A blood test is needed to determine the effect of warfarin on each individual. Everybody reacts differently to warfarin and several dietary and lifestyle factors may affect the way warfarin works. Because of this it is important to monitor how warfarin is working via blood tests to check the INR. Regular monitoring of the INR allows warfarin doses to be tailored to the individual, making for safer, more effective therapy.

There is now a range of options available for INR monitoring. Traditionally, testing takes place at the doctor's surgery or at a pathology laboratory, requiring a blood sample being drawn from a vein and sent for testing. You then visit or call your doctor or healthcare professional to obtain your results and get instructions on your new warfarin dose.

Aside from laboratory monitoring of the INR, there are now portable devices available, which are very similar to the machines used to test blood sugar levels. These machines only involve a finger prick and allow results to be obtained within minutes of the test being done. Some doctor's surgeries now have these monitors in their practice which means their patients will only need one visit to have the test performed and get their results, and any dosage adjustments.

These devices also allow certain people to monitor their own INRs in the comfort of their own home or office. This process is known as self-monitoring.