The blood thinner warfarin (Coumadin) saves lives. It also sends older patients to the emergency room more often than does any other prescription drug. Prescribed to prevent dangerous blood clots—such as after heart attacks, strokes or surgery—it’s a “Goldilocks” drug. A dose that’s too high can lead to life-threatening internal bleeding…one that’s too low can cause blood clots, also life-threatening. Now there’s a better way to find the dose that’s “just right.”

Background: A patient’s response to warfarin depends on his/her genetic makeup (genotype)…in this case, variants of three specific genes. However, prior studies that looked at warfarin dosing based on genotype—rather than on conventional methods that use only clinical factors such as age and weight—have had mixed results.

New study: Researchers at Washington University’s Hospital for Special Surgery in St. Louis and other institutions looked at 1,650 patients age 65 and older who were having hip or knee replacements, two procedures for which warfarin is normally prescribed. The participants were randomly assigned to either genotype-based dosing or clinical dosing of warfarin. The researchers then monitored various adverse events over 30 days from the date of each patient’s operation—major bleeding…presence of a blood clot in a vein (within 60 days)…elevated international normalized ratio (INR, a measurement of clotting ability)…and death. An elevated INR (4 or greater) shows decreased clotting ability and increased risk of bleeding.

Results: Genotype-based dosing was associated with a 27% reduction in adverse events. Among patients in the genotype-based dosing group, 10.8% experienced at least one adverse event compared with 14.7% of patients in the other group.

Bottom line: Because it is effective, inexpensive and administered orally and the blood-thinning effects are reversible, warfarin is frequently prescribed to prevent blood clots. While genotyping isn’t currently routine practice and is not covered by insurance for this use, if you have been prescribed the drug, you may want to talk to your doctor about getting a gene test and paying for it out-of-pocket (starting at about $100).