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In recent years, DNA variants in two different genes have been linked to how much blood thinner a patient needs. Researchers have developed algorithms to predict a proper dose based on those genes as well characteristics such as weight and age. But now new research suggests these formulas may not actually help doctors determine the right dose.

Blood thinners can be difficult to prescribe because each patient can respond differently depending on their physical characteristics, their diet, and other medications they take. Based on that information, doctors tailor the dosage so it is high enough to prevent clotting but not so high that it causes internal bleeding. Once an initial dose is given, blood tests can help a doctor tune in to the best dose.

The hope has been that doctors might be better able to hit the right dose by looking at genes known to affect a patient’s response to the drugs—an approach called pharmacogenetics. Many experts have promoted this approach and some small patient trials have offered hope, writes Bruce Furie, a Harvard specialist in bleeding and...