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Prothrombin Time

Prothrombin Time with INR

Test Highlight

Prothrombin Time with INR


Clinical Use

  • Screen for bleeding disorders

  • Monitor anticoagulant therapy (warfarin)

Clinical Background

The prothrombin time (PT) test assesses the extrinsic and common coagulation pathways, beginning with factor VII and ending with fibrin formation. It can be used to screen for deficiencies in, or inhibitors of, factors II, V, VII, or X as well as to monitor patients who are receiving anticoagulant therapy.


Photo-optical clot detection

Results are reported as seconds and as an international normalized ratio (INR)

Interpretive Information

Prolonged PT results (>11.5 seconds) suggest a potential bleeding disorder that may be caused by a deficiency in factor II, V, VII, or X. Prolonged results are also associated with very low fibrinogen levels, factor-specific inhibitors, therapeutic anticoagulants, or a lupus anticoagulant.

The recommended therapeutic range for oral anticoagulant (warfarin) therapy is based on the INR (Table).

Table. Recommended Therapeutic INR Range



Moderate intensity warfarin therapy 2.0 – 3.0
High intensity warfarin therapy 3.0 – 4.0

a Patients who are being treated with anticoagulant therapy and who have a lupus anticoagulant are better monitored with the factor X chromogenic assay (therapeutic range: 11% to 41%).


Content reviewed 02/2013

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* The tests listed by specialist are a select group of tests offered. For a complete list of Quest Diagnostics tests, please refer to our Directory of Services.