## 4 Discussion  
The present study aimed to examine the role of preoperative and postoperative C-reactive protein (CRP) levels in mediating the association between preoperative depression symptoms and postoperative length of stay in patients undergoing coronary artery bypass grafting (CABG). No significant association between elevated BDI-II (>13) depression symptom scores and more extended (>7 days) postoperative hospital LOS was detected. Although the higher persistent CRP response was significantly associated with a longer postoperative hospital stay, no significant effect of higher BDI-II scores on early (1-3 days post-surgery) or persistent (4-6 days post-surgery) CRP response, or vice versa, was found. In our cohort, 12.3% of patients exhibited mild-to-severe depressive symptoms measured preoperatively by the BDI-II depression inventory. The obtained frequencies are within the range of 0.6% to 60% of previously published data on the prevalence of depression symptoms before CABG surgery [10; 45; 46; 47; 48; 50]. This variability in reported estimates throughout the corresponding literature may be attributed to one or more of the following causes: clinical and demographic differences between patient cohorts, diverse inclusion and exclusion criteria, differences in questionnaires or clinical interviews used for measurement, different timing of measurement, and different cut-off values used for determining high or low levels of depression. [PERSON] et al. reported that CABG patients with elevated preoperative Beck Depression Inventory (BDI) scores had significantly higher odds of a prolonged hospital stay of greater than one week when compared with the non-depressive symptom group [48]. However, no Bonferroni or any other correction for multiple comparisons was applied in their study. The cut-off value for the BDI-II depression score was set at \(\leq\)10 for the non-depressive and >10 for the depressive CABG patient group. Notably, no statistically significant association between binary BDI-II values and hospital LOS was detected when the same cut-off values were applied to our cohort of CABG patients. The negative influence of higher depressive symptom scores on CABG patient outcomes and longer hospital stay was also reported in several other studies [45; 46; 47]. Thus, [PERSON] et al. (2021.) reported that every one-unit increase in preoperative depressive symptoms in their patients' cohort resulted in an increased length of hospital stay by 0.37 days [45]. Furthermore, in their cohort, the female patients had higher levels of depressive symptoms compared to male patients, while being female increased the hospital LOS by 0.18 days [45; 46]. The higher rate of preoperative depression among female CABG patients was also reported by [PERSON] et al. (2012.) and [PERSON] et al. (2014.) [48; 50]. Figure 3: Mediation model of baseline BDI-II score and length of stay through change in persistent CRP. In our patient cohort, female CABG patients were also more prone to preoperative depression and extended hospital stay than male patients. However, that gender discrepancy was not statistically significant after applying the Bonferroni correction for multiple comparisons. Although non-significant, gender related association of prolonged hospital stay with higher depression score was also underlined by separate analysis of male and female patient subgroups within our cohort of CABG patients with females having more than a four times higher odds ratio for prolonged hospital stay compared with the male subgroup that exhibited a negative association. This coincides with the differences in the prevalence of depression between men and women and confirms the theory of the gender gap in depression etiology [10; 48; 50].