Harvard Heart Letter

Genetic help for a blood-thinner balancing act

Genetic help for a blood-thinner balancing act?

There's little evidence yet that a genetic test improves the safety of warfarin.

The "blood thinner" warfarin isn't the easiest drug to use. Unlike aspirin and most other drugs for adults, the amount of warfarin needed varies from person to person. And for the same person, the dose can vary from week to week. The FDA is trying to take out some of the guess work by adding to the drug label a recommendation that people using warfarin have a genetic test.

That's a controversial ruling for several reasons. There isn't yet strong evidence that testing for two warfarin-related genes makes use of the drug safer. The tests cost $400 each. Waiting for the results might delay starting warfarin. The results can be confusing, as when one test suggests the need for a high dose of warfarin and the other indicates the need for a low dose. Some experts worry that doctors might rely too heavily on the genetic tests and not pay enough attention to the myriad other factors that influence how an individual responds to warfarin. Others are concerned that the new wording might prompt lawsuits from patients who do not have the test and later experience problems with warfarin.

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