Patients admitted to the hospital on weekends are more likely to get a preventable illness or injury during their stay than people admitted during the week, a large U.S. study finds.

Even after adjusting for patient characteristics, including the severity of the condition that brought them to the hospital, weekend admission was still linked with more than a 20 percent increased likelihood of hospital-acquired conditions when compared to weekday admissions, lead author Dr. Frank Attenello, a researcher at the University of Southern California, said by email.

Attenello and colleagues analyzed data from more than 350 million admissions from 2002 to 2010 and found that 16.7 million of these stays, or about 5 percent, resulted in at least one avoidable hospital-acquired condition.

Falls were the most common complication, occurring in 14 million admissions and accounting for 85 percent of all hospital-acquired conditions. Pressure sores and catheter-associated urinary tract infections were also common.

Even though most admissions - 81 percent - were on weekdays, preventable complications were more common on weekends. Hospital-acquired conditions occurred in 5.7 percent of weekend admissions, compared to 3.7 percent in people admitted on weekdays.

"This increased hospital-acquired condition rate is significant because we found presence of at least one hospital-acquired condition to be associated with an 83 percent likelihood of increased healthcare cost and a 38 percent increase in the likelihood of a prolonged hospital stay," Attenello said.

The study has some limitations, including its reliance on insurance billing codes, which don't always reflect all of the conditions treated, the authors note. It also counted people admitted on Sundays for elective surgery the following morning as weekend admissions.

With billing codes, it's possible that the study underestimated the number of catheter-associated urinary tract infections and central-line associated bloodstream infections, said Dr. Kumar Dharmarajan, a researcher at Yale University School of Medicine in New Haven, Connecticut who wrote an editorial accompanying the study in the British Medical Journal.

At the same time, conditions such as pressure sores might be overestimated if they were present at the time of admission.

And, because the study doesn't note which day of the week patients experienced a preventable illness or injury, it's difficult to say how conditions on the weekends might contribute to problems, Dharmarajan told Reuters Health in an email.

"It is premature to conclude that factors intrinsic to the hospital on weekends like reduced staffing or the increased number of covering providers are primarily responsible for the greater number of hospital-acquired conditions among weekend admissions," Dharmarajan said. "It is not clear that the weekend is a less safe time for patients."

It's also possible that the study found more complications on weekends because patients admitted then are sicker and in need of more urgent treatment, said Sarah Krein, a researcher at the Ann Arbor VA Center for Clinical Management Research at the University of Michigan.

"It isn't clear whether more aggressive prevention efforts are needed on weekends or whether patients admitted on weekends should be viewed as high-risk for hospital-acquired complications thus warranting extra vigilance throughout the course of their hospital stay," Krein, who wasn't involved in the study, said by email.

Still, the study highlights the need for better prevention, including efforts to avoid the most common complications, such as falls and infections, as well as initiatives at the administrative level that can improve staffing or organizational factors contributing to complications, said Enrique Castro-Sanchez, a researcher at the Center for Infection Prevention and Management at Imperial College, London.

"In light of the enormous number of patient discharges analyzed by the authors in the study period, any reduction in the rate of preventable healthcare conditions is likely to improve the experience of care of millions of patients," Castro-Sanchez, who wasn't involved in the study, said by email.