The causal effect of hospital length of stay on crucial patient out-comes such as readmissions or mortality is under-investigated and therefore unknown for the vast majority of the US population. Existing evidence stems from association studies that are unable to draw causal conclusions. This study leverages Medicare’s two-midnight (2MN) and three-day (3D) rules as two natural experiments to establish causal rela-tionships between hospital length of stay (LOS) and patient outcomes. Using a quasi-experimental regression discontinuity design with data from a large US hospital, we find that the 2MN rule increases LOS by 0.10 days and the 3D rule by 0.21 days, confirming the validity of these rules as instruments for causal inference. However, despite these increases in LOS, there are no significant effects on 90-day mortality or 30-day readmission rates. These findings suggest that while the 2MN and 3D rules effectively extend hospital stays, they do not im-prove patient-related outcomes, indicating an inefficient use of hospital resources.

The authors have declared no competing interest.

National Institute of Allergy and Infectious Diseases New Innovator Award, DP2AI171011

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Medical Information Mart for Intensive Care IV (MIMIC-IV) is a deidentified database comprising health-related data from patients admitted to the Beth Israel Deaconess Medical Center (BIDMC) in Boston, Massachusetts. The creation and dissemination of MIMIC-IV were conducted with strict adherence to ethical standards to ensure patient privacy and compliance with regulatory requirements. Ethical Oversight and Approval: The collection of patient information and the development of MIMIC-IV as a research resource underwent thorough review by the Institutional Review Board (IRB) at BIDMC. The IRB granted a waiver of informed consent and approved the data-sharing initiative, recognizing that the deidentification processes applied to the dataset sufficiently protected patient privacy.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

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The Medical Information Mart for Intensive Care IV (MIMIC-IV) is a publicly accessible, deidentified database encompassing detailed health-related data from patients admitted to the intensive care units (ICUs) and emergency departments of the Beth Israel Deaconess Medical Center in Boston, Massachusetts. The database is designed to support a wide array of research studies and educational initiatives, thereby reducing barriers to conducting clinical research.

https://physionet.org/content/mimiciv/2.2/