An 84-year-old man taking dabigatran 150 mg twice daily for atrial fibrillation is admitted with melena and acute kidney injury (eGFR 16 mL/min/1.73 m²). Results: PT 19.0 s (reference 11.0-13.5 s), APTT 78 s (25-35 s), thrombin time over 150 s (14-19 s), reptilase time 17 s (15-20 s), heparin-calibrated anti-Xa below 0.10 IU/mL, platelets 230 × 10^9/L and D-dimer normal. Fibrinogen is 1.1 g/L (110 mg/dL) by the Clauss method but 3.9 g/L by the PT-derived method, and fibrinogen antigen is 3.7 g/L (reference 2.0-4.0 g/L). The most likely cause of the low Clauss result is:

Dabigatran inhibiting the Clauss thrombin

Heparin contaminating the sample

A dysfunctional fibrinogen molecule

Why is the reptilase time normal in a sample that contains unfractionated heparin?

Batroxobin is not inhibited by antithrombin

The reagent contains protamine to neutralize it

Batroxobin activates factor X, not fibrinogen

Heparin works only in the presence of calcium

In a septic ICU patient, the analyzer's PT-derived fibrinogen is 3.4 g/L (340 mg/dL), but a Clauss fibrinogen on the same sample is 1.2 g/L (120 mg/dL). Reference range 2.0-4.0 g/L. Which result should guide fibrinogen replacement?

The Clauss result; PT-derived values can read falsely high

The PT-derived result; it reflects all clottable protein

The mean of both results, 2.3 g/L (230 mg/dL)

Neither; only a fibrinogen antigen can be trusted

Congenital hypofibrinogenemia is best matched by which pattern of thrombin time, reptilase time and fibrinogen results?

TT and reptilase long; Clauss and antigen both low

TT long, reptilase normal; Clauss and antigen normal

TT and reptilase normal; Clauss and antigen normal

TT and reptilase long; Clauss low, antigen normal

Fibrinogen, thrombin time and D-dimer

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Fibrinogen, thrombin time and D-dimer, part of Coagulation and Hemostasis. The first 9 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
An 84-year-old man taking dabigatran 150 mg twice daily for atrial fibrillation is admitted with melena and acute kidney injury (eGFR 16 mL/min/1.73 m²). Results: PT 19.0 s (reference 11.0-13.5 s), APTT 78 s (25-35 s), thrombin time over 150 s (14-19 s), reptilase time 17 s (15-20 s), heparin-calibrated anti-Xa below 0.10 IU/mL, platelets 230 × 10^9/L and D-dimer normal. Fibrinogen is 1.1 g/L (110 mg/dL) by the Clauss method but 3.9 g/L by the PT-derived method, and fibrinogen antigen is 3.7 g/L (reference 2.0-4.0 g/L). The most likely cause of the low Clauss result is:
1. AHeparin contaminating the sample
2. BDabigatran inhibiting the Clauss thrombin
3. CFibrinogen consumption by DIC
4. DA dysfunctional fibrinogen molecule
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Answer: B · Dabigatran inhibiting the Clauss thrombin
The thrombin time is grossly prolonged while the reptilase time, which uses a snake-venom enzyme that thrombin inhibitors do not block, is normal: the fibrinogen clots normally and something is inhibiting thrombin. The heparin-calibrated anti-Xa below 0.10 IU/mL excludes heparin, which leaves dabigatran, which has accumulated because it is cleared mainly by the kidneys. At very high drug levels, dabigatran can inhibit the reagent thrombin in some Clauss methods and give a falsely low fibrinogen, while the antigen and PT-derived results show that fibrinogen is adequate. A dysfibrinogenemia would prolong the reptilase time too, and DIC would lower the antigen and platelets and raise the D-dimer. A dabigatran level by diluted thrombin time or ecarin assay should be measured, and cryoprecipitate should not be given on the Clauss result alone.
- A: A heparin-calibrated anti-Xa below 0.10 IU/mL excludes heparin.
- C: Platelets, D-dimer and fibrinogen antigen are normal, so nothing is being consumed.
- D: A dysfibrinogen would prolong the reptilase time too, and it is normal.
Question 2 · Easy
Why is the reptilase time normal in a sample that contains unfractionated heparin?
1. AThe reagent contains protamine to neutralize it
2. BBatroxobin is not inhibited by antithrombin
3. CBatroxobin activates factor X, not fibrinogen
4. DHeparin works only in the presence of calcium
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Answer: B · Batroxobin is not inhibited by antithrombin
Reptilase is batroxobin, a thrombin-like enzyme from Bothrops snake venom. It splits fibrinopeptide A from fibrinogen directly and, unlike thrombin, is not inactivated by the heparin-antithrombin complex or by direct thrombin inhibitors. A long thrombin time with a normal reptilase time therefore points to heparin or a thrombin inhibitor, while both are long when fibrinogen is low or abnormal. The reptilase time is also prolonged by fibrin degradation products and many dysfibrinogens.
- A: The reagent has no heparin neutralizer; batroxobin is simply not affected by heparin.
- C: Factor X is activated by Russell viper venom, not by batroxobin.
- D: The heparin-antithrombin complex does not need calcium to inhibit thrombin.
Question 3 · Medium
In a septic ICU patient, the analyzer's PT-derived fibrinogen is 3.4 g/L (340 mg/dL), but a Clauss fibrinogen on the same sample is 1.2 g/L (120 mg/dL). Reference range 2.0-4.0 g/L. Which result should guide fibrinogen replacement?
1. AThe PT-derived result; it reflects all clottable protein
2. BThe Clauss result; PT-derived values can read falsely high
3. CThe mean of both results, 2.3 g/L (230 mg/dL)
4. DNeither; only a fibrinogen antigen can be trusted
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Answer: B · The Clauss result; PT-derived values can read falsely high
PT-derived fibrinogen is calculated from the change in optical density during the PT, not from a dedicated fibrinogen clotting assay. It agrees with Clauss in healthy plasma but can overestimate fibrinogen when the level is low or the protein is abnormal, as in DIC, liver disease, dysfibrinogenemia and after thrombolysis. Most guidelines therefore recommend the Clauss method for decisions such as giving cryoprecipitate or fibrinogen concentrate. The low Clauss value here fits consumption in sepsis-associated DIC.
- A: The optical density change is not a reliable measure of functional fibrinogen in abnormal plasma.
- C: Averaging two methods with different biases gives a meaningless number.
- D: Antigen measures protein, including non-functional fibrinogen, and is not used alone to guide treatment.
Question 4 · Hard
