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The Prothrombin Time (PT) test measures the time taken for blood plasma to clot through the extrinsic and common coagulation pa...
Also known as: PT Test · PT/INR Test · INR Test · Prothrombin Time with INR · Warfarin Monitoring Test
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The Prothrombin Time (PT) test measures the time taken for blood plasma to clot through the extrinsic and common coagulation pathways — activated by tissue factor (thromboplastin). It is the primary test for monitoring warfarin (Coumadin) anticoagulation therapy and is a sensitive indicator of liver synthetic function, Vitamin K status, and extrinsic pathway factor deficiencies (particularly Factor VII). The PT is reported alongside the INR (International Normalised Ratio) — a standardised value that allows comparison of PT results across different laboratories and reagents. No fasting required.
Key benefits of this package include:
What Does the Prothrombin Time (PT) Test Measure?
| Level / Result | Possible Interpretation |
|---|---|
| INR 0.8–1.2 (Normal — not on anticoagulation) | Normal extrinsic and common pathway coagulation function |
| INR 1.2–2.0 (Mildly prolonged) | Early liver disease, mild Vitamin K deficiency, or subtherapeutic warfarin — investigate cause |
| INR 2.0–3.0 (Therapeutic — warfarin for AF/DVT/PE) | Correct anticoagulation range for most warfarin indications — continue current dose |
| INR 2.5–3.5 (Therapeutic — mechanical heart valves) | Target range for high-risk mechanical valve patients |
| INR 3.0–4.0 (Supratherapeutic — warfarin) | Excess anticoagulation — increased bleeding risk; warfarin dose reduction required |
| INR > 4.0 (Dangerous — warfarin or liver disease) | High spontaneous bleeding risk — dose hold or reversal (Vitamin K, FFP) may be required urgently |
| INR > 5.0 (Critical) | Severe liver failure or massive warfarin overdose — urgent medical intervention required |
Important Note: PT/INR results are highly context-dependent. An INR of 2.5 is dangerous in someone not on anticoagulation (suggesting severe liver failure or factor deficiency) but is the correct therapeutic target for a warfarin patient with a mechanical heart valve. Always interpret PT/INR in the context of anticoagulation therapy, liver disease status, Vitamin K intake, and current medications. All results must be reviewed by a physician.
PT/INR in Liver Disease — More Than Just Anticoagulation: In patients with chronic liver disease, PT/INR is a critical marker of liver synthetic capacity. As the liver loses functional mass — in cirrhosis, acute hepatitis, or liver failure — its ability to produce coagulation factors declines, prolonging PT. The MELD score (Model for End-stage Liver Disease) incorporates INR alongside bilirubin and creatinine to assess liver failure severity and prioritise liver transplantation. A rising INR in a cirrhotic patient signals decompensation and requires urgent hepatology assessment — it is not merely a coagulation concern.
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