The influence of length of stay in the ICU on power of discrimination of a multipurpose severity score (SAPS)

Objective: To assess how the power of discrimination of a multipurpose severity score (Simplified Acute Physiology Score; SAPS) changes in relation to the length of stay (LOS) in the intensive care unit (ICU). Design: III order to compute the SAPS probability, a model derived from logistic regression was developed in a cohort of 8059 patients. Measures of calibration (goodness-of-fit statistics) and discrimination [receiver operating characteristic (ROC) curve and relative area under the curve (AUC)] were adopted in a developmental set (5389 patients) and a validation set (2670 patients), bell? randomly selected. Once the legit was developed and the model validated, the whole database (8059 patients) was again assembled. To evaluate the accuracy of first-day SAPS probability over time, area under the ROC curve was computed for each of the Initial 10 days of ICU care and for day 15. Setting: 24 Italian ICUs. Patients: A total of 8059 patients out of 10 065 consecutive admissions over a period of 3 years (1990-1992) were included in this study, Patients whose SAPS was not correctly compiled (n = 687), patients younger than IS years (II = 442), and patients whose LOS was less than 24 h (II = 877) were excluded from this analysis. Interventions: None, Measurements and results: The logistic model gave good results in terms of calibration and discrimination, both ill the developmental set (goodness-of-fit: chi(2) = 9.24, p = 0.32; AUC = 0.79 +/- 0.01) and in the validation, set (goodness-of-fit: chi(2) = 8.95, p = 0.537: AUC = 0.78 +/- 0.01). The AUC for the whole database showed a loss in discrimination closely related to LOS: 0.79 +/- 0.01 at a day 1 and 0.59 +/- 0.02 at day 15. Conclusion: The logistic model that we developed meets high standards for discrimination and calibration. However, SAPS loses its discriminative power over time; accuracy of prediction is maintained at an acceptable level only in patients who stay in the ICU no longer than 5 days, The stay in the ICU represents a complex variable, which is not predictable, that influences the performance of SAPS on the first day.