Nearly half of all atrial fibrillation patients at the highest risk for stroke are not being prescribed treatment that could avoid it, a new study finds.

One in four adults over age 40 is at risk for atrial fibrillation (AF) with a projection of nearly six million people in the nation having the condition by 2050.  AF can cause sluggish blood flow in the heart, which can result in a blood clot traveling to the brain and causing a stroke.  As a result, people with AF can have up to a seven-fold risk of suffering a stroke.

A University of California-led research team reviewed the records of 400,000 people over four years. They found that as stroke risk factor scores generally increased, cardiologists were more likely to prescribe blood thinners, but also that just under half (48 percent) of AF patients at the highest risk for stroke were not prescribed treatment.

The study’s authors said there are likely multiple reasons behind the practice, including the perceived risk of prescribing blood thinners in sicker patients.

“It may be thought of as too dangerous for these sicker patients, but we still know that in most of these patients the benefits of blood thinning to reduce the risk of stroke outweigh the risks of bleeding,” says Dr. Jonathan C. Hsu, the lead author of the study, which appears online in JAMA Cardiology.