Not all U.S. stroke patients eligible for a clot-busting treatment actually receive it - and the odds of getting this therapy may depend on where they live, a large study finds.

Researchers found wide variation in treatment for ischemic stroke, which results from an obstruction in a blood vessel supplying the brain. Patients who are diagnosed quickly may have less disability afterward if they receive an intravenous therapy known as thrombolysis to dissolve clots.

Because this therapy generally must be completed within 4.5 hours of the onset of stroke symptoms, a major obstacle to treatment can be getting to the hospital fast enough, said lead study author Dr. James Burke, a neurologist at the University of Michigan and the VA Ann Arbor Health System.

“For a variety of reasons, only a minority of patients get to the hospital within the first couple hours of a stroke,” Burke said by email. Patients might not recognize symptoms or call 911 soon enough, and even when they do seek help quickly they might not end up at a hospital that’s equipped to provide thrombolysis, he added.

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, and slurred or impaired speech.

Within three hours of stroke symptoms starting, patients may be able to receive thrombolysis if they haven’t recently had a heart attack, stroke or head trauma and don’t have other medical complications that could make this treatment unsafe. Some patients under 80 may be able to receive treatment within 4.5 hours.

Burke and colleagues examined 844,241 hospital admissions for ischemic stroke from 2007 to 2010 among U.S. patients insured by Medicare, the government health insurance program for the elderly.

They sorted patients into 3,436 different hospital service areas based on home postal code to assess regional variation in thrombolysis treatment rates.

Patients were 78 years old on average. About 57 percent were women, and most were white. The majority had high blood pressure and many also had diabetes, high cholesterol or an irregular heart rhythm.

Overall, just 3.9 percent of these patients received thrombolysis, the researchers report in the journal Stroke. The treatment wasn’t given at all in 20 percent of regions, and it was more likely to occur in places with higher population density.

In the 20 regions with the highest rates of thrombolysis, roughly 10 to 14 percent of patients received the treatment.

After accounting for the number of strokes in each region, the proportion of patients receiving thrombolysis ranged from 2.2 percent in the bottom fifth of regions to 5.9 percent in the top fifth.

Older patients, women and minorities were less likely to receive the treatment. Regions with the lowest proportion of college graduates also had a smaller percentage of people treated with thrombolysis.

Not every patient with stroke should receive thrombolysis. One study in Cincinnati estimated that about 6 percent of stroke patients would be eligible, but the new findings of higher rates in the highest-performing regions suggest that more patients could benefit if they could be transported more quickly to hospitals where thrombolysis is available, the authors say.

By boosting use of the treatment in regions where it’s least likely to happen up to the level in places where the therapy is most common, researchers estimated that an additional 92,847 stroke patients might get thrombolysis, averting disability for 8,078 of them.

“Prompt recognition and reaction to warning signs and effective emergency service systems can minimize delays in pre-hospital dispatch, assessment and transport, and ultimately increase the number of stroke patients reaching the hospital and being prepared for thrombolytic therapy within the 4.5-hour time window,” Dr. Maurizio Paciaroni, a stroke specialist at the University of Perugia in Italy who wasn’t involved in the study, said by email.

The best outcomes are for patients who receive thrombolysis within the first hour after the blood vessel becomes blocked, said Dr. Brian Silver, director of the Comprehensive Stroke Center at Rhode Island Hospital and researcher at Brown University.

“When patients don’t receive this treatment, they are up to 50 percent less likely to have a better outcome,” Silver, who wasn’t involved with the study, said by email. “This means, for some, residual speech difficulties, paralysis, vision loss, cognitive impairment and depression.”