APTT & PT testing - principles, interpretation and emergency use
Overview
APTT (activated partial thromboplastin time) and PT (prothrombin time, usually reported as INR) are the common laboratory coagulation screening tests used in the emergency department.
They assess different limbs of the clotting cascade, guide monitoring of some anticoagulants, and help trigger reversal and transfusion actions in major haemorrhage. Understanding what each test measures, their limitations, and how they fit into local major‑bleed and reversal pathways is essential to safe, timely care (NICE, MHRA).
What the tests measure
APTT - intrinsic + common pathways
- Sensitive to deficiencies of factors XII, XI, IX, VIII and common‑pathway factors I (fibrinogen), II (prothrombin), V and X.
- Common ED uses: monitoring unfractionated heparin (UFH); investigation of suspected intrinsic factor deficiencies (haemophilia) or inhibitors.
PT / INR - extrinsic + common pathways
- Sensitive to factor VII and the common pathway (I, II, V, X).
- Reported as INR to standardise PT for vitamin‑K antagonist (VKA, warfarin) monitoring.
- Common ED uses: warfarin monitoring, screening for vitamin‑K dependent factor dysfunction and assessment of liver synthetic failure.