INR (warfarin testing)

What is the international normalised ratio (INR)?

The prothrombin time (PT) is a measurement of blood clotting. The PT is used to calculate the INR which is used to assess warfarin dosage.

Why is the prothrombin time & the INR important?

When a blood vessel is damaged, bleeding occurs. The body responds to this injury in several ways. Firstly, the damaged vessel narrows to reduce the amount of blood which flows through it. Secondly, the coagulation (clotting) cascade is initiated, which involves multiple different clotting factors and proteins. The clotting cascade finished by producing a network of fibres called fibrin which crosslink over the broken area. Thirdly, platelets are activated and cling to the fibrin meshwork to produce a clot.

The clotting cascade involves two separate pathways - the extrinsic and intrinsic pathways. The prothrombin time (PT) tests the function of the extrinsic pathway. Clotting factors I (fibrinogen), II (thrombin), V, VII, and X are all involved in the extrinsic pathway and are tested using the PT.

Using data from the healthy population, the PT can be used to calculate the INR. Essentially, this calculation is performed by dividing the patient’s PT by the average of the INR in the population where the patient lives. People who have a normal PT will have an INR around 1.0.

When & why should I get tested?

People with bleeding problems or who have increased risk of clotting may also benefit from this test. The INR is most commonly used in assessing the dosage of warfarin, a drug taken to reduce clotting (thin the blood).

How is the sample collected for testing? Are any special precautions necessary?

This test is performed using a blood sample from a vein, usually taken into a tube containing an anticoagulant. No special circumstances are required for this test.

What levels of INR are considered normal?

Each laboratory has slightly different equipment and techniques. Results from one laboratory are not necessarily comparable to those issued by another laboratory. It is recommended to use the normal range data issued by the laboratory where the test was done. The information below is given as a guide.

INR

Interpretation

 

0.8-1.3

Reference range (95% of healthy adults have results within this range).

2.0-3.0

Most common target for people taking warfarin.

What conditions cause high levels of INR?

High levels of INR are associated with:

  • Disorders affecting clotting factors I (fibrinogen), II (thrombin), V, VII, and X
  • Warfarin usage
  • Usually, people on warfarin area advised to aim for a target INR of 2-3. In some people who are at particularly high risk of clotting, for example with certain types of prosthetic heart valves, the INR target may be higher, for example, 2.5-3.5.
  • Vitamin K deficiency
  • Liver disease
  • Disseminated intravascular coagulation

What conditions cause low levels of INR?

Low levels of INR are not associated with any particular disease. There may be a slightly increased risk of clotting.

References

1. Carl Burtis, Edward Ashwood & David Bruns, Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 5th Edition, 2013.

 

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