Study subjects and design

This was a retrospective, observational study at a tertiary academic medical center (NewYork-Presbyterian/Weill Cornell Medical Center, New York, NY, USA). We included adult patients with positive SARS-CoV-2 testing by RT-PCR who were hospitalized with a COVID-19-related illness from March 3, 2020 to May 15, 2020. For Cohort 1, we excluded hospital transfers, prior hospitalization within 30 days, patients with chronic kidney disease (defined as baseline serum creatinine ≥ 2.0 mg/dL or presence of end-stage renal disease), and any extra-pulmonary infection without an isolated pathogen (Fig. 1A). For Cohort 2, we excluded hospital transfers, prior hospitalization within 30 days, patients with ‘do not resuscitate ± do not intubate (DNR ± DNI)’ status and intensive care unit (ICU) admission, microbiologically confirmed infections, or any extra-pulmonary infections without an isolated pathogen (Fig. 1B).

Study population. PCT pro-calcitonin, DNR do not resuscitate, DNI do no intubate, ICU intensive care unit

The study was approved by the Institutional Review Board of Weill Cornell Medical College. Informed consent was waived, and no animals were included in the study.For Cohort 1, PCT levels were compared between absent/low-suspicion and proven bacterial co-infection groups as defined below and stratified by admission to ICUs. For Cohort 2, clinical outcomes were compared between patients given antibiotics upon presentation and those not given antibiotics stratified by PCT ≥ 0.25 µg/L, a cutoff most commonly adopted in previous PCT studies among non-ICU population [2–4].