BDNF and COVID-19: Test of ROC curve revealed that MMP-9/BDNF ratio can be treated as suitable marker for the disease stage assessment, from I (mild) stage upon II (moderate) (sensitivity 78.57%, specificity 72.73%; cut off: 564.8), stage III (severe) (sensitivity 88.24%, specificity 81.82%; cut off: 889.6) and IV (sensitivity 66.67%, specificity 77.27%; cut off: 598.5) in COVID-19 patients (Figure 2).