What is the difference between Prothrombin Time (PT) and activated Partial Thromboplastin Time (aPTT)?

Differences Between Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT)

PT and aPTT are distinct coagulation tests that measure different pathways of the coagulation cascade, with PT primarily assessing the extrinsic pathway and aPTT evaluating the intrinsic pathway.

Basic Differences

• PT measures the extrinsic and common pathways of coagulation (factors II, V, VII, X, and fibrinogen), while aPTT measures the intrinsic and common pathways (factors II, V, VIII, IX, X, XI, XII, and fibrinogen) 1
• PT results are often reported as International Normalized Ratio (INR) when monitoring vitamin K antagonist therapy (e.g., warfarin), while aPTT is typically reported in seconds or as a ratio compared to normal control 2, 3
• PT is more sensitive to deficiencies in factors VII, X, and II, while aPTT is more sensitive to deficiencies in factors VIII, IX, XI, and XII 4, 5

Clinical Applications

PT/INR Applications

• Primary test for monitoring vitamin K antagonist therapy (warfarin) 2
• Screening for liver disease (as the liver produces most clotting factors) 6
• Assessment of synthetic liver function 5
• Detection of factor VII deficiency 4
• Evaluation of the extrinsic coagulation pathway 1

aPTT Applications

• Monitoring unfractionated heparin therapy 1
• Screening for hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency) 7
• Detection of lupus anticoagulant and other inhibitors 8
• Evaluation of the intrinsic coagulation pathway 1
• Qualitative assessment of dabigatran (direct thrombin inhibitor) presence 1

Reference Ranges and Interpretation

• Normal PT range is typically 11-13.5 seconds or INR of 0.8-1.2 for those not on anticoagulation therapy 3
• Normal aPTT range is typically 25-35 seconds, though this varies by laboratory and reagent 8
• For emergency neurosurgery, maintaining PT/aPTT <1.5 times normal control is recommended 1
• During massive transfusion protocols, both PT and aPTT should be monitored with a target ratio of <1.5 normal control 1

Medication Monitoring

• PT/INR is the standard test for monitoring warfarin therapy with target ranges typically between 2.0-3.0 or 2.5-3.5 depending on indication 2
• aPTT is used for monitoring unfractionated heparin with a typical target of 1.5-2.5 times the baseline value 1
• For direct oral anticoagulants (DOACs):
  • Dabigatran (a direct thrombin inhibitor) primarily affects aPTT 1
  • Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) primarily affect PT 1

Limitations

• Neither test fully reflects the complexity of hemostasis as they only measure the initiation phase of blood coagulation (first 4% of thrombin production) 1
• PT/INR was specifically designed for monitoring vitamin K antagonists and lacks validation as a general predictor of bleeding risk in other clinical scenarios 6
• aPTT can be affected by many preanalytical variables including sample collection technique and processing 8, 7
• Both tests may appear normal while overall coagulation is abnormal, or vice versa 1

Advanced Testing Considerations

• When more comprehensive assessment is needed, viscoelastic testing methods like thromboelastography (TEG) or rotational thromboelastometry (ROTEM) provide better evaluation of overall clot formation and stability 1
• For specific DOAC measurement:
  • Dabigatran: Diluted thrombin time, ecarin clotting time, or ecarin chromogenic assay are more accurate than aPTT 1
  • Factor Xa inhibitors: Anti-Xa assays calibrated for the specific drug provide more accurate measurement than PT 1

Common Pitfalls

• Relying solely on PT/INR or aPTT for overall assessment of bleeding risk 6
• Using INR to assess bleeding risk in non-warfarin patients 6
• Drawing samples from heparinized lines, which can artificially prolong results 1
• Failure to consider timing of anticoagulant administration relative to blood sampling 1
• Not accounting for developmental differences in hemostasis when interpreting results in neonates and children 1