How Warfarin Is Monitored

INR Testing

INR stands for International Normalized Ratio. This is a blood test that allows us to determine if we are giving you the correct amount of warfarin. It does this by measuring the activity of certain clotting substances in your blood.

As long as you remain on warfarin therapy, you will need to have regular INR tests to determine if the dose of warfarin you need has changed. If you do not receive necessary INR tests, warfarin can be a very dangerous drug. Taking too much warfarin can cause serious bleeding, and taking too little could lead to a blood clot.

Testing Procedure

First, a small sample of blood is drawn via a finger poke. This sample is placed on a testing strip which has several chemicals on it that combine with your blood. The machine then calculates how long it takes your blood to form a clot. This is your INR result.

INR Results

Each patient will have a goal INR range which is determined by your doctor. You may want to write your goal range down. Your goal range is the area where you receive the most warfarin benefit (clot prevention) while having the least risk (bleeding problems).

People who are not on warfarin will have an INR around 1.0. Warfarin prolongs the time it takes your blood to clot, raising the INR result. INR results below your goal range mean that your blood is thicker than desirable. INR results above your goal range mean that your blood is thinner than desirable.

Frequency of Testing

When you first start warfarin, you will have to have your blood tested one to two times per week for several weeks. Then INR tests will be needed every week until your results are stable. Once your INR is steady, it is recommended that INR tests be completed once a month.