"Excess" deaths in NHS hospitals

9 August 2013
  • The government investigates hospitals with elevated mortality rates, but - contrary to media reports - "excess" deaths aren't necessarily a result of poor care.
  • There are two main ways of measuring mortality rates at hospitals and these indicators produce different results.
  • There will be always be some hospitals with a mortality rate that's above 'average' because that's the nature of averages.

Numerous investigations into "excess" deaths

There have been several official inquiries into reports of "excess" deaths at England's hospitals. The first of these was the review into child heart surgery deaths at Bristol Royal Infirmary. More recently, the Francis report examined the higher than average death rates at Mid Staffs hospitals. This prompted Sir Bruce Keogh's investigation of  14 other NHS trusts.

Excess doesn't mean "avoidable" or "needless"

Mortality rates are useful smoke signals that can warn of potential problems at a hospital. However, the number of "excess" deaths, as measured by either the HSMR or the SHMI, is not proof of how many patients have died from poor treatment.

As Robert Francis QC, who led the inquiry into the Mid Staffs scandal, has explained:

 "...it is in my view misleading and a potential misuse of the figures to extrapolate from them a conclusion that any particular number, or range of numbers of deaths were caused or contributed to by inadequate care." 

The only way to judge whether or not a death has been caused by negligence is to review the notes for an individual's case. However, most of the media has run with the idea that an excess death is an avoidable one. 

Mortality rate measures don't always tell the full story

There are two measures for a hospital's mortality rate: the Standardised Hospital Mortality Indicator (SHMI) and the Hospital Standardised Mortality Ratio (HSMR). Each indicator uses a different methodology.

The main difference is this: while the SHMI looks at each hospital in isolation, the HSMR compares one hospital with all the others in the country.

A hospital's mortality rate is judged against a baseline of 100. An SHMI or HMSR of 125 means the number of deaths is 25 per cent higher than we would expect (conversely, a score of 75 would suggest the mortality rate is 25% lower).

- The SHMI compares the number of actual deaths to the number of deaths that would be expected at a particular hospital, either during a patient's stay or within 30 days of their discharge. The expected number of deaths is calculated by looking at the case mix of that individual hospital. 

-The HSMR calculates the number of deaths likely to occur at an average NHS hospital. By definition, this means about half of all hospitals will be above average, and about half will be below. Having said this, many of these mortality rates will be in the range of chance variation - in other words, it wouldn't take much for a hospital to be the other side of the average line.

Unsurprisingly, the SHMI and HSMR often produce different results. A hospital that's an "outlier" on the HSMR will not necessarily be flagged by the SHMI, which is usually more precise. After a review, the government is planning to adopt the SHMI as the sole mortality rate indicator for NHS hospitals.


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