Question 30

What  is Standardised  Mortality Ratio?    Outline  the  limitations  of using  this  ratio to compare the performance of Intensive Care Units.

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College Answer

Standardised Mortality Ratio is defined as the observed mortality rate/expected mortality rate. Need to estimate expected mortality rate using a scoring system (eg. APACHE II or III, SAPS II or MPM). Better than comparison of non-adjusted mortality data.

The potential limitations of the system are multiple including: inconsistencies and inaccuracies associated with collection of data and scoring (eg. GCS, recording of parameters); problems of missing  data  limiting  inclusion  of  all  patients;  problems  of  patient  mix  not  adequately accounted for by the original population used for calculation of formulae (eg. transferred patients or delays before admission); small numbers of patients (increasing the error of the SMR estimate); accuracy of the prediction model; relying on mortality as a surrogate marker for quality of care; cost of use of proprietary system; etc.


Definition of the SMR

  • This is the ratio of the observed hospital mortality vs. predicted hospital mortality for a specified time period.
  • One can use this to compare hospitals and ICUs
  • One needs to first calculate the predicted hospital mortality using an illness severity scoring system.
  • An SMR of 1 means the mortality is as expected.
  • An SMR of < 1 is better than expected, and >1 is worse than expected.

Limitations of the SMR

  • Acceptable deviations from the SMR are not defined
  • Suffers from inaccuracies associated with data collection
  • SMR may be influenced by ICU admission and discharge practices (eg. discharging patients who are palliated, or admitting patients who are inevitably going to die).
  • Accuracy of the SMR as a quality assessment tool may be influenced by patients who have been predominantly cared for at another ICU, and who have been received as a transfer.
  • Mortality is not a surrogate for quality of care
  • The populations used to calculate the predicted hospital mortality are potentially non-representative (i.e. the population may also contains a number of dying critically ill patients, or it may contain an unusually large proportion of people in robust health).

Limitations of comparing ICUs with the SMR:

  • The SMR assumes all pre-ICU care is identical
  • Ignores differences in case mix
  • Sample sizes need to be large enough to obey the laws of logistic regression
  • Data is assumed to be flawless and complete


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Wolfe, Robert A. "The standardized mortality ratio revisited: improvements, innovations, and limitations.American Journal of Kidney Diseases 24.2 (1994): 290-297.


Gaffey, William R. "A critique of the standardized mortality ratio." Journal of Occupational and Environmental Medicine 18.3 (1976): 157-160.


Jones, Michael E., and Anthony J. Swerdlow. "Bias in the standardized mortality ratio when using general population rates to estimate expected number of deaths." American journal of epidemiology 148.10 (1998): 1012-1017.


van Gestel, Yvette RBM, et al. "The hospital standardized mortality ratio fallacy: a narrative review." Medical care 50.8 (2012): 662-667.


Combes, Alain, et al. "Adverse effect on a referral intensive care unit's performance of accepting patients transferred from another intensive care unit*."Critical care medicine 33.4 (2005): 705-710.