Prothrombin Time (PT / INR)

Also known as: PT, INR, International Normalised Ratio

Prothrombin time measures how long blood takes to form a clot via the extrinsic coagulation pathway. The INR standardises PT results across laboratories and is essential for monitoring warfarin therapy.

Specimen

Venous blood (citrate tube — must be filled exactly to the line)

Turnaround

1–2 hours

Fasting

Not required

Reference Ranges

Measurement	Reference Range	Unit
PT (normal)	11 – 13.5	seconds
INR (normal, not on warfarin)	0.8 – 1.2	—
INR target (atrial fibrillation on warfarin)	2 – 3	—
INR target (mechanical heart valve)	2.5 – 3.5	—

Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.

What This Test Measures

Prothrombin time tests the extrinsic coagulation pathway — the chain of clotting factors (VII, X, V, II, fibrinogen) initiated by tissue factor exposure. PT is reported in seconds and as INR. The INR corrects for variability in the thromboplastin reagent used, making results comparable between laboratories worldwide. Factors II, VII, IX, and X are vitamin K-dependent — warfarin and liver disease disrupt their synthesis, prolonging PT.

When Is This Test Ordered?

Monitoring warfarin therapy
Evaluating liver function (PT is one of the most sensitive tests of synthetic liver function)
Pre-operative bleeding risk assessment
Investigating unexplained bruising or bleeding
Diagnosing disseminated intravascular coagulation (DIC)

When Results Are High

Prolonged PT/elevated INR means clotting is slower than normal — bleeding risk is higher. In warfarin patients, INR above 4.0 significantly raises haemorrhage risk; above 5.0 requires dose reduction or reversal. In non-anticoagulated patients, an elevated INR suggests liver disease (failed factor synthesis), vitamin K deficiency, or factor deficiency.

When Results Are Low

INR below 0.8 is rarely significant unless a patient on warfarin is subtherapeutic — their anticoagulant effect is insufficient to prevent thrombosis.

Factors That Can Affect Your Result

Vitamin K in food (leafy greens) competes with warfarin — changes in vegetable intake change INR
Dozens of drugs interact with warfarin — always check interactions when any medication is added
Liver disease impairs synthesis of all clotting factors, prolonging PT
The citrate tube must be filled exactly to the mark — underfilling prolongs PT artefactually

Tracking This Test Over Time

Warfarin monitoring requires a consistent INR testing schedule. Check within the first week of any new medication or significant dietary change. In stable patients with three consecutive in-range INRs, extend testing intervals to every 4–6 weeks.

Track your PT / INR in Looms

Related Tests

D-Dimer

D-dimer is a fibrin degradation product released when a blood clot dissolves. It is primarily used as a rule-out test for deep vein thrombosis (DVT) and pulmonary embolism (PE) — a low result in a low-risk patient effectively excludes these diagnoses.

Medical disclaimer

This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.

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