Performance of the mortality probability models in assessing severity of illness during the first week in the intensive care unit

Objective: To extend the Mortality Probability Models (MPM) II severity system to time periods between 4 and 7 days after admission to the intensive care unit (ICU). Design: Prospective inception cohort. Setting: Fifteen adult medical and surgical ICUs in Spain. Patients: A total of 1,441 patients aged ≥ 18 yrs consecutively admitted from April 1, 1995 through July 31, 1995. Interventions: Prospective data collection during the stay of the patient in the ICU. Data collected included demographic information, length-of-stay and vital status at both ICU and hospital discharge, as well as all variables necessary for computing the MPM II system at admission and during the first 7 days of stay in the ICU Measurements and Main Results: Calibration and discrimination of the four existing MPM II models (MPM0, MPM24, MPM48, and MPM72) were assessed in the study database. The MPM II system overestimated the mortality of patients with probabilities of death ≥ 0.4. The MPM24 model was customized. Models for time periods between 48 hrs and 7 days (MPM48 to MPM(d7)) were obtained using the same strategy that was used to develop the original MPM48 and the MPM72 models. The variable coefficients of the MPM24 model were kept fixed and the constant terms of the MPM48 to MPM(d7) models were estimated by logistic regression. The constant term stabilized after the fourth day of admission and it was similar to the constant term of the MPM72 model. The customized MPM72 performed very well for days 4 to 7 after admission to the ICU. Conclusions: If the patient's condition stays the same day after day, the probability of dying in the hospital increases until 72 hrs, and then stabilizes. A severity measure that performs well at 72 hrs can be a useful tool for measuring severity at later time periods.