the boy can be cleared for surgery. What is the best next step?
1. AMeasure factor VIII activity directly
2. BPerform a 1:1 mixing study
3. CRepeat the APTT with longer activation
4. DReport that hemophilia A is excluded
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Answer: A · Measure factor VIII activity directly
His mother has a 50% chance of carrying her brother's F8 variant, so the boy has about a 25% chance of hemophilia A (50% if she is a carrier), and affected males in one family usually have similar factor VIII levels. A factor VIII in the low 30s IU/dL would leave this APTT within the reference interval, because the reagent prolongs only below about 30 IU/dL; a normal APTT therefore does not exclude mild hemophilia. Factor VIII activity should be measured directly before surgery, with VWF tests if it is low. A mixing study investigates a prolonged APTT, and a longer activation time corrects only prekallikrein deficiency. This is why each laboratory should know the factor sensitivity of its APTT reagent.
- B: A mixing study investigates a prolonged APTT, not a normal one.
- C: Longer activation helps only in prekallikrein deficiency.
- D: This reagent stays normal until factor VIII falls below about 30 IU/dL.
Question 3 · Medium
A grossly lipemic sample gives an optical error flag for the PT and no result. What is the best next step?
1. AAdd extra calcium chloride to the test
2. BTest it on a mechanical clot-detection analyzer
3. CDilute the plasma 1:2 with saline and retest
4. DReport the PT with a comment on lipemia
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Answer: B · Test it on a mechanical clot-detection analyzer
Turbidity from lipids blocks or scatters light, so a photo-optical analyzer may fail to detect the change caused by fibrin formation. Mechanical (electromechanical) clot detection senses the clot physically and is not affected by turbidity, so it is the usual solution; high-speed centrifugation to clear the lipid is another option where validated. Diluting the plasma would dilute the clotting factors and prolong the PT. No result should be reported when the analyzer could not detect a valid clot.
- A: Calcium is already in excess in the reagent and does not help with optical interference.
- C: Diluting the plasma lowers the factor levels and falsely prolongs the PT.
- D: No valid result was obtained, so there is nothing to report.
Question 4 · Hard
An anticoagulation-clinic sample clots in 48.0 s. For this reagent lot, the ISI is 1.20 and the laboratory's MNPT (geometric mean of normal PTs) is 12.0 s. Calculated to one decimal place, the INR is:
1. A3.3
2. B5.3
3. C4.0
4. D4.8
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Answer: B · 5.3
INR = (patient PT ÷ MNPT)^ISI = (48.0 ÷ 12.0)^1.20 = 4.0^1.20. Using logarithms, 1.20 × ln 4.0 = 1.20 × 1.386 = 1.664, and e^1.664 = 5.3. The PT ratio must be raised to the power of the ISI, not multiplied or divided by it. An INR of 5.3 is well above the usual 2.0-3.0 target, and many laboratories treat it as a critical value.
- A: 3.3 divides the PT ratio by the ISI.
- C: 4.0 is the PT ratio alone, without the ISI correction.
- D: 4.8 multiplies the PT ratio by the ISI instead of raising it to that power.
Question 5 · Hard
After a new thromboplastin lot is introduced, the laboratory forgets to update the MNPT. The old MNPT was 11.0 s; the correct MNPT for the new lot is 12.5 s. What happens to patients' reported INRs?
1. AThey are all falsely low
2. BOnly INRs below 2.0 are affected
3. CThey are all falsely high
4. DThey are unaffected, as the ISI corrects for it
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Answer: C · They are all falsely high
The INR is (patient PT ÷ MNPT)^ISI, so an MNPT that is too small makes every PT ratio too large. For example, a PT of 30 s gives a ratio of 30 ÷ 11.0 = 2.73 instead of 30 ÷ 12.5 = 2.40, and the error grows further when the ratio is raised to the ISI. Patients would appear over-anticoagulated, and warfarin doses could be cut unnecessarily. The MNPT must be re-established with every new lot of reagent.
- A: A smaller MNPT raises the PT ratio, so INRs read high, not low.
- B: Every patient's PT ratio is affected, at all INR levels.
- D: The ISI corrects for reagent responsiveness, not for a wrong MNPT.
Question 6 · Easy
In a photo-optical coagulation analyzer, the clotting time is the point at which:
1. AA chromogenic substrate releases a yellow color
2. BLight transmission or scatter changes as fibrin forms
3. CA steel ball in the cuvette changes its movement
4. DThe electrical impedance between two electrodes rises
Answer and explanation in LearnFree account
Question 7 · Medium
Compared with a thromboplastin with an ISI of 2.0, a thromboplastin with an ISI of 1.0 is:
1. AOnly suitable for heparin monitoring
2. BUnsuitable for calculating an INR
3. CLess responsive to low vitamin K-dependent factors
4. DMore responsive to low vitamin K-dependent factors
Answer and explanation in LearnFree account
Question 8 · Hard
A laboratory moves its thromboplastin to a new analyzer model but keeps using the ISI assigned for the old analyzer. INRs on certified plasmas now differ from their assigned values. What should the laboratory do?
1. AUse the ISI assigned for the new analyzer or calibrate locally
2. BCorrect INRs by the average difference seen on QC
3. CChange to a new lot of reagent with a higher ISI
4. DKeep the old ISI, as the ISI depends only on the reagent
Answer and explanation in LearnFree account
Question 9 · Hard
A 66-year-old man with nephrotic-range proteinuria and a lambda light-chain paraprotein bruises easily. PT and APTT are prolonged and both correct in a 1:1 mix. Factor X is 14% (reference 50-150%) and all other factors are normal. What is the most likely cause?
1. AAn autoantibody against factor X
2. BFactor X binding to amyloid fibrils
3. CInherited factor X deficiency
4. DVitamin K deficiency from poor diet
Answer and explanation in LearnFree account
Question 10 · Medium
A healthy term newborn has an APTT of 44 s; the adult reference range is 25-35 s. The best interpretation is:
1. AHemophilia until proven otherwise
2. BLikely normal for age; use neonatal reference ranges
3. CA sign of vitamin K deficiency bleeding
4. DA lupus anticoagulant from the mother
Answer and explanation in LearnFree account
Question 11 · Hard
Plasma from a patient with cirrhosis, not on warfarin, gives an INR of 1.8 with one thromboplastin and 2.5 with another; both systems are correctly calibrated and pass QC. What best explains the difference?
1. ACirrhotic plasma has a raised factor VIII level
2. BISI is calibrated with plasma from warfarin patients
3. CThe two reagents use different mean normal PTs
4. DOne of the two analyzers has a calibration fault
Answer and explanation in LearnFree account
Question 12 · Hard
An outpatient whose coagulation results were normal last week now has 'no clot detected' for both PT and APTT and an unmeasurable Clauss fibrinogen. The specimen arrived from an outside clinic as an aliquot labeled 'plasma'. What is the most likely explanation?
1. ACongenital afibrinogenemia
2. BA strong lupus anticoagulant
3. CAcute disseminated intravascular coagulation
4. DThe aliquot is serum, not citrated plasma
Answer and explanation in LearnFree account
Question 13 · Easy
The mean normal prothrombin time (MNPT) used in the INR calculation is:
1. AThe PT of the normal control plasma on that day
2. BThe normal PT printed by the reagent manufacturer
3. CThe geometric mean PT of at least 20 healthy adults
4. DThe midpoint of the laboratory's PT reference range
Answer and explanation in LearnFree account
Question 14 · Medium
A laboratory introduces a new lot of thromboplastin. Before it reports INRs with the new lot, what must it do?
1. AKeep the old MNPT if QC is in range
2. BEstablish a new MNPT and use the new lot's ISI
3. CConvert old INRs using a correction factor
4. DRun QC only and keep the previous ISI
Answer and explanation in LearnFree account
Question 15 · Medium
In late pregnancy, a short APTT on a well-collected sample is most often due to:
1. ALowered antithrombin levels
2. BLowered fibrinogen levels
3. CActivation during the draw
4. DRaised factor VIII levels
Answer and explanation in LearnFree account
Question 16 · Medium
The same plasma from a patient with a lupus anticoagulant gives an APTT of 52 s with one reagent and 33 s with another (both reference 25-35 s). The difference is mainly due to the reagents':
1. ACalcium chloride concentration
2. BPhospholipid content and composition
3. CBuffer pH and ionic strength
4. DTissue factor content
Answer and explanation in LearnFree account
Question 17 · Hard
A 19-year-old man has had heavy bleeding after dental extractions. PT 16.5 s (reference 11.0-13.5 s) and APTT 52 s (25-35 s) both correct in a 1:1 mix; thrombin time and fibrinogen are normal. Factors: II 95%, V 12%, VII 98%, VIII 10%, IX 100%, X 96% (all reference 50-150%). What is the most likely diagnosis?
1. AEarly hepatocellular synthetic failure
2. BDisseminated intravascular coagulation
3. CVitamin K deficiency or warfarin effect
4. DCombined factor V and factor VIII deficiency
Answer and explanation in LearnFree account
Question 18 · Easy
Besides a contact activator such as silica or ellagic acid, the APTT reagent contains:
1. ATissue factor with phospholipid
2. BPhospholipid without tissue factor
3. CRussell viper venom and phospholipid
4. DThrombin in a buffered solution
Answer and explanation in LearnFree account
Question 19 · Medium
The analyzer reports the clotting time as the moment the light-scatter signal has risen by half of its total change. From the clot curves shown, what PT does it report for the patient?
Figure: MedExams
1. A12 s
2. B27 s
3. C21 s
4. D24 s
Answer and explanation in LearnFree account

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PT/INR and APTT

A grossly lipemic sample gives an optical error flag for the PT and no result. What is the best next step?

An anticoagulation-clinic sample clots in 48.0 s. For this reagent lot, the ISI is 1.20 and the laboratory's MNPT (geometric mean of normal PTs) is 12.0 s. Calculated to one decimal place, the INR is:

After a new thromboplastin lot is introduced, the laboratory forgets to update the MNPT. The old MNPT was 11.0 s; the correct MNPT for the new lot is 12.5 s. What happens to patients' reported INRs?

In a photo-optical coagulation analyzer, the clotting time is the point at which:

Compared with a thromboplastin with an ISI of 2.0, a thromboplastin with an ISI of 1.0 is:

A laboratory moves its thromboplastin to a new analyzer model but keeps using the ISI assigned for the old analyzer. INRs on certified plasmas now differ from their assigned values. What should the laboratory do?

A 66-year-old man with nephrotic-range proteinuria and a lambda light-chain paraprotein bruises easily. PT and APTT are prolonged and both correct in a 1:1 mix. Factor X is 14% (reference 50-150%) and all other factors are normal. What is the most likely cause?

A healthy term newborn has an APTT of 44 s; the adult reference range is 25-35 s. The best interpretation is:

Plasma from a patient with cirrhosis, not on warfarin, gives an INR of 1.8 with one thromboplastin and 2.5 with another; both systems are correctly calibrated and pass QC. What best explains the difference?

An outpatient whose coagulation results were normal last week now has 'no clot detected' for both PT and APTT and an unmeasurable Clauss fibrinogen. The specimen arrived from an outside clinic as an aliquot labeled 'plasma'. What is the most likely explanation?

The mean normal prothrombin time (MNPT) used in the INR calculation is:

A laboratory introduces a new lot of thromboplastin. Before it reports INRs with the new lot, what must it do?

In late pregnancy, a short APTT on a well-collected sample is most often due to:

The same plasma from a patient with a lupus anticoagulant gives an APTT of 52 s with one reagent and 33 s with another (both reference 25-35 s). The difference is mainly due to the reagents':

Besides a contact activator such as silica or ellagic acid, the APTT reagent contains:

The analyzer reports the clotting time as the moment the light-scatter signal has risen by half of its total change. From the clot curves shown, what PT does it report for the patient?