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Blog: Standardized mortality ratios may be a bad measure of hospital quality

Standardized mortality ratios are widely used to evaluate avoidable deaths in healthcare. But a new study from England questions the accuracy of the measure as a reflection of healthcare quality and advises against using the measure altogether.

Researchers conducted a retrospective review of 3,400 patient cases of death from 34 acute trusts (divisions of the National Health Service that manage hospital care) and found the proportion of avoidable deaths was 3.6%. They found no significant association between a hospital's standard mortality ratio and the proportion of avoidable deaths throughout the rest of the trust.

“The small proportion of deaths judged to be avoidable means that any metric based on mortality is unlikely to reflect the quality of a hospital,” concluded researchers from the London School of Hygiene & Tropical Medicine and the Imperial College London.

The lack of association between hospital- and trust-wide standardized mortality ratios “reflects methodological shortcomings in both metrics,” they wrote in findings published Tuesday in BJM, the journal of the British Medical Association.

Standardized mortality looks at the ratio of the number of observed deaths to the number expected in a given population. It is one of a dizzying array of metrics used to evaluate quality among U.S. hospitals as well. Variations in mortality rates may reflect that a hospital is struggling in other areas, such as coordination of care, patient-safety policies and staffing, according to the CMS.

Authors of the BMJ study concluded that the measure is “potentially misleading” and should be restricted to assessing the quality of care for conditions with high case fatality for which good quality clinical data exists.

Risk-adjusted mortality rates for specific conditions such as pneumonia, chronic obstructive pulmonary disease, heart attack and stroke are posted publicly in the U.S. on the CMS' Hospital Compare website, a consumer-facing page created to help consumers evaluate the quality of healthcare facilities. Those measures are adjusted for factors like patients' age and current and previous illnesses, which may increase the patient's risk of dying.

Still, the measure can cause confusion. The topic became a point of controversy after a recent CNN story investigating a seemingly high mortality rate among pediatric heart surgery patients at a Florida hospital. The dispute “highlights just how hard-to-parse health statistics can be,” wrote authors of a recap of that incident in the Columbia Journalism Review.


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