While crude mortality rates are important, it is very hard to use this information to compare and contrast what’s happening between hospitals. This is because every hospital is different, both in the treatments and operations that it offers and the make-up of its local population.

A hospital that carries out higher-risk operations, such as organ transplants or sees more patients that are elderly and/or come from areas of greater poverty, will have a crude mortality rate that is very different to one that doesn’t provide such higher-risk operations and/or whose local population is generally younger and more affluent.

This is why statisticians interested in comparing mortality rates between hospitals sought to find a new statistical way to allow them to do just that. The one now used most commonly is called the hospital standardised mortality ratio – or HSMR for short – which is published nationally by Dr Foster Intelligence.

The HSMR scoring system works by taking a hospital’s crude mortality rate and adjusting it for a variety of factors – population size, age profile, level of poverty, range of treatments and operations provided, etc. The idea is that by taking these factors into account for each hospital, it is possible to calculate two scores – the mortality rate that would be expected for any given hospital and its actual observed rate. It is the difference between these two rates that is important when it comes to HSMR.

Nationally, the expected HSMR score for hospitals is set as being 100. It is important to remember that this figure does not represent deaths or percentages – it is just a baseline number used to compare performance. The Trust’s published HSMR scores for the last few years are set out in the chart below.

Data source – Dr Foster

When thinking about HSMR scores, it’s tempting to view these figures simplistically. For example a hospital with a score of, say, 115 has 15% more deaths than average, while one with a score of 95% has 5% less deaths on average. This is not the case with HSMR scores as they are not percentages, rather an indication to help clinicians and managers understand how their services are performing.

Indeed there are several reasons why any individual HSMR score needs to be treated with caution, namely:

  1. The quality of the clinical coding – every clinical procedure undertaken in the NHS has its own unique code and unless these are used properly on our computer records, this can have a direct effect on the resulting HSMR score.
  2. Compared to other parts of the country, Hertfordshire has fewer hospice beds and community-based services that help people be with their families and loved ones when they die. As a result, we have more people who die in our hospitals due to a lack of appropriate community-based support.  Again, this can affect our HSMR score.
  3. Clinical quality issues – while these have not been a significant issue for the Trust, they are understandably, and rightly, what most people get concerned about. Running a service where more patients may be dying than would be expected is a key clinical quality issue to which the Trust pays particular attention. A rising HSMR for a particular clinical procedure is an early warning indicator that something might not be right.  In most cases it proves to be caused by a coding or other non-clinical issues. But sometimes, it can be a pointer to something more serious. Early investigation allows the Trust to take appropriate and speedy corrective action.
  4. The complexity of the data being measured, combined with natural random variation, mean that HSMR scores are never absolute figures. Indeed, the experts behind the HSMR system suggest that any individual score could vary by as much as +/- 7%. So, statically speaking, an NHS trust with a HMSR score of 94 could well have an identical performance to one with a score of 106 and vice versa.

So what does this performance tell us?

First of all, we’re no longer dealing with actual deaths, but rather whether or not what’s recorded against an individual hospital looks to be above or below average. And the important word here is looks. Why?  Because experts stress that when using the HSMR scoring system care must be taken regarding how it should be used and interpreted.

For example, scores well above 100 suggest that there may be a need to investigate whether or not there is an underlying clinical problem that needs to be addressed. This does not mean that people can, or should, assume that a clinical quality of care issue exists. It could be that the data on which the calculation was based wasn’t as accurate as it should have been. But there again, it could point to a specific clinical issue that needs attention.

Until investigated thoroughly, it is often impossible to tell what the cause is of a higher or lower than expected HSMR score. 

Working through the Trust’s scores

It is possible to shed some light on what might be going on behind this important statistic by looking at the Trust’s annual HSMR performance for the last few years. Until 2017/18, HSMR had remained relative steady and consistently below the 100 average mark. The rise in 2017/18 coincides with the introduction of our new patient administration system. The introduction of new IT systems are generally accepted to have a short term negative impact on HSMR, due to there being a temporary reduction in the recording of some information affecting the calculation of this score. HSMR for the current year is on course to fall back in line with pre 2017/18 levels.