Congenital hypofibrinogenemia is best matched by which pattern of thrombin time, reptilase time and fibrinogen results?
1. ATT long, reptilase normal; Clauss and antigen normal
2. BTT and reptilase normal; Clauss and antigen normal
3. CTT and reptilase long; Clauss and antigen both low
4. DTT and reptilase long; Clauss low, antigen normal
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Answer: C · TT and reptilase long; Clauss and antigen both low
In hypofibrinogenemia the fibrinogen molecule is normal but too little is made, so the functional (Clauss) and immunological (antigen) levels fall in proportion, with an activity:antigen ratio near 1. Both the thrombin time and the reptilase time lengthen because there is little substrate for either enzyme. In dysfibrinogenemia the antigen is normal or clearly higher than the Clauss result, giving a low ratio. A long thrombin time with a normal reptilase time points to heparin, not a fibrinogen disorder.
- A: A long TT with a normal reptilase time and normal fibrinogen suggests heparin or a thrombin inhibitor.
- B: Normal TT, reptilase and fibrinogen results exclude a significant fibrinogen disorder.
- D: Low Clauss with normal antigen is the pattern of dysfibrinogenemia.
Question 5 · Hard
A Clauss fibrinogen curve is calibrated for a 1:10 plasma dilution. A sample clots too slowly to read at 1:10, so it is retested at 1:5, and the curve gives 0.90 g/L (90 mg/dL). What fibrinogen should be reported?
1. A0.45 g/L (45 mg/dL)
2. B1.80 g/L (180 mg/dL)
3. C0.90 g/L (90 mg/dL)
4. D4.50 g/L (450 mg/dL)
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Answer: A · 0.45 g/L (45 mg/dL)
At 1:5 the test holds twice as much plasma as the 1:10 dilution the curve assumes, so the value read from the curve is twice the true level. Correct by multiplying by 5/10: 0.90 x 0.5 = 0.45 g/L (45 mg/dL). Analyzers apply this factor automatically in a low-fibrinogen repeat protocol, but a manual rerun must be corrected by hand. A level this low usually calls for fibrinogen replacement in a bleeding patient.
- B: Doubling inverts the correction: 1:5 is less dilute than 1:10, so the reading must be halved.
- C: Reporting the curve value ignores the change in dilution.
- D: Multiplying by 5 treats the curve value as if it were not already a plasma value.
Question 6 · Medium
The Clauss fibrinogen method dilutes the plasma about 1:10 and adds a high concentration of thrombin. What is the main purpose of this design?
1. ATo neutralize any heparin in the plasma completely
2. BTo make fibrinogen the only rate-limiting factor
3. CTo measure fibrinogen antigen as well as activity
4. DTo activate factor XIII and cross-link the fibrin
Answer and explanation in LearnFree account
Question 7 · Easy
In the thrombin time test, undiluted citrated plasma is clotted by adding:
1. ARussell viper venom and phospholipid
2. BTissue factor, phospholipid and calcium
3. CA contact activator, phospholipid and calcium
4. DA low concentration of thrombin
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Question 8 · Easy
Most automated D-dimer assays on coagulation analyzers detect D-dimer by:
1. AA clot-based clotting time
2. BProtamine sulfate precipitation
3. CA chromogenic substrate for plasmin
4. DLatex-enhanced immunoturbidimetry
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Question 9 · Medium
A 52-year-old man has had a swollen left calf for 3 weeks, and another clinic started rivaroxaban 10 days ago. His D-dimer is 0.35 mg/L FEU (exclusion cut-off 0.50). The best interpretation is:
1. ARivaroxaban interferes with the latex antibody reaction
2. BDVT is excluded because D-dimer has a high NPV
3. CThe sample should be repeated in an EDTA tube
4. DThe negative result cannot exclude DVT in this setting
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Fibrinogen, thrombin time and D-dimer

Why is the reptilase time normal in a sample that contains unfractionated heparin?

In a septic ICU patient, the analyzer's PT-derived fibrinogen is 3.4 g/L (340 mg/dL), but a Clauss fibrinogen on the same sample is 1.2 g/L (120 mg/dL). Reference range 2.0-4.0 g/L. Which result should guide fibrinogen replacement?

Congenital hypofibrinogenemia is best matched by which pattern of thrombin time, reptilase time and fibrinogen results?

A Clauss fibrinogen curve is calibrated for a 1:10 plasma dilution. A sample clots too slowly to read at 1:10, so it is retested at 1:5, and the curve gives 0.90 g/L (90 mg/dL). What fibrinogen should be reported?

The Clauss fibrinogen method dilutes the plasma about 1:10 and adds a high concentration of thrombin. What is the main purpose of this design?

In the thrombin time test, undiluted citrated plasma is clotted by adding:

Most automated D-dimer assays on coagulation analyzers detect D-dimer by:

A 52-year-old man has had a swollen left calf for 3 weeks, and another clinic started rivaroxaban 10 days ago. His D-dimer is 0.35 mg/L FEU (exclusion cut-off 0.50). The best interpretation is: