Patients with critical COVID-19 are hyper-coagulable and optimal thromboprophylaxis treatment differs with stage and severity. The most commonly used drug for thromboprophylaxis in the intensive care unit (ICU) is low-molecular-weight heparin (LMWH). In contrast to unfractionated heparins, the effects of LMWH usually do not require monitoring. Exceptions from this are when elimination of LMWH is impaired, extremes in age and weight, to identify deviations from predicted pharmacokinetics, and if there is an unexpected clinical response. The unexpected high incidence of thromboembolic complications among patients with critical COVID-19 compared to critically ill non-COVID-19 patients could motivate monitoring. The activity of LMWH is monitored by quantifying the presence of anti-Factor Xa (aFXa). The aim of this study is to investigate if the level and the monitoring frequency of aFXa is associated to mortality, thrombosis and bleeding in patients with critical COVID-19 treated with LMWH and therefore could be used as a potential tool to guide LMWH-treatment.

Inclusion Criteria:

• Patients with critical COVID-19 and respiratory failure
• Positive SARS-CoV-2 test
• LMWH-effect monitored by aFXa, as a trough or peak value, after at least four doses at a twice daily regime

Exclusion Criteria:

• Diagnosed thromboembolism or major bleeding during their COVID-illness prior to ICU-admission or as a reason or contributing factor for the ICU-admission.
• Transferred to or from an ICU in other hospital/region making data collection regarding baseline data/outcomes not possible

All patients with critical COVID-19 and respiratory failure admitted to ICU at Södersjukhuset, Karolinska Sjukhuset and Skånes Universitetssjukhus, in Sweden, between March 1 2020 and May 31 2021 are eligible.

Patients will be included on their first admission to the ICU if admitted more than once. First admission can be replaced by the second one if the first one is considered very short/insignificant compared to the second one. If a patient is transferred from one ICU to another this will be considered the same admission but they will be grouped according to their initial ICU.