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Home :: International normalized ratio

International Normalized Ratio

The international normalized ratio (INR) system is viewed as the best means of standardizing measurement of prothrombin time to monitor oral anticoagulant therapy. It isn't used as a screening test for coagulopathies.


  • To evaluate effectiveness of oral anticoagulant therapy.

Patient preparation

  • Explain to the patient that this test is used to determine the effectiveness of his oral anticoagulant therapy.
  • Tell the patient that a blood sample will be taken. Explain who will perform the venipuncture and when.
  • Reassure him that drawing a blood sample will take less than 3 minutes.
  • Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 7-ml blue-top tube.
  • If a hematoma develops at the venipuncture site, apply warm soaks.
  • Completely fill the collection tube; otherwise, an excess of citrate appears in the sample.
  • Gently invert the tube several times to thoroughly mix the sample and the anticoagulant.
  • To prevent hemolysis, avoid excessive probing during venipuncture and handle the sample gently.
  • Put the sample on ice, and send it to the laboratory promptly.

Reference values

Normal INR for those receiving warfarin therapy is 2.0 to 3.0. For those with mechanical prosthetic heart valves, an INR of2.5 to 3.5 is suggested.

Abnormal findings

Increased INR values may indicate disseminated intravascular coagulation, cirrhosis, hepatitis, vitamin K deficiency, salicylate intoxication, uncontrolled oral anticoagulation, or massive blood transfusion.

Interfering factors

  • Failure to fill the collection tube completely, to adequately mix the sample and the anticoagulant, or to send the sample to the laboratory immediately.
  • Hemolysis due to excessive probing at the venipuncture site or to rough handling of the sample.

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