During a 7-hour spinal fusion, a 54-year-old woman with an estimated blood volume of 4.5 L loses about 5 L of blood. It is replaced with 7 units of red cells and 5 L of crystalloid; no plasma or platelets are given, and no heparin is used. She now oozes from the wound. Before surgery: platelets 260 × 10^3/µL (260 × 10^9/L), PT 12.1 s, APTT 29 s, fibrinogen 310 mg/dL (3.1 g/L). Now: platelets 92 × 10^3/µL (92 × 10^9/L), PT 19.6 s (reference 11.0-13.5 s), APTT 49 s (reference 25-35 s), fibrinogen 98 mg/dL (0.98 g/L; reference 200-400), thrombin time 21 s (reference 14-19 s), D-dimer 1.1 µg/mL FEU (1.1 mg/L FEU; reference below 0.5). There are no schistocytes. What is the most likely cause?

Dilution of platelets and clotting factors

Heparin contamination of the sample

Acute disseminated intravascular coagulation

Which components make up the prothrombin time reagent?

Tissue factor, phospholipid and calcium

Bovine thrombin in a buffered solution

Contact activator, phospholipid and calcium

Snake venom that clots fibrinogen

Lab: PT/INR, APTT, fibrinogen, thrombin time, D-dimer

13 ASCP MLS practice questions on Lab: PT/INR, APTT, fibrinogen, thrombin time, D-dimer, part of Hematology. The first 13 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
During a 7-hour spinal fusion, a 54-year-old woman with an estimated blood volume of 4.5 L loses about 5 L of blood. It is replaced with 7 units of red cells and 5 L of crystalloid; no plasma or platelets are given, and no heparin is used. She now oozes from the wound. Before surgery: platelets 260 × 10^3/µL (260 × 10^9/L), PT 12.1 s, APTT 29 s, fibrinogen 310 mg/dL (3.1 g/L). Now: platelets 92 × 10^3/µL (92 × 10^9/L), PT 19.6 s (reference 11.0-13.5 s), APTT 49 s (reference 25-35 s), fibrinogen 98 mg/dL (0.98 g/L; reference 200-400), thrombin time 21 s (reference 14-19 s), D-dimer 1.1 µg/mL FEU (1.1 mg/L FEU; reference below 0.5). There are no schistocytes. What is the most likely cause?
1. APrimary hyperfibrinolysis
2. BDilution of platelets and clotting factors
3. CHeparin contamination of the sample
4. DAcute disseminated intravascular coagulation
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Answer: B · Dilution of platelets and clotting factors
She has lost and replaced more than one blood volume with red cells and crystalloid, which contain no clotting factors or platelets, so platelets, fibrinogen and the factors measured by the PT and APTT have all fallen together. The D-dimer is only slightly raised, as expected after major surgery, and there are no schistocytes, which argues against DIC, where the D-dimer is usually markedly raised. Primary hyperfibrinolysis lowers fibrinogen but leaves the platelet count normal. Heparin would prolong the thrombin time far more than this, and none was used. Treatment is plasma, platelets and fibrinogen replacement guided by these results.
- A: Primary hyperfibrinolysis lowers fibrinogen but not the platelet count.
- C: No heparin was used, and heparin would prolong the thrombin time much more.
- D: DIC usually gives a markedly raised D-dimer and often schistocytes; here the D-dimer is barely raised.
Question 2 · Easy
Which components make up the prothrombin time reagent?
1. ABovine thrombin in a buffered solution
2. BTissue factor, phospholipid and calcium
3. CContact activator, phospholipid and calcium
4. DSnake venom that clots fibrinogen
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Answer: B · Tissue factor, phospholipid and calcium
Thromboplastin reagent supplies tissue factor and phospholipid, and calcium reverses the citrate in the sample. Tissue factor binds factor VIIa and starts the extrinsic pathway, and the time to a fibrin clot is recorded. The APTT uses a contact activator such as silica or ellagic acid with phospholipid, then calcium. Thrombin and batroxobin are the reagents for the thrombin and reptilase times.
- A: Thrombin alone is the thrombin time reagent.
- C: A contact activator with phospholipid is the APTT reagent.
- D: Snake venom (batroxobin) is the reptilase time reagent.
Question 3 · Hard
A patient on warfarin has a PT of 25.0 s. The mean normal PT is 12.5 s and the thromboplastin ISI is 1.4. What is the INR?
1. A2.8
2. B2.6
3. C1.6
4. D2.0
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Answer: B · 2.6
INR = (patient PT ÷ mean normal PT)^ISI. The PT ratio is 25.0 ÷ 12.5 = 2.0, and 2.0^1.4 = 2.6. Using the ratio alone gives 2.0, multiplying it by the ISI gives 2.8, and using 1 ÷ ISI as the exponent gives 1.6. The mean normal PT is the geometric mean from at least 20 healthy people, tested with the same reagent lot and instrument.
- A: 2.8 multiplies the ratio by the ISI instead of using it as an exponent.
- C: 1.6 uses 1 ÷ ISI as the exponent.
- D: 2.0 is the PT ratio without the ISI exponent.
Question 4 · Hard
A healthy man with no bleeding history has an APTT of 62 s (reference 25-35) before surgery. When incubation of his plasma with the silica activator is extended from 3 to 10 minutes, the APTT falls to 33 s. Which deficiency is most likely?
1. AFactor XII
2. BPrekallikrein
3. CFactor XI
4. DFactor VIII
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Answer: B · Prekallikrein
Prekallikrein (Fletcher factor) speeds the activation of factor XII on the contact surface. Without it, factor XII still activates itself slowly, so a longer incubation with the activator lets activation catch up and the APTT shortens toward normal. Factor XII, factor XI and factor VIII deficiencies do not correct in this way. Like factor XII deficiency, prekallikrein deficiency causes no bleeding, so surgery need not be delayed.
- A: Factor XII deficiency stays prolonged with a longer incubation.
- C: Factor XI deficiency does not correct with a longer incubation and can cause bleeding.
- D: Factor VIII deficiency does not correct with a longer incubation and causes bleeding.
Question 5 · Easy
A prolonged PT with a normal APTT points to a deficiency of factor:
1. AVIII
2. BVII
3. CXII
4. DX
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Answer: B · VII
The PT tests the extrinsic and common pathways (factor VII, then X, V, II and fibrinogen), and the APTT tests the intrinsic and common pathways. Factor VII is the only factor measured by the PT alone, so its deficiency prolongs the PT and leaves the APTT normal. Factor VII has the shortest half-life of the vitamin K-dependent factors, so early warfarin therapy and early vitamin K deficiency give the same pattern.
- A: Factor VIII deficiency prolongs the APTT only.
- C: Factor XII deficiency prolongs the APTT only.
- D: Factor X is in the common pathway, so both tests are prolonged.
Question 6 · Medium
The INR was introduced mainly to:
1. AMake warfarin PT results comparable between labs
2. BMonitor unfractionated heparin therapy
3. CStandardize the PT in patients with liver disease
4. DCorrect the PT for a raised patient hematocrit
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Question 7 · Hard
A 72-year-old woman with a low clinical probability of pulmonary embolism has a D-dimer of 620 ng/mL FEU (0.62 mg/L FEU). The fixed cutoff is 500 ng/mL FEU, and the hospital uses an age-adjusted cutoff of age × 10 ng/mL FEU after age 50. How should the result be read?
1. AAbove the cutoff; PE is confirmed
2. BBelow her cutoff; PE is excluded
3. CBorderline; repeat the test tomorrow
4. DAbove the cutoff; imaging is needed
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Question 8 · Hard
A 2-week-old boy bled from the umbilical stump, and a scalp cut later reopened after it had first stopped bleeding. Platelet count, PT, APTT, thrombin time, fibrinogen and VWF studies are all normal. Which factor deficiency is most likely?
1. AFactor XI
2. BFactor XIII
3. CFactor VII
4. DFactor XII
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Question 9 · Medium
A specimen collected after a slow, difficult venipuncture gives an APTT of 20 s (reference 25-35). The most likely cause is:
1. AA lupus anticoagulant in the plasma
2. BHeparin contamination from an indwelling line
3. CFactor XII deficiency in the patient
4. DClotting factors activated during collection
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Question 10 · Medium
In suspected venous thromboembolism, a D-dimer test is used mainly to exclude disease because it has a high:
1. APositive predictive value
2. BCorrelation with clot size
3. CSpecificity for thrombosis
4. DNegative predictive value
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Question 11 · Medium
PT 18.5 s (reference 11.0-13.5), APTT 47 s (25-35), thrombin time 16 s (14-19) and fibrinogen 310 mg/dL (3.1 g/L; 200-400). Which single factor deficiency fits these results?
1. AFactor IX
2. BFactor VII
3. CFactor XIII
4. DFactor X
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Question 12 · Medium
In the Clauss fibrinogen assay, diluted plasma is clotted with excess thrombin. The clotting time is:
1. ADirectly related to fibrinogen level
2. BInversely related to fibrinogen level
3. CUnrelated to the fibrinogen level
4. DRelated mainly to factor XIII level
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Question 13 · Hard
Clauss fibrinogen is 75 mg/dL (0.75 g/L; reference 200-400), but fibrinogen antigen is 285 mg/dL (2.85 g/L). The thrombin time is 31 s (14-19) and the reptilase time 29 s (15-20). The most likely cause is:
1. AHeparin in the sample
2. BHypofibrinogenemia
3. CAfibrinogenemia
4. DDysfibrinogenemia
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Lab: PT/INR, APTT, fibrinogen, thrombin time, D-dimer

Which components make up the prothrombin time reagent?

A patient on warfarin has a PT of 25.0 s. The mean normal PT is 12.5 s and the thromboplastin ISI is 1.4. What is the INR?

A healthy man with no bleeding history has an APTT of 62 s (reference 25-35) before surgery. When incubation of his plasma with the silica activator is extended from 3 to 10 minutes, the APTT falls to 33 s. Which deficiency is most likely?

A prolonged PT with a normal APTT points to a deficiency of factor:

The INR was introduced mainly to:

A 72-year-old woman with a low clinical probability of pulmonary embolism has a D-dimer of 620 ng/mL FEU (0.62 mg/L FEU). The fixed cutoff is 500 ng/mL FEU, and the hospital uses an age-adjusted cutoff of age × 10 ng/mL FEU after age 50. How should the result be read?

A 2-week-old boy bled from the umbilical stump, and a scalp cut later reopened after it had first stopped bleeding. Platelet count, PT, APTT, thrombin time, fibrinogen and VWF studies are all normal. Which factor deficiency is most likely?

A specimen collected after a slow, difficult venipuncture gives an APTT of 20 s (reference 25-35). The most likely cause is:

In suspected venous thromboembolism, a D-dimer test is used mainly to exclude disease because it has a high:

PT 18.5 s (reference 11.0-13.5), APTT 47 s (25-35), thrombin time 16 s (14-19) and fibrinogen 310 mg/dL (3.1 g/L; 200-400). Which single factor deficiency fits these results?

In the Clauss fibrinogen assay, diluted plasma is clotted with excess thrombin. The clotting time is:

Clauss fibrinogen is 75 mg/dL (0.75 g/L; reference 200-400), but fibrinogen antigen is 285 mg/dL (2.85 g/L). The thrombin time is 31 s (14-19) and the reptilase time 29 s (15-20). The most likely cause is: