From the Guidelines
PT (Prothrombin Time) and APTT (Activated Partial Thromboplastin Time) are two distinct blood clotting tests that measure different aspects of the coagulation cascade, with PT evaluating the extrinsic and common pathways and APTT assessing the intrinsic and common pathways. The key difference between PT and APTT lies in the coagulation factors they evaluate and the clinical contexts in which they are used. PT measures factors I, II, V, VII, and X, and is commonly used to monitor warfarin therapy and assess liver function, with results often reported as INR (International Normalized Ratio) for standardization 1. APTT, on the other hand, measures factors I, II, V, VIII, IX, X, XI, and XII, and is used to monitor heparin therapy and screen for hemophilia and other bleeding disorders.
Key Differences and Clinical Applications
- PT typically takes 11-13.5 seconds in healthy individuals and is used to assess the extrinsic pathway, with prolonged PT suggesting issues such as vitamin K deficiency or warfarin effect 1.
- APTT normally ranges from 25-35 seconds and is used to assess the intrinsic pathway, with prolonged APTT indicating problems such as hemophilia A or B 1.
- The choice between PT and APTT depends on the clinical context, with PT preferred for monitoring warfarin therapy and APTT preferred for monitoring heparin therapy 1.
- Both tests are complementary and often ordered together for a comprehensive assessment of coagulation function, especially in patients on anticoagulant therapy or with suspected bleeding disorders 1.
Interpretation and Limitations
- A normal aPTT and/or PT cannot rule out the effect of direct oral anticoagulants (DOACs), highlighting the need for careful interpretation and consideration of clinical context 1.
- Viscoelastic coagulation tests, such as thromboelastography (TEG) and rotational thromboelastometry (ROTEM), can provide rapid detection of coagulation disorders and may be useful in guiding reversal therapies in patients on DOACs 1.
- The limitations of PT and APTT in monitoring DOACs underscore the importance of using these tests in conjunction with clinical judgment and other diagnostic tools to ensure optimal patient care 1.
From the Research
Difference between PT and APTT
- PT (Prothrombin Time) measures the extrinsic pathway of coagulation, whereas APTT (Activated Partial Thromboplastin Time) measures the intrinsic pathway of coagulation 2.
- PT is used to monitor patients on warfarin therapy, while APTT is used to monitor patients on heparin therapy 3, 4.
- APTT is affected by warfarin, and the effects of warfarin and heparin on APTT are additive 3.
- PT and APTT are both used as global screening tests for abnormalities in the coagulation pathways and for monitoring anticoagulant therapy, but they have limitations as they are artificial in vitro systems 5.
Clinical Implications
- Abnormal PT and APTT results can confound monitoring of anticoagulant therapies, contributing to treatment failure 6.
- Warfarin can markedly affect APTT, leading to subtherapeutic heparin levels if heparin dose is decreased in response to a high APTT 3.
- PT and APTT are significantly increased in patients with acute myocardial infarction on anticoagulation therapy, with PT being more sensitive for predicting blood clotting tendency 2.
Monitoring Anticoagulation Therapy
- Time in the therapeutic range (TTR) can be used to estimate the quality of anticoagulation with continuous unfractionated heparin, using APTT monitoring 4.
- Different methodologies can be used to estimate TTR, such as percentage of APTT values in range (%aIR) and a modification of the Rosendaal method (mod-Rosendaal) 4.