Standardised Mortality Ratio

Reviewed and revised 26 August 2015

OVERVIEW

• Standardised Mortality Ratio SMR is the ratio of the observed or actual hospital mortality and the predicted hospital mortality for a specified time period.
• Requires an estimate of predicted mortality rate using a scoring system (e.g. APACHE, ANZROD, SAPS, MPM…)
• Interpretation:
  • a value of 1 is considered normal or as expected
  • > 1 worse than expected, and
  • < 1 better than expected

USE

Used as a surrogate for good quality of care

• comparison of different ICUs
• monitor improvements or decline in ICU care over time

BENEFITS

• widely used
• quantitative
• better than a comparison of non-adjusted mortality data
• mortality is a hard endpoint that is clinically meaningful

LIMITATIONS

General

• acceptable deviations from SMR are not defined, therefore whether a unit significantly deviates from normal is unquantifiable
• needs to be considered in the context of case-mix and calibration
• inconsistencies and inaccuracies associated with data collection and scoring
• missing data limits inclusion of all patients
• original population used to calculate formulae possibly not generalisable
• model used for prediction may be inaccurate
• relies on mortality as a surrogate for quality care
• cost
• ideally samples should be very large
• does not account for post-ICU mortality or morbidity that may be related to ICU care

Comparison between different ICUs are limited by the following assumptions:

• that all pre-ICU care identical and that it has no impact on ICU or hospital outcome
• patients from different hospitals are all drawn from the same case-mix
• sample sizes are large enough to obey the laws of logistic regression
• data acquisition is flawless with respect to the rules of scoring systems and consistent between units

REASONS FOR A CHANGE IN SMR FOR AN ICU

Potential explanations:

• Need to ensure data entry is correct and accurate and consistent with prior practice (i.e. comparable)
• Issues like quantifying GCS accurately will have an impact on APACHE scores and consequently SMR. Quantification of GCS is a major source of inaccuracy in APACHE scores. Also source of admission and diagnosis;
• SMR reflects system wide performance rather than ICU performance alone, because based upon hospital mortality, not ICU mortality. Look at pre ICU and post ICU facilities in the hospital
• SMR affected by case-mix, so changes in case mix may account for increase in SMR and increased other hospital admissions
• Has there been increased practice variation and a deviation from clinical protocols in the ICU?
• Lead time bias (pre ICU care) has been shown to impact on SMR and this needs to be considered
• Any unusual events such as influenza epidemic or terrorist attack?
• Assess for changes in personnel, staffing levels, protocols and equipment
• New services introduced? (e.g. ECMO retrieval)