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Prothrombin Time (PT/INR): Indications, Interpretation, and Anticoagulation Monitoring

The Prothrombin Time (PT) measures the extrinsic and common coagulation pathways. Results are standardized as the International Normalized Ratio (INR) to harmonize across thromboplastin reagents.

Physiology

PT assesses Factors VII, X, V, II (prothrombin), and fibrinogen. Tissue factor (thromboplastin) + Ca2+ → Factor VIIa → Factor Xa → Thrombin → Fibrin clot.

Clinical Indications

  • Warfarin Monitoring: Target INR 2.0–3.0 (most indications); 2.5–3.5 (mechanical mitral valve, recurrent VTE).
  • Liver Disease: Liver synthesizes all factors except VIII. PT prolongs early (Factor VII shortest half-life: 4–6 hrs).
  • Vitamin K Deficiency: Malabsorption, antibiotics, warfarin, neonatal (vitamin K prophylaxis prevents VKDB).
  • DIC: Consumption of factors + fibrinolysis → ↑ PT, ↓ fibrinogen, ↑ D-dimer, thrombocytopenia.
  • Factor Deficiencies (VII, X, V, II, Fibrinogen): Congenital or acquired.
  • Preoperative Screening: High-risk surgery, liver disease, anticoagulant use.

INR Interpretation

INRClinical Significance
0.9–1.1Normal (off anticoagulation)
1.2–1.5Mild prolongation (early warfarin, mild liver disease, vit K deficiency)
2.0–3.0Therapeutic for most warfarin indications
2.5–3.5High-intensity (mechanical mitral valve, antiphospholipid syndrome)
>4.0Supratherapeutic – bleeding risk; hold/reverse warfarin
>5.0High bleeding risk; vitamin K indicated

Warfarin Dosing Adjustment

  • Use validated nomograms or computer algorithms.
  • Frequent INR checks initially (daily → weekly → monthly stable).
  • Assess adherence, diet (vitamin K), drug interactions (antibiotics, amiodarone, NSAIDs), illness.

Limitations

PT/INR does not monitor heparin (use aPTT/anti-Xa) or DOACs (use drug-specific calibrated assays). Lupus anticoagulant can artifactually prolong PT.

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