Prolidase is essential for the construction and degradation of collagen-containing connective tissue, and changes in its activity are present in many diseases involving fibrotic processes. Prolidase activity may be a useful marker in the early detection and monitoring of these diseases (22). In a study evaluating idiopathic and ischemic dilated cardiomyopathies, which exhibited the same histopathological feature of fibrosis, serum prolidase activity was found to be significantly lower in both groups compared to healthy volunteers (23).
Chronic obstructive pulmonary disease is an inflammatory lung disease in which irreversible fibrotic changes occur in the airways. In a study including 60 COPD patients, lower plasma prolidase activity was observed in COPD patients compared to the control group (p < 0.05) (24). In our study, no significant relationship was observed between 3rd month total fibrosis score and serum prolidase levels. When evaluated in terms of fibrotic changes, although there was no significant difference between the groups, they were observed to be higher in the fibrotic group compared to the other group at the beginning. This can also be explained by the fact that it is an early period for changes in prolidase activity during the fibrosis process. Significant changes may be found when evaluated in the future.
In terms of lung fibrotic diseases, MMP-1/MMP-7 were identified as potential peripheral blood biomarkers in idiopathic pulmonary fibrosis (IPF). In a study, MMP1 and MMP7 levels were found to be significantly higher in serum and bronchoalveolar lavage fluid in IPF patients compared to healthy controls(25). MMP-7 levels were associated with early fibrotic changes on CT scan in asymptomatic patients with familial interstitial pneumonia (7).
Our study has some limitations. These results may have emerged considering that patients infected with COVID-19 may have been at different stages of the disease at hospital admission and hospitalisation. Although the biomarkers included in our main hypothesis were found to be higher in the fibrotic group, no significant statistical difference was observed.
Studies conducted for COVID-19 showed that males have higher mortality or severe infection rates than females. While some studies reported a higher incidence of COVID-19 infection in males, some studies found no difference in the incidence of COVID-19 infection (26).
In a study involving 72,314 suspected or confirmed COVID-19 cases in China, it was found that 51.4% of the patients were male, with 22,981 cases, and mortality was significantly higher in males than females (27).
In a study conducted with 14,712 patients with COVID-19, all-cause mortality rate was higher in males than females (p < 0.001) (28). In our study, the number of male patients (n = 39) was high in patients with COVID-19 infection and the proportion of male patients (n = 33) was high in the severe disease group.
In a meta-analysis of 2018 patients evaluating pulmonary fibrosis after COVID-19, the mean age of fibrotic patients was 59 years, while the mean age of non-fibrotic patients was 48.5 years (p = 0.003), and there was no significant difference between fibrotic and non-fibrotic patients in terms of BMI (25.23 and 24.75, respectively), evaluated by only two studies, and no significant gender difference (53.8% male and 46.2% female) was observed between groups (29).
In our study, age (p = 0.401) and body mass index (p = 0.169) were not significantly associated with the 3rd month fibrosis total score. There was no significant difference between Group 1 and Group 2 in terms of age and body mass index, and no significant difference was found in terms of gender (p > 0.05).
Although the symptoms of patients at hospital admission differ in COVID-19, the common symptoms are similar in most studies.
In a study evaluating the data of 1099 COVID-19 patients, fever was present in 43.8% of the patients during hospital admission, developed in 88.7% of the patients at hospitalisation, and the second most common symptom was cough (67.8%), followed by nausea or vomiting (5.0%) and diarrhea (3.8%) (30).
In one of the first studies on COVID-19, common symptoms at the onset of the disease were fever (98%), cough (76%), and myalgia or fatigue (44%), with less common symptoms were phlegm (28%), headache (8%), and diarrhea (3%). It was observed that 22 of 40 patients (55%) developed dyspnea (31).
In a study including 10 studies and a total of 1995 cases, clinical symptoms of COVID-19 patients were fever (88.5%), cough (68.6%), myalgia-fatigue (35.8%), phlegm (28.2%), and shortness of breath (21.9%) (32). According to a meta-analysis, among COVID-19 symptoms, cough (47.4%), chest pain (27.6%) and fever (72.4%) were more common in the fibrotic group compared to the non-fibrotic group (p < 0.05) (29).
In our study, the most common symptoms at hospital admission were cough (57.4%), fever (52.9%) and fatigue (52.9%), respectively. Similar to the literature, cough and fever symptoms were observed frequently in our patients.
