Final answer:
A patient who is taking warfarin must routinely take a blood test known as international normalized ratio (INR) to measure how well the blood clots.
Step-by-step explanation:
A patient who is taking warfarin must routinely take a blood test known as international normalized ratio (INR) to measure how well the blood clots. The INR measures the speed of a particular pathway of coagulation, comparing it to normal. If the INR is increased, it means it is taking longer than usual for blood to clot.