The marathon working shifts expected of first-year medical residents, called interns, are putting patients in danger, according to the first study to identify mistakes that injured patients. After five 24-hour-plus shifts a month, the study found that interns were seven times more likely to harm a patient through error than if they had not worked any long shifts, and four times more likely to make a mistake that resulted in a patient's death.

"These data suggest that fatigue contributes to tens of thousands of injuries to patients and thousands of deaths" annually, says Charles Czeisler of Harvard Medical School, who led the study. "Academic medicine is failing both doctors and patients."

The new findings are based on responses from 2,737 interns nationwide who voluntarily submitted monthly reports on their work hours and fatigue-related errors. The questionnaires, crafted by Czeisler and his colleagues, asked interns if they had made any harmful or fatal errors that they felt were the result of fatigue or sleep deprivation as well as if they had erred due to other causes.

Based on a total of 17,003 reports, the researchers found that working even a few prolonged shifts led to significantly more errors. After one to four extended shifts, interns were 3.5 times more likely to report that they had made a fatal error and were 8.7 times more likely to report they had inadvertently harmed a patient. Marathon shifts also made the interns more likely to doze off during surgery, lectures and while examining patients alone or with other doctors, the monthly reports showed.

More stringent guidelines adopted three years ago still allow residents to work up to nine 30-hour shifts per month, but earlier this year Czeisler's gropu found that interns routinely violated many of the new guidelines.

In absolute terms, 1 to 2 percent of the monthly reports included a harmful error, and 0.3 to 0.4 percent included a fatal error. Czeisler based his estimate of mistake-related deaths on the number of interns (at least 100,000) now practicing in the U.S. and the assumption that more experienced residents are erring at similar rates.

The study "exposes interns' extended shifts as a weakness in the design of the residency system," writes public health researcher Mariana Szklo-Coxe of the University of Wisconsin in an editorial that accompanies the new study, which was published online this week by PLoS Medicine.

To reduce medical errors, she recommends that hospitals or health officials require residents to take naps and work only 14-hour shifts, which studies have found to reduce errors and are closer to the limits imposed in European countries.