Introduction

PT (Prothrombin Time) and INR (International Normalised Ratio) are laboratory tests used to assess the extrinsic and common pathways of the blood coagulation system.

PT measures the time it takes for blood to clot, while INR standardises this measurement globally, allowing consistent interpretation across different laboratories.

These tests are crucial for evaluating bleeding disorders, liver function, and anticoagulant therapy monitoring, especially in patients receiving warfarin.

Applications

1. Monitoring Warfarin Therapy

• PT/INR is the standard test for patients on warfarin anticoagulation, ensuring the drug is effective and safe.
• Therapeutic INR range typically: 2.0–3.0 (varies by condition).

2. Detection of Coagulation Disorders

Used to diagnose:

• Vitamin K deficiency
• Coagulation factor deficiencies (II, V, VII, X)
• Disseminated intravascular coagulation (DIC)
• Inherited bleeding disorders (rare)

3. Liver Function Assessment

Liver produces most clotting factors → prolonged PT/INR indicates:

• Liver failure
• Cirrhosis
• Hepatitis

4. Preoperative Screening

Helps evaluate bleeding risk before:

• Surgery
• Invasive procedures

5. Evaluation of Bleeding Symptoms

Useful in cases of:

• Bruising
• Nosebleeds
• Gum bleeding
• Heavy menstrual bleeding

Keynotes

• PT measures the clotting time; INR standardises results regardless of lab reagents.
• Normal PT varies; normal INR ≈ 1.0 in healthy individuals.
• Prolonged PT/INR suggests defective clotting, most commonly due to warfarin therapy, vitamin K deficiency, or liver dysfunction.
• Always interpret INR in the context of clinical history and medications.
• Critical INR values (>5) increase the risk of serious bleeding; values <2 increase the risk of clots in warfarin patients.
• Not useful for monitoring heparin therapy (use aPTT instead).

Further Readings

• https://medlineplus.gov/lab-tests/prothrombin-time-test-and-inr-ptinr/
• https://www.testing.com/tests/prothrombin-time-and-international-normalized-ratio-ptinr/
• https://www.ganeshdiagnostic.com/blog/prothrombin-time-pt-inr-test-crucial-tool-for-assessing-bleeding-disorder
• https://www.ncbi.nlm.nih.gov/books/NBK507707/
• https://emedicine.medscape.com/article/2086058-overview
• https://www.heart.org/en/health-topics/arrhythmia/prevention–treatment-of-arrhythmia/a-patients-guide-to-taking-warfarin
• https://www.healthline.com/health/prothrombin-time-pt
• https://www.liliumdiagnostics.ca/products/pt-and-inr-prothrombin
• https://medicallabnotes.com/pt-inr-and-aptt-test-introductiontest-result-unit-normal-range-test-method-clinical-significance-and-keynotes/
• https://www.mountsinai.org/health-library/tests/prothrombin-time-pt