External validity of a mortality prediction model in patients after open abdominal aortic aneurysm repair using multi-level methodology

Objectives. Evaluation of the prognostic ability of the APACHE-AAA model in an independent group of post-operative (open) Abdominal Aortic Aneurysm (AAA) patients. Methods. The model was applied to predict in-hospital mortality in 541 patients (325 elective and 216 emergencies; 489 from Oxford; 52 from Lewisham). Multi-level modelling was used to adjust for both the local structure and process of care and patient case-mix. Model performance was assessed using goodness-of-fit and subgroup analyses. Results. The model's predictive ability to discriminate between dead and alive patients was very good (ROC area = 0.84). The model achieved a good fit across all strata of risk (Hosmer-Lemeshow C-test (8, N = 476) = 7.777, p = 0.456) and in all subgroups. The model was able to rank the ICUs according to their performance independently of the patient case-mix. Conclusion. The APACHE-AAA model accurately predicted in-hospital mortality in a population of patients independent of the one used to develop it, confirming its validity. The multi-level methodology employed has shown that patient outcome is not only a function of the patient case-mix but instead predictive models should also adjust for the individual hospital-related factors (structure and process of care). (C) 2007 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved.