During plasma coagulation, soluble fibrin is generated. D-Dimers are released as characteristic degeneration products of cross-linked fibrin. Low D-Dimer concentrations can be used to exclude venous thrombic events (VTE) such as deep vein thrombosis of the leg (DVT) and pulmonary embolism (PE). On the other hand, increased D-Dimer levels indicate the activation of coagulation and following fibrinolytic processes demanding for further diagnostic and therapeutic approaches.
The annual incidence of VTE is approximately one in 1000 adults. In addition to inherited risk factors for venous thromboembolism, numerous acquired risk factors for venous thromboembolism exist, including infections and inflammatory diseases. Even before the outbreak of the coronavirus pandemic, increased D-Dimer levels were reported in influenza like infections due to the activation of coagulation by respiratory viruses. 
The recently published IFCC Guidelines on COVID-19 strongly suggest D-Dimer testing in patients with COVID-19, since studies on SARS-CoV-2 revealed a high correlation between severity and outcome of COVID-19 in patients with increased D-Dimer levels. [2–6]
Despite from activated coagulation in COVID-19, even disseminated intravascular coagulation (DIC) can develop in severe cases. [3, 4] Marietta et al. related increased D-Dimer levels as a predictor of developing acute respiratory distress in COVID-19, mentioning the probability of micro pulmonary embolism especially in severe forms of COVID-19. 
Recently performed autopsies on 12 patients with COVID-19 showed deep vein thrombosis in seven patients whereof four had pulmonary embolism (PE) as main pathological findings. In two of these cases PE was directly stated in the clinical cause of death. 
- Van Wissen M, Keller TT, Van Gorp ECM, Gerdes VEA, Meijers JCM, Van Doornum GJJ, et al. Acute respiratory tract infection leads to procoagulant changes in human subjects: Letters to the Editor. Journal of Thrombosis and Haemostasis. 2011;9:1432–4.
- IFCC Information Guide on COVID-19 [Internet]. IFCC Information Guide on COVID-19. 2020. Available from: https://www.ifcc.org/ifcc-news/2020-03-26-ifcc-information-guide-on-covid-19/
- Tang N, Li D, Wang X, Sun Z. Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia. J Thromb Haemost. 2020;18:844–7.
- Lippi G, Favaloro EJ. D-dimer is Associated with Severity of Coronavirus Disease 2019: A Pooled Analysis. Thromb Haemost. 2020;s-0040-1709650.
- Lippi G, Plebani M, Henry BM. Thrombocytopenia is associated with severe coronavirus disease 2019 (COVID-19) infections: A meta-analysis. Clinica Chimica Acta. 2020;506:145–8.
- Zhou F, Yu T, Du R, Fan G, Liu Y, Liu Z, et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study. The Lancet. 2020;395:1054–62.
- Marietta M, Ageno W, Artoni A, De Candia E, Gresele P, Marchetti M, et al. COVID-19 and haemostasis: a position paper from Italian Society on Thrombosis and Haemostasis (SISET). Blood Transfus. 2020;
- Wichmann D, Sperhake J-P, Lütgehetmann M, Steurer S, Edler C, Heinemann A, et al. Autopsy Findings and Venous Thromboembolism in Patients With COVID-19: A Prospective Cohort Study. Ann Intern Med [Internet]. 2020 [cited 2020 May 12]; Available from: annals.org/aim/fullarticle/2765934/autopsy-findings-venous-thromboembolism-patients-covid-19-prospective-cohort-study