What are the differences between prothrombin time (PT) / international normalized ratio (INR) and partial thromboplastin time (PTT)?

PT/INR vs PTT: Key Differences

PT/INR and PTT are fundamentally different coagulation tests that assess distinct pathways of the clotting cascade—PT/INR evaluates the extrinsic and common pathways (factors VII, X, V, II, and fibrinogen) while PTT assesses the intrinsic and common pathways (factors XII, XI, IX, VIII, X, V, II, and fibrinogen).

What Each Test Measures

PT/INR (Prothrombin Time/International Normalized Ratio)

• Measures: Tissue factor-induced coagulation time using thromboplastin reagent 1
• Reported as: Clotting time in seconds, which is then converted to INR using the formula: INR = (PT/MNPT)^ISI 1
• Primary validated use: Monitoring vitamin K antagonist (VKA) therapy like warfarin 2
• Pathway tested: Extrinsic pathway (Factor VII) and common pathway

PTT/APTT (Partial Thromboplastin Time/Activated PTT)

• Measures: Intrinsic pathway clotting time
• Reported as: Clotting time in seconds or ratio to normal
• Primary uses: Monitoring heparin therapy, detecting intrinsic pathway deficiencies (hemophilia A/B, von Willebrand disease)
• Pathway tested: Intrinsic pathway (Factors XII, XI, IX, VIII) and common pathway

Clinical Applications and Sensitivity

For Trauma and Acute Coagulopathy

In acute trauma settings, PT/INR is significantly more reliable than APTT for detecting low coagulation factor levels, with 84% sensitivity compared to only 50% for APTT 3. This is critical because:

• APTT shows false positives with lupus anticoagulants and heparin contamination
• APTT shows false negatives when Factor VIII is elevated (common in acute illness/trauma)
• PT/INR cutoff ≥1.5× normal is more dependable for identifying true factor deficiencies 3

For Preoperative Bleeding Risk Assessment

Neither PT/INR nor APTT reliably predicts perioperative bleeding in patients without known bleeding risk factors 4, 5. The evidence shows:

• Weak or inconsistent association between these tests and actual bleeding events in most surgical settings 5
• Exception: PT may have predictive value in patients with advanced liver disease undergoing major abdominal surgery or liver transplantation 5
• A bleeding history and physical examination are superior screening tools 4

Critical Limitations of INR

The INR Was Designed ONLY for VKA Monitoring

The INR standardization was developed and validated exclusively for patients on stable vitamin K antagonist therapy 2. The standardization process specifically excluded patients with:

• Underlying bleeding tendencies
• Acute medical illness
• Liver disease coagulopathy
• Other anticoagulant agents

Misuse in Clinical Practice

Despite this narrow validation, INR is widely misused as a general coagulopathy screen, lacking both validity and evidence for non-VKA indications 2. Common inappropriate uses include:

• Assessing bleeding risk in liver disease (where it's a poor predictor)
• Guiding plasma transfusion decisions
• Preoperative screening in non-anticoagulated patients
• General hemostatic screening

There is no high-quality evidence supporting treatment of INR <2.0 in patients not receiving VKA therapy 2.

Test Variability and Reporting

PT/INR Variability

• PT results vary significantly based on thromboplastin source, even with identical samples 2
• INR was created to standardize PT across different thromboplastins for VKA monitoring 1
• In dilutional coagulopathy, diagnostic decisions can depend entirely on which PT reagent is used 6

Temperature Effects

Hyperthermia significantly prolongs both tests 7:

• At 43°C: PT increases ~30%, INR increases ~33%, APTT increases ~40% from baseline
• Standard laboratory testing at 37°C may not reflect in vivo coagulation in febrile patients

Storage Stability

For accurate results 8:

• Freezer storage: Stable for PT, INR, and APTT up to 36 hours
• Refrigerator: PT/INR stable 24 hours; APTT deteriorates at 12 hours
• Room temperature: All tests deteriorate at 12 hours

Practical Algorithm for Test Selection

Choose PT/INR when:

• Monitoring warfarin/VKA therapy (only validated indication for INR)
• Evaluating extrinsic pathway defects (Factor VII deficiency)
• Assessing acute trauma coagulopathy (more sensitive than APTT)
• Screening for liver synthetic dysfunction (though not predictive of bleeding)

Choose APTT when:

• Monitoring unfractionated heparin therapy
• Evaluating intrinsic pathway defects (hemophilia A/B, Factor XI deficiency)
• Screening for lupus anticoagulant (though causes false prolongation)
• Assessing von Willebrand disease workup

Order both tests when:

• Evaluating unexplained bleeding with unknown etiology
• Assessing common pathway defects (Factors X, V, II, fibrinogen)
• Initial coagulopathy workup requiring comprehensive assessment

Key Pitfalls to Avoid

1. Never use INR to guide plasma transfusion in non-VKA patients—no evidence of benefit, definite harms 2
2. Don't rely on APTT in trauma settings—high false positive/negative rate 3
3. Don't use either test for routine preoperative screening without bleeding history 4, 5
4. Remember reagent variability—cutoffs must be laboratory-specific 6
5. Consider clinical context—elevated Factor VIII in acute illness falsely normalizes APTT 3