Thrombin Time Fibrinogen to Fibrin — FRCA Primary MCQ

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HardPhysiology - Gastrointestinal & HepaticThrombin Time Fibrinogen to FibrinFRCA Primary

Blood drawn from an arterial line shows a markedly prolonged thrombin time, but reptilase time and fibrinogen concentration are normal. Which explanation is most likely?

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Correct answer: A — Heparin contamination from the arterial line

The best answer is “Heparin contamination from the arterial line”. Thrombin time is prolonged when heparin inhibits added thrombin, whereas reptilase cleaves fibrinogen through a heparin-insensitive mechanism. A long thrombin time with normal reptilase time therefore points to heparin contamination, particularly from a line. Low or dysfunctional fibrinogen generally prolongs both tests; factor VIII deficiency affects aPTT rather than thrombin time.

Reference: Coagulation testing in the perioperative period: https://pmc.ncbi.nlm.nih.gov/articles/PMC4260302/; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf