Patients
We included all patients ≥18 years old with confirmed COVID-19 admitted to one of the six participating hospitals in the Netherlands (1 academic hospital (Radboudumc, Nijmegen), 5 general hospitals (Bernhoven hospital, Uden; Sint Maartenskliniek, Nijmegen; Canisius Wilhelmina hospital, Nijmegen; Rijnstate hospital, Arnhem; Jeroen Bosch hospital, ‘s-Hertogenbosch)) between March 1 and May 31, 2020. The diagnosis COVID-19 was made with a real-time reverse transcriptase polymerase chain reaction (RT-PCR) positive for SARS-CoV-2 on a deep naso-oropharyngeal swab. In addition, patients with negative PCR but with clinical symptoms consistent with COVID-19 and a CT-scan of the chest showing a very high suspicion of typical pulmonary involvement of COVID-19 (COVID-19 reporting and data system score of 5 defined by the Dutch Radiology Society) were included.29 Patients were excluded when COVID-19 was not RT-PCR or radiographically confirmed, or when patients had insufficient clinical documentation because they were transferred to or from another hospital due to capacity constraints.
The index date was the day of hospital admission. Patients were followed until hospital discharge or death. Data on the occurrence of thrombotic events, length of hospital stay, ICU admission, type of oxygen ventilation, and mortality were obtained from the patients’ records (EPIC, EPIC Systems Corporation, Verona, Wisconsin, USA; HiX, ChipSoft, Amsterdam, The Netherlands; xCare EPD, NEXUS, Nieuwegein, The Netherlands) and recorded in our database using a standardized case report form in the good clinical practice (GCP)-compliant data management system Castor (Castor Electronic Data Collection, Amsterdam, the Netherlands). CT pulmonary angiograms (CTPAs) and Doppler ultrasonography were performed at the discretion of the treating clinician. Common indications for CTPA included high D-dimer levels and/or progressive hypoxemia. The study was carried out in the Netherlands in accordance with the applicable rules concerning the review of research ethics committees and informed consent. The Institutional Review Boards of the participating hospitals waived the need for informed consent due to the observational nature of this study.