ALEX CHADWICK, host:

Back now with DAY TO DAY.

Dear listeners, when you and I go to the doctor, often, we worry. And it turns out that when we show up at the office, often the doctors worry, too. What exactly do we need in terms of medication? It's not always an easy answer. You need a particular drug, but how much of it?

Dr. Sydney Spiesel is a professor at Yale Medical School, and he has pediatrics practice in Woodbridge, Connecticut. And he joins us regularly on DAY TO DAY to talk about medical news.

Syd, welcome back.

Dr. SYDNEY SPIESEL (Yale Medical School): Thank you.

CHADWICK: You're writing in your column in Slate that there's one medication in particular that causes problems for doctors with what to prescribe, exactly. What is that?

Dr. SPIESEL: Well, there's a drug called warfarin. And warfarin is a drug which is called a blood thinner, which really means it's a medication that regulates blood clotting.

CHADWICK: And so what do you prescribe this for?

Dr. SPIESEL: Well, doctors prescribe this as a way of treating heart attacks, as a way of treating and preventing stroke.

CHADWICK: And what is it that is so tricky about prescribing this particular drug? How come it's especially challenging for you?

Dr. SPIESEL: Well, you have a very narrow therapeutic range in which it works. That's one of the issues. So you got to get the dose just right. But individual people have very individual responses to this drug so that some people, for example, eliminate the drug very quickly. Other people eliminate it very slowly.

And so getting the dose just right means you have to know - the best way to do it would be to know something about an individual patient's way of dealing with the drug, and that's determined by their genetics.

CHADWICK: So, you're saying - let's say you have Patient X, a 6-foot tall, 55-year-old white male, and you might have another patient, identical characteristics, but they would have different reactions to this drug, different amounts for this same drug.

Dr. SPIESEL: That's right. It would - for these two patients, they might have very different doses, which would give the same degree of regulation of their blood clotting.

CHADWICK: And you say that you think that genetic testing is the answer to this. This is a particularly appropriate place for genetic testing?

Dr. SPIESEL: Well, it's an appropriate place for genetic testing only because in recent years, we've found and isolated the two genes which seem to regulate the level of warfarin in the blood, and because this is a drug which causes so much trouble.

CHADWICK: So you have this sort of balance here, another medical test that may help with prescribing a particular medication. And on the other side of that scale, it seems every time I go to the doctor, there's another test and another test and another test. And that's all that's driving up a lot of the cost of medicine, isn't it?

Dr. SPIESEL: Very much so. But sometimes you have to - you can look and you can say, well, here's a test which might be expensive, but it also might save money. And this is one of the first examples of a test of a person's genetic make up in the areas called pharmacogenomics, in which we can really say with a fair amount of certainty that it would really - that even though the tests might be fairly expensive, it will save money.

And here's the reason. Right now, every year, the poor regulation of blood clotting - until you get it just right by doing repeated lab tests and adjusting the dose and lab tests and adjusting the dose - probably contributes to an excess of 85,000 bleeding events, which often would cause hospitalization. It also is implicated in about 17,000 strokes a year. These are the number of problems that would be prevented if we do this testing. And the estimated savings, even given the additional cost of the testing, was estimated at $1.1 billion a year. So this is an investment with a good return on itself.

CHADWICK: Is this going to expand to other drugs as well, do you think?

Dr. SPIESEL: I'm sure it will. I'm sure that this is just the leading edge. In fact, this is, for the first time, the FDA has recommended that these tests be used as you adjust the dose of warfarin. I'm sure that as we learn more about drugs, there are many more drugs that are going to wind up in the same pile.

CHADWICK: Opinion from Dr. Sydney Spiesel of the Yale Medical School. You can read his Medical Examiner column at Slate.com.

Syd, thank you again.

Dr. SPIESEL: Thank you.

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