INR (International Normalized Ratio) - Clinical Correlations

What the INR Measures

INR Reference Ranges and Correlation with Conditions

• Quick Compendium of Clinical Pathology, pp. 2217-2247
• Harrison's Principles of Internal Medicine 22E, p. 343

Diagnostic Flowchart (PT/INR vs. aPTT)

Key Clinical Correlations

1. Warfarin (Vitamin K Antagonist)

• INR is the primary monitoring tool for warfarin therapy
• Target typically INR 2.0-3.0; takes 3-5 days to reach therapeutic range (heparin bridging recommended)
• Factors affecting INR on warfarin:
  • Pharmacogenomics: VKORC1 (encodes the warfarin target, vitamin K epoxide reductase) and CYP2C9 (warfarin metabolism) have the greatest genetic impact
  • Diet: vitamin K intake (green leafy vegetables lower INR)
  • Comorbidities and drug interactions
• Reversal guidelines (PCC dosing): INR 2-4 → 25 U/kg; INR 4-6 → 35 U/kg; INR >6 → 50 U/kg (max weight 100 kg)
• Asymptomatic supratherapeutic INR (no bleeding): hold warfarin ± oral vitamin K; daily INR monitoring until back in range

2. Liver Disease

• INR (and PT) reflects hepatic synthetic function - a prolonged INR in a non-anticoagulated patient strongly suggests dysfunctional liver - Morgan and Mikhail's, p. 1334
• Coagulopathy seen primarily in severe liver disease (significant hepatic reserve must be lost first)
• Characteristically: INR/PT disproportionately longer than aPTT (because factor VII, with the shortest half-life, is most prominently affected first)
• Important caveat: the INR is NOT calibrated for liver disease - it was validated only for patients on VKA therapy. The ISI (International Sensitivity Index) used to calculate INR does not harmonize across labs for liver disease. This causes interlaboratory variation in MELD scores
• Factor VIII is typically normal or elevated in liver disease (made by endothelial cells, not hepatocytes) - this can help distinguish liver disease from DIC, in which factor VIII is low
• Despite prolonged INR, liver disease creates a rebalanced hemostasis - some patients may actually be hypercoagulable (due to concurrent protein C/S deficiency)

3. Vitamin K Deficiency

• Prolongs PT/INR (and sometimes aPTT) by impairing gamma-carboxylation of factors II, VII, IX, and X
• Pattern: PT/INR disproportionately longer than aPTT (same as liver disease, because factor VII has shortest half-life)
• Key distinguishing point: factor V is normal in pure vitamin K deficiency, whereas both factors V and VII are low in liver disease
• Common causes: malnutrition, prolonged antibiotic use (kills gut flora), malabsorption (fat-soluble vitamin)
• Responds to vitamin K replacement (if INR fails to correct → consider PIVKA-II measurement to confirm)

4. Disseminated Intravascular Coagulation (DIC)

• Both PT/INR and aPTT are prolonged (combined pattern)
• Additional markers: thrombocytopenia, reduced fibrinogen, elevated D-dimer and fibrin degradation products
• Requires an inciting cause: sepsis, major trauma, obstetric crisis, shock
• Factor VIII is LOW (consumed) - helps distinguish from liver disease

5. Isolated Factor VII Deficiency

• Prolonged PT/INR only (aPTT normal) - because factor VII is the only extrinsic pathway factor
• Congenital deficiency is rare; acquired usually from early liver disease or early vitamin K deficiency

INR and Procedural/Invasive Risk

Limitations of INR

• Not calibrated for non-warfarin states - the ISI is validated only for VKA-treated patients; using INR in liver disease or other acquired coagulopathies introduces significant interlaboratory variation - Tietz Textbook of Laboratory Medicine, 7th ed.
• Does not reflect anticoagulant factors (protein C, protein S, antithrombin III) - so a patient with INR of 3 (not on warfarin) may still be in a net procoagulant state if anticoagulant proteins are more depleted than coagulant factors
• DOACs (apixaban, rivaroxaban, etc.) do not use INR for monitoring - these are monitored with anti-Xa assays

Summary Table: PT/INR Pattern by Condition