The drug warfarin interferes with blood clotting. To use warfarin safely, your response to the drug needs to be checked every few weeks with a blood test. The test measures how quickly the blood clots. This result is called the “prothrombin time.”

The prothrombin time depends on the way the test is done. So its result must be changed to a standardized number called the “international normalized ratio” (INR) before your doctor interprets the result. Based on the INR, your doctor can tell whether your warfarin dose is in a safe range.

There are small monitors where you can test a drop of blood and get a result for the INR at home. These monitors pick up changes in electrical current that happen when blood is in the process of clotting. Using these results, your doctor can adjust your warfarin dose, if needed. The monitors are small, about the size of a paperback book.