Mortality rates are an unreliable metric for assessing hospital quality, study finds

Is quality in the eye of the beholder?

Researchers at Harvard Medical School and Massachusetts General Hospital have found wide disparities among four common measures of hospital-wide mortality rates, with competing methods yielding both higher- and lower-than-expected rates for the same Massachusetts hospitals during the same year.

The findings, published Dec. 23 in a special article in the New England Journal of Medicine, stoke a simmering debate over the value of hospital-wide mortality rates as a yardstick for health care quality. The measure, which compares a hospital's actual patient death rate to statistical predictions, is reported publicly in countries including England, Canada and Denmark, but some hospitals and policy experts have questioned its value due to the complexity and variability of diagnoses.

"It's troubling that four different methods for calculating hospital mortality rates as a measure of quality should yield such different results," said lead author David M. Shahian, HMS professor of surgery at Massachusetts General Hospital. "Measurement theory—not to mention plain common sense—suggests there is a problem."

The potential of performance evaluation to improve both the quality and the cost of health care has fueled interest in provider "report cards," including mandates by state and federal law.

In 2008, the Massachusetts Division of Health Care Finance and Policy engaged researchers Shahian; Lisa I. Iezzoni, HMS professor of medicine at Mass General; and Sharon-Lise T. Normand, HMS professor of health care policy (biostatistics) and professor in the Department of Biostatistics at Harvard School of Public Health, to evaluate four vendor-created measures of hospital-wide mortality. The state was looking for a means to measure hospital quality for the public report cards mandated under its 2006 health care reform law.

The researchers compared four measures of hospital-wide mortality provided by commercial vendors. These vendors all believed that their hospital-wide mortality measures were an accurate reflection of hospital quality. Each vendor received identical data—three years of patient discharge data from all 83 general acute care hospitals in Massachusetts, representing 2.5 million discharges—and used the data to calculate each hospital's mortality rate. The researchers then compared results.

"The results of the horse race are that it's really not clear who won," said Iezzoni, who also is the director of the Mongan Institute for Health Policy at Mass General. "The problem is that we were measuring the success of each of these measures against a gold standard we simply do not have: an objective measure of hospital quality."

Without that gold standard, the researchers were left to compare vendor tools with one another. Their conclusion: Methods and results varied widely. For example, every tool excluded some discharges from its calculations based on the details of each. But where one tool excluded 5 percent of all discharges, another excluded 72 percent.