Stages of CAC. The spectrum of coagulation abnormalities in COVID-19 parallels disease severity. In stage 1, patients exhibit mild symptoms and evidence of mild localized inflammation and coagulopathy with elevations in D-dimer approximately 2 to 3 times the upper limit of normal. Stage 2 is characterized by more severe symptoms requiring supplemental oxygen with pulmonary inflammation and intravascular coagulation activation in the lungs as well as some systemic involvement. This is reflected in further D-dimer elevation to 3 to 6 times the upper limit of normal. Patients progressing to stage 3 require critical care with respiratory and other organ support and demonstrate severe systemic inflammation and coagulopathy with markedly elevated D-dimer and fibrinogen, prolonged PT, thrombocytopenia, and high incidence of thromboembolism. A proportion of these patients will develop overt DIC as a result of their COVID-19 illness or other superimposed infections.¹⁸