Many hospitals overestimate how quickly they give stroke patients a clot-busting treatment designed to help minimize damage, a U.S. study suggests.

Researchers asked hospital staff how fast they administered an intravenous (IV) therapy known as thrombolysis to dissolve clots and compared the answers to stroke registry data with the actual times.

Only 29 percent of hospitals had an accurate sense of their own speed, and the slower hospitals mostly overestimated their own performance, the study found.

“Everyone likes to think that they are doing better,” senior study author Dr. Bimal Shah, a researcher at Duke University School of Medicine, said by email. “Not acting quickly makes the prognosis for stroke patients worse.”

Globally, 15 million people suffer strokes each year; five million of them die and another five million are left permanently disabled, according to the World Health Organization.

Initial symptoms can include drooping or numbness on one side of the face, lack of feeling or mobility in one arm or slurred, impaired speech.

Thrombolysis can be used within three hours of when stroke symptoms start, as long as patients haven’t recently had a heart attack, stroke or head injury and don’t have other medical complications that might make the treatment unsafe. Some patients under 80 may be able to receive the treatment within 4.5 hours, according to past research.

The drugs delivered in IV thrombolysis can be effective for ischemic stroke, which results from an obstruction in a blood vessel supplying the brain and is the most common type of stroke.

Because speed is crucial, Shah and colleagues focused on what they describe as the “door-to-needle” time, or the time from when patients enter the hospital door to when IV therapy begins.

Overall, the 141 hospitals in the study admitted 48,201 stroke patients. The researchers ranked the hospitals based on the percentage of patients who received the IV therapy within an hour of arrival.

The top-performing hospitals tended to treat more ischemic stroke patients each year and to perform thrombolysis in more patients. Staff at these facilities estimated that more than 60 percent of patients were treated within an hour of arrival, while in reality it was about 57 percent.

The gap between perception and reality was far bigger for hospitals that were generally slower.

Among the lowest-performing hospitals, staff surveyed generally thought that at least 20 percent of treated patients got the therapy within an hour. In reality, none did.

Despite their lack of speed, 85 percent of the low-performing hospitals reported their performance as average or above, with almost 5 percent of them ranking themselves as superior in comparison with other hospitals nationwide.

One limitation of the study is that it only included hospitals that participate in voluntary national stroke registries, the researchers note in the Journal of the American Heart Association. In addition, only one staff person at each hospital was interviewed for the study, making it possible the results would differ based on the individual participating.

Moreover, hospitals aren’t the only players in this race against the clock, Shah said. Patients need to call for an ambulance immediately when they notice any potential symptoms of a stroke.

“The common mistake is to delay seeking medical attention when first exhibiting symptoms,” Shah said.

How long it takes patients with ischemic stroke to receive thrombolysis is one of the best predictors of their outcomes, said Dr. Shazam Hussain, head of the stroke program at Cleveland Clinic in Ohio.

Less than half of patients in the U.S. are treated within the recommended door-to-needle time of 60 minutes, Hussain, who wasn’t involved in the study, said by email.

“This study sheds important light on one of the reasons why overall we are struggling meeting this target,” Hussain said. “Hospitals and practitioners feel they are performing well and may not be devoting the proper resources to fully analyze their data to see the true situation.”