CHICAGO - About one in three people in the United States will encounter some kind of mistake during a hospital stay, US researchers said on Thursday.

The finding, which is based on a new tool for measuring hospital errors, is about 10 times higher than estimates using older methods, suggesting much work remains in efforts to improve health quality.

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"Without doubt, we've seen improvements in health care over the past decade, and even pockets of excellence, but overall progress has been agonizingly slow," said Susan Dentzer, editor-in-chief of Health Affairs, which published several studies on a special issue on patient safety.

The special issue came 10 years after an influential Institute of Medicine report that found significant gaps in health quality.

"It's clear that we still have a great deal of work to do in order to achieve a health care system that is consistently high-quality -- that is, safe, effective, patient-centered, efficient, timely, and devoid of disparities based on race or ethnicity," Dentzer said.

Medical errors can range from bedsores to objects left in the body after surgery to life-threatening staph infections.

A study by David Classen of the University of Utah and colleagues compared a new quality yardstick developed at the Institute for Healthcare Improvement in Massachusetts, with two common older methods of detecting errors -- reports of errors voluntarily included in the medical record and an older method for assessing errors developed by the US Agency for Healthcare Research and Quality, or AHR.

"A key challenge has been agreeing on a yardstick for measuring the safety of care in hospitals," the researchers wrote.

To find the best yardstick, the team tested three methods of tracking errors on the same set of medical records from three different hospitals.

Among the 795 patient records reviewed, voluntary reporting detected four problems, the Agency for Healthcare Research's quality indicator found 35, and the Institute for Healthcare Improvement's tool detected 354 events -- 10 times more than AHR's method.

"Our findings indicate that two methods commonly used by most care delivery organizations and supported by policy makers to measure the safety of care ... fail to detect more than 90 percent of the adverse events that occur among hospitalized patients," the team wrote.

The findings suggest there may be many errors that go undetected.

In a separate study in the same issue, a team led by Jill Van Den Bos and colleagues at the Denver Health practice of the Milliman Inc consulting firm, used insurance claims to estimate the annual cost of medical errors that harm patients to be $17.1 billion in 2008 dollars.

They found that 10 types of errors accounted for more than two-thirds of the total cost, with the most common ones being pressure ulcers or bedsores, postoperative infections and persistent back pain following back surgery. The researchers recommended that those three types of errors receive top priority for intervention and improvement.