SummaryVirchow's triad describes three factors that contribute to the development of venous thrombosis: hypercoagulability, stasis and endothelial injury. Yet, extensive review of the historical literature casts doubt on the existence of a triad described by Virchow in the form it is currently quoted throughout contemporary medical literature. Certainly his work involved extensive study of venous thrombosis and pulmonary embolism, with these two terms being coined by Virchow, but a triad of factors relating to the development of venous thrombosis is elusive. Interestingly, Virchow only began to be routinely credited with this triad one hundred years after publication of his work on venous thrombosis. This acknowledgement coincided with the accumulation of experimental evidence for the role these factors play in thrombogenesis. Controversial as the origins of Virchow's triad might be, it is apt given his substantial contribution to our knowledge of venous thromboembolism, and the fact that the triad continues to be clinically relevant today that a triad pertaining to Virchow should remain.

The association of severe COVID-19 with an increased risk of VTE has resulted in specific guidelines for its prevention and management. The VTE risk appears highest in those with critical care admission. The need for post discharge thromboprophylaxis remains controversial and this is reflected in the conflicting recommendations of expert guidelines. Our local protocol provides thromboprophylaxis to COVID-19 patients during admission only. We report post-discharge VTE data from an ongoing quality improvement programme incorporating root cause analysis of hospital-associated VTE (HA-VTE). Following 1,877 hospital discharges associated with COVID-19, there were 9 episodes of HA-VTE diagnosed within 42 days, to give a post-discharge rate of 4.8 per 1000 discharges. Over 2019, following 18,159 discharges associated with a medical admission; there were 56 episodes of HA-VTE within 42 days (3.1 per 1000 discharges). The odds ratio for post-discharge HA-VTE associated with COVID-19 compared to 2019 was 1.6 (95% CI 0.77-3.1). Hospitalisation with COVID-19 does not appear to increase the risk of post-discharge HA-VTE compared to hospitalisation with other acute medical illness. Given the risk-benefit ratio of post discharge thromboprophylaxis remains uncertain, randomised controlled trials to evaluate the role of continuing thromboprophylaxis in patients with COVID-19 following hospital discharge are required.

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Background: Coronavirus disease 2019 (COVID-19) is characterised by dyspnoea and abnormal coagulation parameters, including raised D-dimer. Data suggests a high incidence of pulmonary embolism (PE) in ventilated patients with COVID-19. Objectives: To determine the incidence of PE in hospitalised patients with COVID-19 and the diagnostic yield of Computer Tomography Pulmonary Angiography (CTPA) for PE. We also examined the utility of D-dimer and conventional pre-test probability for diagnosis of PE in COVID-19. Patients/methods: Retrospective review of single-centre data of all CTPA studies in patients with suspected or confirmed COVID-19 identified from Electronic Patient Records (EPR). Results: There were 1477 patients admitted with COVID-19 and 214 CTPA scans performed, of which n = 180 (84%) were requested outside of critical care. The diagnostic yield for PE was 37%. The overall proportion of PE in patients with COVID-19 was 5.4%. The proportions with Wells score of ≥4 ('PE likely') was 33/134 (25%) without PE vs 20/80 (25%) with PE (P = 0.951). The median National Early Warning-2 (NEWS2) score (illness severity) was 5 (interquartile range [IQR] 3-9) in PE group vs 4 (IQR 2-7) in those without PE (P = 0.133). Ddimer was higher in PE (median 8000 ng/mL; IQR 4665-8000 ng/mL) than non-PE (2060 ng/mL, IQR 1210-4410 ng/mL, P < 0.001). In the 'low probability' group, D-dimer was higher (P < 0.001) in those with PE but had a limited role in excluding PE. Conclusions: Even outside of the critical care environment, PE in hospitalised patients with COVID-19 is common. Of note, approaching half of PE events were diagnosed on hospital admission. More data are needed to identify an optimal diagnostic pathway in patients with COVID-19. Randomised controlled trials of intensified thromboprophylaxis are urgently needed.

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