External validation of the Simplified Mortality Score for the Intensive Care Unit (SMS-ICU)

The Simplified Mortality Score for the Intensive Care Unit (SMS-ICU) is a clinical prediction model, which estimates the risk of 90-day mortality in acutely ill adult ICU patients using 7 readily available variables. We aimed to externally validate the SMS-ICU and compare its discrimination with existing prediction models used with 90-day mortality as the outcome.

We externally validated the SMS-ICU using data from 3282 patients included in the Stress Ulcer Prophylaxis in the Intensive Care Unit trial, which randomised acutely ill adult ICU patients with risk factors for gastrointestinal bleeding to prophylactic pantoprazole or placebo in 33 ICUs in Europe. We assessed discrimination, calibration and overall performance of the SMS-ICU and compared discrimination with the commonly used and more complex SAPS II and SOFA scores.

Mortality at day 90 was 30.7%. The discrimination (area under the receiver operating characteristic curve) for the SMS-ICU was 0.67 (95% CI: 0.65-0.69), as compared with 0.68 (95% CI: 0.66-0.70, P = 0.35) for SAPS II and 0.63 (95% CI: 0.61-0.65, P < 0.001) for the SOFA score. Calibration (intercept and slope) was 0.001 and 0.786, respectively, and Nagelkerke's R² (overall performance) was 0.06. The proportions of missing data for the SMS-ICU, SAPS II and SOFA scores were 0.2%, 8.5% and 6.8%, respectively.

Discrimination for 90-day mortality of the SMS-ICU in this cohort was poor, but similar to SAPS II and better than that of the SOFA score with markedly less missing data.