“Among adults undergoing surgical procedures, the odds of adverse postoperative outcomes, including death, readmission, and complications in the short and long term, were increased by 5 percent for patients undergoing surgery immediately preceding the weekend,” reports the paper, published this week in JAMA Network Open. “This weekend effect was seen across multiple subspecialties, in particular among patients undergoing elective operations.”

It’s a conclusion drawn from a huge dataset: the study followed nearly 430,000 patients in Ontario, Canada, undergoing any of 25 common surgical procedures on either a Monday or a Friday between 2007 and 2019. 

But it wasn’t just the sample size that made the difference – in an effort to untangle potentially conflicting results from previous investigations, the team went deep as well as wide. “We studied both elective and emergent procedures, analyzed the specific contribution of physician factors (such as age, experience etc.), and looked at short-term (30 day), intermediate-term (90 day), and long-term (1 year) outcomes on the weekend effect,” Vatsala Mundra, a clinical research fellow at Houston Methodist Urology and one of the authors of the study, told Gizmodo this Tuesday.

The result – that surgery on a Friday does seem to be more risky than an alternative day – is one that has been seen in many previous studies. Some of those produced quite striking differences in outcome: in the Netherlands, for example, a 2015 study found a 20 percent increase in 30-day mortality for patients going under the knife on a Friday rather than a Monday; a 2018 meta-analysis of more than 8 million patients worldwide found an even higher risk, finding Friday surgeries to be 24 percent more deadly than Mondays.

Those figures might seem dramatic compared to this week’s 5 percent increase – and of course, differences in methodology and sample selection will play into that – but the mere fact that so many studies have drawn the same conclusion is testament to the effect’s existence, the team suggest.

All of which raises an important question: why?

According to the researchers, the answer may be surprisingly straightforward. “In a study quantifying workforce staffing, numbers for all staff members (doctors, nurses, and other clinical staff) were shown to sharply decline over the weekend,” the study points out. “This may contribute to the observed weekend effect via a failure-to-rescue mechanism – that is, a short-staffed weekend team may be less likely to detect and act on acute complications early in their evolution, leading to a higher complication rate for patients.”

But the issue isn’t only how many staff are there – which staff is also important. By adjusting their analysis to account for physician characteristics, the team found an interesting confounding variable: Fridays often see more junior surgeons, with less experience under their belt, in the operating room compared to Mondays, with “diminished access to more senior colleagues or consultants” heading into the weekend, the study notes. Even those most eminently qualified of the weekend healthcare workers are likely working with a disadvantage, the authors point out, since “weekend teams may be less familiar with the patients than the weekday team previously managing care.”

Add in a reduced availability of certain tests, tools, and interventions that would normally be available during the week, and the fact that patients are less likely to be discharged on Saturday or Sunday, and it’s easy to see where the “weekend effect” comes from. Two days of overstretched, less confident staff, with a reduced ability to diagnose and treat patients, and less familiar with the cases and patients they’re in charge of? It’s almost amazing the bump in risk isn’t larger. 

Still, the team aren’t done figuring out precisely what factors are to blame – and, in turn, what can be done to mitigate them. 

“Our findings underscore the need for a critical examination of current surgical scheduling practices and resource allocation,” the authors write. “It is important for health care systems to assess how this phenomenon may impact their practices to ensure that patients receive excellent care irrespective of the day.”

The study is published in JAMA Network Open.