Looking for Out-of-Distribution Environments in Critical Care: A case study with the eICU Database

Generalizing to new populations and domains in machine learning is still an open problem which has seen increased interest recently. In particular, clinical models show a significant performance drop when tested in settings not seen during training, e.g., new hospitals or population demographics. Recently proposed models for domain generalisation promise to alleviate this problem by learning invariant characteristics across environments, however, there is still scepticism about whether they improve over traditional training. In this work, we take a principled approach to identifying Out of Distribution (OoD) environments, motivated by the problem of cross-hospital generalization in critical care. We propose model-based and heuristic approaches to identify OoD environments and systematically compare models with different levels of held-out information. In particular, based on the assumption that models with access to OoD data should outperform other models, we train models across a range of experimental setups that include leave-one-hospital-out training and cross-sectional feature splits. We find that access to OoD data does not translate to increased performance, pointing to inherent limitations in defining potential OoD environments in the eICU Database potentially due to data harmonisation and sampling. Echoing similar results with other popular clinical benchmarks in the literature, new approaches are required to evaluate robust models in critical care.