## 4 Discussion
PCT, ferritin, fibrinogen, ESR, PT, and D-dimer were found to be the most important features according to the F12 metric for the histogram-based gradient-boosting model operated with single features in the classification of surviving and deceased patient-groups (Figure 6 and Table 5). In addition, it was observed that PCT and ferritin were the most important feature-pairs in the identification of living and deceased patients (precision > 0.98, recall > 0.98, F12 > 0.98 in both living and deceased patients) (Table 6 and Figure 12). In addition, other feature pairs run with the HGB model produced an F12 value of \(\geq\) 0.94 in identifying patient-groups (Table 6). Accordingly, our HGB model, which was trained with SMOTE, was found to largely accurately identify living and deceased COVID-19 patients. In addition, the performance of the HGB model with a single feature (F12) in the classification of patient groups was found to be more successful than the classification made by considering one and two cut-off-values (Figure 11). In addition, the approach of identifying patient groups based on the relationship structure (Spearman) of the characteristics of the patient groups produced the lowest F12 results (Figure 11). In this study, in order to determine the critical-risk levels of the features in COVID-19 mortality, the lethal levels of the features were determined with one- and two-threshold approaches (see Section 2.4), and patient groups were classified according to these values [Figures 7 and 8 and Tables A2 and A3]. According to the one-threshold approach, PT values greater than 13.50 Sec (F12 = 0.58), D-dimer values greater than 1009.99 \(\upmu\)g/L (F12 = 0.64), INR values greater than 1.51 (F12 = 0.58), amylase values greater than 76.79 mg/dL (F12 = 0.61) and CK-MB values greater than 18.86 U/L (F12 = 0.60) were found to be lethal critical-levels for COVID-19 mortality. According to the two-threshold approach, it was found that the majority of patients who died had fibrinogen values between 349.98 g/L and 379.05 g/L (F12 = 0.68), D-dimer values between 1009.99 \(\upmu\)g/L and 10,742.71 \(\upmu\)g/L (F12 = 0.65), ESR values between 36.12 and 56.62 (F12 = 0.70), ferritin values between 376.2 \(\upmu\)g/L and 396.0 \(\upmu\)g/L (F12 = 0.91) and PCT values between 0.2 \(\upmu\)g/L and 5.2 \(\upmu\)g/L (F12 = 0.95). It can be said that the determined value ranges of these features are the most important lethal-risk levels. It is noteworthy that procalcitonin and ferritin are the most important feature pairs in the detection of surviving and non-surviving COVID-19 patients, according to both the HGB classifier and the two-threshold approach. Similar to the findings in this study, many studies have supported the view that any significant increase in PCT levels reflects the development of a critical condition in COVID-19 [55; 56; 57; 58; 59]. [PERSON] et al. [60] stated that, due to the characteristic structure of PCT in bacterial and viral infections, it may play a role in the prognosis of COVID-19. [PERSON] et al. [55] noted that despite several limitations, elevated PCT levels can be used as a rapid indicator of criticality, a worsening clinical-picture, and even mortality, in COVID-19. Similarly, [PERSON] et al. [58] stated in a meta-analysis that procalcitonin levels above 0.5 \(\upmu\)g/L were correlated with a 5-fold greater risk of serious infection in COVID-19 patients. In another study, [PERSON] et al. [61] showed that more than 96% of COVID-19 patients with low disease-severity had serum procalcitonin levels of less than 0.5 \(\upmu\)g/L, and that these patients had better clinical outcomes. Additionally, [PERSON] et al. noted that PCT levels above 0.5 \(\upmu\)g/L are associated with a more serious COVID-19 illness or secondary bacterial infection [61]. A meta-analysis involving Caucasians and South Asians found a strong association between PCT and the severity of COVID-19 [55]. This multiethnic assessment further reinforces the importance of PCT as a prognostic biomarker in cases of COVID-19. [PERSON] et al.