Prothrombin Time (PT)

Prothrombin time (PT) measures how long blood plasma takes to clot after adding tissue factor; it evaluates the extrinsic and common coagulation pathways. It’s reported in seconds and often as an INR for monitoring warfarin. Normal PT is about 11–13.5 seconds (INR 0.8–1.2). PT helps assess bleeding risk, monitor anticoagulant therapy, and detect liver disease or vitamin K deficiency.

The normal INR (prothrombin time) for someone not on anticoagulants is roughly 0.8–1.2. For people on warfarin, typical therapeutic targets are 2.0–3.0 (most indications) or 2.5–3.5 (certain mechanical heart valves). INR below this range suggests faster clotting; above it indicates higher bleeding risk. Lab reference ranges and individual targets can vary, so discuss results with your clinician.

A 1.5 INR means blood takes slightly longer than normal to clot but is usually below the therapeutic target for most anticoagulation goals (commonly 2.0–3.0). For people on warfarin, 1.5 is subtherapeutic and may raise thrombosis risk; clinicians might raise the dose or monitor more closely. Interpretation depends on indication, bleeding risk and whether you’re on anticoagulants—consult your clinician.

The prothrombin time (PT) ratio is the patient's PT divided by a control PT; the normal value is about 0.8–1.2 (approximately 1.0). Small laboratory variation exists. Clinicians usually use the INR for anticoagulation monitoring—normal INR is around 0.9–1.2, while therapeutic INR targets are higher depending on indication. If outside the range, repeat testing and clinical review are advised.

A high prothrombin time means blood takes longer to clot, indicating increased bleeding risk — easy bruising, prolonged bleeding from cuts, or internal hemorrhage. Causes include liver disease, vitamin K deficiency, anticoagulant therapy, factor deficiencies, or consumption coagulopathy. It prompts further evaluation (INR, liver tests) and treatment of the underlying cause, such as adjusting anticoagulants, vitamin K, or plasma products.

A critical PT (prothrombin time) value is a result far outside the normal range that signals a high risk of bleeding or clotting and requires immediate clinical action. It is often defined as PT >20 seconds or an INR substantially above therapeutic range (commonly >4–5, with higher thresholds prompting urgent reversal). Such results prompt rapid clinician notification and interventions like anticoagulant adjustment or reversal.