warfarinfo

Warfarin Institute of America

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THE INTERNATIONAL NORMALIZED RATIO (INR) SYSTEM

Several years ago, it was noticed that the prothrombin time (PT) would vary from laboratory to laboratory and even with the same analyzer from time to time. The problem was traced to varying sensitivities of the materials used to perform the test.

A mathematical formula was devised to take this variance into account. This is called the International Normalized Ratio or INR.

If your INR is in the desired range you have the greatest chance of avoiding a clot with the least chance of developing major bleeding. Major bleeding is usually defined as needing two units of blood or more.

There are some problems with the system. It is not a very good predictor of who will have minor bleeding. It measures clotting time, so it is not a good indicator of some other factors in the blood.

Despite its limitations, it is the standard of care. If your warfarin monitoring is being done by the PT, then you are getting less than optimum care.

It is no longer necessary to have your blood tested by having someone draw blood from your vein and waiting hours to a week for the results. There are monitors which require only one drop of blood from a finger-stick that give the results in two minutes or less. Insist on this for your care.

The standard of care for frequency of testing has been no less frequently than every four weeks. However, because of the great increase in the number of people using warfarin, this has placed a strain on some systems. Because of a shortage of qualified personnel, I have had to shift the interval for relatively stable patients to six weeks. I have found that this has produced no increase in either minor major bleeding. Lidstone et al. reported a similar increase in the number of people in the UK using warfarin and the subsequent strain on the National Health Service. They concluded that selected patients could have the interval between tests safely extended to 14 weeks, with the potential for longer intervals.

Reference: Lidstone V. et al. INR: intervals of measurement can safely extend to 14 weeks. Clin Lab Haematol 2000;22:291-293.

Print My Own INR Monitoring Chart

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Last updated January 1, 2006