Stroke-preventing devices are not being tested in people who resemble the patients most likely to receive them, a new study shows.

The researchers looked at tests being done as part of “post-market surveillance studies,” which test products that are already approved and in general use by the public.

They found that patients who had small mesh tubes, called stents, placed into their neck in these studies were healthier than other patients in a broader nationwide registry of people who received stents.

“We were surprised,” said Dr. Robert Yeh, the lead author of the study and a cardiologist at Massachusetts General Hospital and Harvard Medical School.

“The intention and presumption of most post-marketing studies is that they represent the group of patients that we see in clinical practice.”

Yet the patients in the larger registry database who weren’t in the post-marketing studies were 50 percent more likely to have had a stroke in the past, and were four times as likely to die in the hospital, Yeh said.

“When we look at these results (from post-marketing surveillance) we should be a little bit careful in applying them to the patient in front of us,” Yeh told Reuters Health.

People in the study were having a stent put into a major blood vessel in the neck that had become narrowed, to widen it and prevent plaque from moving into the brain and causing a stroke.

The blood vessel is known as the carotid artery. When carotid artery stents were first approved, the Food and Drug Administration required that companies monitor their safety and effectiveness after they came on the market.

Like stents that are placed in heart arteries, carotid artery stents are implanted through an incision in the skin - usually in the groin - without surgery.

Yeh and his colleagues looked at the health characteristics before and after stenting for 7,115 patients in the national registry; roughly half of them participated in surveillance studies.

They found that four of every 10 patients in the surveillance studies had experienced a stroke or mini-stroke before they had the stent.

In contrast, six of every 10 patients who were not involved in any studies had had a stroke or mini-stroke (also called a transient ischemic attack, or a TIA).