Objective To perform bilateral lower extremity venous ultrasound in patients with severe or critical COVID-19

and to assess the mortality risk in patients with severe or critical COVID-19 and venous thrombosis of lower extremity by D-dimer test.Methods All 32 patients with severe or critical COVID-19 underwent bilateral lower extremity venous ultrasound and D-dimer test.The incidence of thrombus

D-dimer abnormality and the mortality risk were analyzed.The ROC curve was used to analyze the value of D-dimer in predicting the mortality risk in patients with severe or critical COVID-19 and venous thrombosis of lower extremity.Results Among the 32 patients

21 patients（65.6%） had acute venous thrombosis of the lower extremity

and 12 of them died（57.1%）.Each patient underwent an average of 6D-dimer tests

28（87.5%） had elevated D-dimer levels after each D-dimer test.Taken D-dimer standard deviation of 11.310μg/ml as cut-off value

the sensitivity and specificity for predicting the mortality risk in patients with severe or critical COVID-19 and venous thrombosis of lower extremity were 91.7% and 66.7%

respectively.The area under the curve was 0.759（95%CI:0.524-0.995

P<0.05）.Conclusion The D-dimer level elevated and fluctuated abnormally in patients with severe or critical COVID-19 and venous thrombosis of lower extremity is suggestive of mortality risk.More attention should be paid to the D-dimer level fluctuation during treatment

and bilateral lower extremity venous ultrasound examination should be performed early to identity patients with acute vein thrombosis.