Harvard Heart Letter

Snail's pace for home INR testing

FDA-approved machines allow you to test your blood's clotting potential at home. Why do so few people who take warfarin use them?

Everyone who takes the blood-thinning drug warfarin (Coumadin) — and there are several million of you out there — walks a medication tightrope every day. Take too little warfarin, and you leave yourself open to potentially deadly or disabling blood clots. Take too much, and you're at risk of developing serious bleeding in the stomach or a hemorrhagic stroke (bleeding in the brain). The right amount, though, offers protection against blood clots with a low risk of bleeding problems.

The trouble is, what's "right" is often a moving target. Changes in diet, medications, and health affect how the body reacts to warfarin. A blood test called the prothrombin time, which measures how long it takes blood to clot, tells you if you're in your target range. In a healthy person who isn't taking warfarin or other anticlotting medicines, the normal prothrombin time is 11 to 13 seconds. Clotting times are usually reported as a standardized comparison against normal, called the international normalized ratio, or INR. If your INR is 2, then it takes your blood twice as long to clot compared with normal blood.

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