Association Between Hospital Case Burden and Mortality Risk Among Patients Hospitalized With COVID-19: A Retrospective Cohort Study of U.S. Hospitals

Patients with critical illness and/or respiratory failure tend to display better outcomes at hospitals that manage higher volumes of these patients. However, during a COVID-19 pandemic, this relationship may have been altered by the strain of higher patient volume on resources and personnel. A retrospective cohort study will be conducted using a large administrative database to understand the volume-outcome relationship among patients hospitalized with COVID-19. Understanding the COVID-19 volume-outcome relationship in U.S. hospitals may help identify the optimal care setting and transfer thresholds, and inform triage policies and risk-adjustment.

The Premier Healthcare Database (PHD), an all-payer administrative data repository covering approximately 20% of U.S. hospitalizations from 48 states will be queried for adult (18+ years) in-patient encounters that recorded diagnosis coding for COVID-19 March 1st through August 31st, 2020 at continuously-reporting hospitals. Hospitals with <15 COVID-19 encounters over the study period will be excluded from the primary cohort to preserve statistical reliability.

A surge index will be calculated for each hospital month that incorporates care-complexity-weighted COVID-19 case burden and will be normalized for nominal (2019) bed capacity and non-COVID-19 case burden and will represent the primary covariate. Hospital months will be stratified by the percentile of the surge index (e.g. <50th, 50-75th, 75-90th, 90-95th and ≥95th percentile).

The impact of COVID-19-case burden (surge index) on the risk-adjusted odds ratio (aOR) of mortality (or discharge to hospice) among overall COVID-19 cohorts will be determined using a hierarchical generalized linear mixed model controlling for case-mix, treatment-related, time-varying and other hospital-level factors. Variables will be selected based on their representing "baseline" status upon presentation to the hospital to minimize effect of mediators.

Variables for case mix adjustment: age, gender, race/ethnicity, comorbidity burden, insurance status, admission source and acuity, acute organ failure score and code status present-on-admission, use of pharmacologic agents (remdesivir, corticosteroids, hydroxychloroquine and azithromycin), severity of hypoxia (acute respiratory failure codes, need for non invasive positive pressure ventilation, need for invasive mechanical ventilation), vasopressor use within two days of admission respectively.