(Bloomberg) -- Privatizing England’s National Health Service may have led to a decline in the quality of patient care, according to research that links increased outsourcing to for-profit providers with higher death rates in the public system. 

Legislation passed in 2012, allowing greater outsourcing of health services is associated with a rise in deaths that should have been avoidable, a group of University of Oxford researchers said Wednesday, in a study published in The Lancet Public Health journal. 

Using a database of outsourcing contracts, the authors found that a 1% annual increase of outsourcing of services to for-profit providers corresponded with an increase in “treatable mortality” of 0.38%. Overall, changes to outsourcing since 2014 were linked to an additional 557 deaths across 173 clinical commissioning groups in England.

The authors acknowledged limitations with their study, saying they can’t rule out that other factors could explain a rise in mortality. It was also difficult to precisely measure outsourcing that took place before the new policy came into effect. Additionally, outsourcing files analyzed in the study did not contain details of specific services provided by suppliers, they said. 

“While more research is needed to determine the precise causes of the declining quality of health-care in England, our findings suggest that further increases in NHS privatisation would be a mistake,” said Aaron Reeves, a professor in the department of social policy and intervention at Oxford. 

One potential explanation for the findings is that private providers receiving NHS contracts could be delivering lower quality care, the researchers said. Another, they said, could be that for-profit providers selectively treat profitable cases, concentrating patients with more difficult treatments in public settings. 

“Despite the inability to establish a causal mechanism, the study adds to the evidence base examining the impacts of privatization on the health system in England,” said Andrew Street, a professor in health economics at the London School of Economics and Political Science in a statement. He was not involved in the study.

“For-profit providers may secure greater profit for their shareholders by being innovative and quick to adopt the latest technologies. But a faster route to making profits might be to compromise quality,” Street said. The way forward is to enforce strict quality standards through contracts and to make sure the performance is monitored, he added. 

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