Hypercoagulability in COVID-19 has been attributed to immunothrombosis, a process that involves the formation of neutrophils extracellular traps (NETs). The moment of the COVID-19 evolution in which immunothrombosis mechanisms are triggered is not established. Aim: To describe the kinetics of NETs release during COVID-19 hospitalization associating with thrombosis and death.

We quantified citrullinated H3 and inflammatory cytokines (TNF-α, IL-6), markers of NETs release, on 4 time points during COVID-19 hospitalization (admission, day 4, day 8 and last day) between May and July 2020. The association between changes in these markers levels and clinical outcomes was determined.

101 patients were included, the median days in-hospital were 15, 62% were men, 27% were obese, 43% were diabetic, 54% were hypertensive, 59% were critically ill, 11% had a thrombotic event and 21% died. IL-6 levels were high on admission in survivors (median 25.32, IQR 24.19-28.15) and non-survivors (median 24.19, IQR 12.51-27.19), but gradually decreased on day 4 (median 12.07, IQR 6.32-17.81), day 8 (median 9.34, IQR 5.18-17.59) and last day (median 8.64, IQR 4.81-14.89) in survivors. TNF-α levels remained 2 times higher in non-survivors: admission (median 1.60, IQR 0.64-2.26), day 4 (median 1.78, IQR 1.02-2.60), day 8 (median 1.65, IQR 0.93-2.5), last day (median 2.41, IQR 1.31-4.06); than in survivors: admission (median 0.81, IQR 0.52-1.26), day 4 (median 0.84, IQR 0.44-1.16), day 8 (median 0.72, IQR 0.44-1.24), last day (median 0.69, IQR 0.4-1.14). CitH3 levels were similar between non-survivors at the beginning of hospitalization: admission (median 1.03, IQR 0.43-4.34), day 4 (median 1.1, IQR 0.65-3.45); as for survivors: admission (median 1.20, IQR 0.45-2.60), day 4 (median 1.27, IQR 0.64-3.29). On day 8, citH3 increased by 3-fold (median 3.80, IQR 1.98-10.15) in non-survivors and 2-fold (median 2.60, IQR 1.22-5.01) in survivors. While IL-6 and TNF-α levels were similar between patients with and without thrombosis, citH3 levels increased shortly on day 4, before the occurrence of a thrombotic event: admission (median 1.64, IQR 0.44-4.14), day 4 (median 3.21, IQR 2.57-9.31); but it didn't change on non-thrombotic event patients: admission (median 1.05, IQR 0.44-2.50), day 4 (median 1.06, IQR 0.58-2.95).

Markers of inflammation and immunothrombosis were associated with poor outcomes in COVID-19; however, these disorders were detected in different moments during COVID-19 course. While an increased inflammatory response was observed since the beginning of hospitalization, markers of immunothrombosis arose latter during the course of the disease. Acknowledgment of the time-course of immunothrombosis development in COVID-19 is important for planning therapeutic strategies against this pathological process.