In a study of 80 COVID-19 patients, the frequencies of myalgia and fatigue were 46.1% and 50%, respectively. Laboratory data showed a significant increase in creatinine kinase (CK) level and lymphocyte count in patients with myalgia symptoms (p < 0.05). It was reported that caution should be exercised regarding muscle damage due to increased CK in these patients, and that extremely high values may indicate a progressive myopathic process (33). In our study, when the relationship between the symptoms at hospital admission and the 3rd month fibrosis total score was evaluated, the score was higher in patients with muscle-joint pain symptoms compared to the group without symptoms (p = 0.031). The 3rd month fibrosis total score of patients with fatigue symptoms was found to be significantly higher than the group without symptoms (p = 0.006). When Group 1 and Group 2 were compared in terms of symptoms, muscle-joint pain (p = 0.015) and fatigue symptoms were more common in Group 1 (p = 0.029). There was no difference in other symptoms (p > 0.05).
COVID-19 pneumonia was more severe in some patient groups compared to other patients, and many studies on this subject showed that comorbidities play a role in this situation. In a study evaluating 191 COVID-19 patients, hypertension was found to be the most common comorbidity, followed by diabetes and coronary heart disease (34).
In a study evaluating 654 COVID-19 patients in our country, it was observed that the most common comorbidities were hypertension, diabetes mellitus and cardiovascular diseases, respectively (25.3%, 12.4%, 10.7%) (35).
In a study in which 81 patients were evaluated for pulmonary fibrosis, the fibrosis group had higher rates of DM, hypertension, chronic lung disease, chronic liver disease, cardiovascular and cerebrovascular diseases than the non-fibrosis group. Abnormal parameters including leukocytosis, neutrophilia, lymphopenia, eosinopenia, high CRP and d-Dimer were found to be significantly different between the groups (36). In our study, the most common comorbidities were noted as DM (44.1%,30), respiratory disease (42.6%,29) and hypertension (41.2%,28), respectively. No significant difference was observed between the groups in terms of comorbidities (p > 0.05). There was also no significant correlation between comorbidities and 3rd month fibrosis total score (p > 0.05).
A study conducted for SARS showed a positive correlation between peak LDH level and pulmonary fibrosis at hospital admission (37). In a study conducted in patients with MERS infection, a higher serum LDH peak was detected in those with persistent abnormalities on chest radiographs at a mean follow-up of 42 days (38). Serum LDH level was used as a marker of disease severity after acute lung injury, and it was found to be a marker of pulmonary tissue destruction and was associated with mortality risk (39). High LDH levels in COVID-19 patients was associated with disease severity, and the time until LDH normalized was shown to be positively correlated with early CT resolution (17). In our study, LDH values at hospital admission in the group with fibrotic changes were significantly higher than the group without fibrotic changes (p = 0.012). Fibrinogen value was also significantly higher in Group 1 compared to Group 2 at hospital admission (p = 0.002). In the light of these data, LDH values were noted to be high in the fibrotic group at hospital admission, in line with the literature.
In a study evaluating 227 COVID-19 patients, pulmonary fibrosis was found to be significantly associated with some coagulation indices and fibrinogen (40). Similarly, in our study, it was observed that the fibrinogen value in blood parameters at hospital admission was higher in Group 1 than in Group 2 (p = 0.008).
Huang et al. indicated that neutrophil, neutrophil-to-lymphocyte ratio (NLR), C-reactive protein (CRP), and LDH levels were significantly above the normal range during the first four weeks after hospitalisation in patients with extensive fibrosis at two-month follow-up. Conversely, CRP and LDH were shown to increase within two weeks, but then sharply decreased to normal levels in the non-fibrosis group (36). In our study, there was a significant and positive relationship between neutrophil percentage and 3rd month fibrosis total score (p = 0.040).
In another study evaluating pulmonary fibrosis after COVID-19, some predictive factors such as old age and smoking were reported. The incidence of post-pulmonary fibrosis was found to be much higher in patients with a smoking history than in non-smokers, and post-pulmonary fibrosis was observed to develop in 18 out of 30 smokers (41). In our study, no significant relationship was found between smoking exposure (secondhand smoker) (p = 0.793) and 3rd month fibrosis total score. There was no difference between the groups in terms of smoking and smoking exposure (secondhand smoker) (p = 0.584), and our study was different in this respect.
A significant and positive relationship was detected between the patients' length of stay in the clinic (p < 0.001) and the 3rd month fibrosis total score. It was observed that the length of stay of the patients in the clinic was longer in Group 1 compared to Group 2 (p < 0.001). In a study in which fibrotic and non-fibrotic groups were evaluated after COVID-19, a statistically significant difference was observed between both groups in terms of ICU hospitalisation and length of hospital stay (p˂0.001), and the length of stay in the fibrotic group was found to be longer than in the non-fibrotic group (23.26 ± 20.89 vs. 8.56 ± 7.03 days) (42). The data in the relevant study were found to support our results.
Some limitations of our study are that it was single-center study conducted with a small number of patients. On the other hand, the fact that patients may have different disease stages during admission may have affected our results.