aPTT Intrinsic Pathway — FRCA Primary MCQ

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HardPhysiology - Gastrointestinal & HepaticaPTT Intrinsic PathwayFRCA Primary

Preoperative testing shows an isolated prolonged aPTT. A 1:1 mix with normal plasma corrects immediately but becomes prolonged after two hours at 37°C. Which explanation best fits this pattern?

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Correct answer: E — A time-dependent factor VIII inhibitor

The best answer is “A time-dependent factor VIII inhibitor”. A factor deficiency typically corrects after mixing because normal plasma supplies the missing factor. Loss of correction after incubation indicates a time- and temperature-dependent inhibitor, classically an acquired factor VIII inhibitor. Factor VII affects PT, platelet number does not directly prolong aPTT, and anticoagulant contamination should be excluded with medication history and thrombin or anti-Xa testing rather than assumed to vanish during incubation.

Reference: Isolated prolongation of activated partial thromboplastin time: diagnostic review: https://pmc.ncbi.nlm.nih.gov/articles/PMC10305031/; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf