Question 13

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.

Discussion

This question closely resembles Question 30 from the second paper of 2006.

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

Liddell, F. D. "Simple exact analysis of the standardised mortality ratio."Journal of Epidemiology and Community Health 38.1 (1984): 85-88.

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.