Basic coagulation tests: aPTT, PT/INR, D-dimer and fibrinogen
What the PT/INR, APTT, fibrinogen, thrombin time and D-dimer measure, the patterns that point to each cause, mixing studies, sample artefacts, and when a result should change what you do.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- The PT tests factors VII, X, V, II and fibrinogen. The INR standardises the PT for warfarin monitoring only.
- The APTT tests XII, XI, IX, VIII and the common pathway (X, V, II, fibrinogen). It is long with factor deficiency, heparin and lupus anticoagulant.
- Fibrinogen (Clauss method) is an acute phase protein and the first factor to fall to critical levels in major bleeding.
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Sources
- RCPA Manual (coagulation tests)AU/NZRoyal College of Pathologists of Australasia
- Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis: update of the guidelines for lupus anticoagulant detection and interpretationDevreese KMJ et al., J Thromb Haemost 2020;18:2828-39, 2020
- Updated recommendations for warfarin reversal in the setting of four-factor prothrombin complex concentrateAU/NZMedical Journal of Australia (Robinson et al., 2025;222:49-51), 2025
- Patient blood management guideline for adults with critical bleedingAU/NZNational Blood Authority, 2023, updated 2025
- Age-adjusted D-dimer cutoff levels to rule out pulmonary embolism: the ADJUST-PE studyRighini M et al., JAMA 2014;311:1117-24, 2014
- Guidelines on the assessment of bleeding risk prior to surgery or invasive proceduresChee YL et al., British Committee for Standards in Haematology, Br J Haematol 2008;140:496-504, 2008
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy