In our study, adult CAP was more common in males, which may be related to their anatomy, socioeconomics, behaviour, and lifestyle [3]. Regarding clinical manifestations, this study suggests that fever, cough, expectoration, and shortness of breath are the most common clinical symptoms, whereas nasal congestion, runny nose, sore throat, and headaches are relatively rare. These results are similar to those of other studies [4, 5]. Concomitantly, compared to bacterial infections, viral infections were more likely to induce headaches and muscle soreness. In a study of 304 adult patients with CAP, myalgia was detected in 58%, 40%, and 29% of patients with influenza virus, adenovirus, and respiratory syncytial virus infections, respectively. In addition, headaches were detected in 55%, 41%, and 25% of patients with influenza virus, rhinovirus, and adenovirus infections, respectively [6].
In terms of laboratory examination, the results showed that the PCT levels in the bacterial infection group were significantly higher than those in the viral infection group, which was similar to previous studies. PCT is encoded by CALC-1. Bacterial infection promotes increased PCT secretion through intracellular toxins and cytokines (IL-6, TNF-α, etc.), while the γ-interferon produced by the virus-infected body can block this process. Most scholars suggest that when PCT level of < 0.1 µg/L, bacterial infection can be excluded, which can reduce antibiotic exposure time and antibiotic-related adverse reactions [7–10]. In addition, the ESR level and NAP score in the bacterial infection group were significantly higher than those in the viral infection group, which was also observed by Kubota et al [11, 12]. The degree of NAP activation in bacterial infection is higher than that in viral infection; thus, there are different integral values [13]. Therefore, PCT, ESR, and NAP scores are helpful for the rapid differentiation of bacterial and viral infections. Wang et al. suggested that ESR can be mainly used for the early diagnosis and disease evaluation of Mycoplasma pneumoniae infection [14, 15]. This study showed that the ESR level of the atypical pathogen infection group was higher than that of the viral infection group, indicating that there were more patients with severe disease in the atypical pathogen infection group. In addition to inflammatory indicators, this study found that there were differences in the myocardial enzyme spectrum among the groups. The LDH level in the mixed infection group was significantly higher than that in the bacterial infection group, which may be related to the majority of severe pneumonia cases in the mixed infection group. In addition, the level of CK in the viral infection group was significantly higher than that in the bacterial infection group, which corroborates the report of Voiriot et al. [16], indicating that toxin levels that easily cause myocardial cell damage are produced in the body after viral infection.
In terms of imaging manifestations, this study found that there were more patients with consolidation in the atypical pathogen infection group than in the other groups. However, previous studies have shown that patients with Mycoplasma pneumoniae and Chlamydia pneumoniae infections are more likely to have centrilobular nodules, tree-in-bud sign, and ground-glass opacity on imaging, while bronchial wall thickening and pleural effusion are rare [17, 18]. The results of this study differed in that the latter were considered to be related to the majority of patients with severe disease in the atypical pathogen infection group, who had a wide range of lesions and were more likely to progress to consolidation.
The prediction model obtained in our study found that patients with fever, muscle soreness, and insignificant elevation of PCT levels were more likely to be infected with a virus. Previously, several scholars had conducted research on model prediction of respiratory tract infection pathogens. Liu et al. [19] analysed the clinical characteristics of adult outpatients with CAP in Beijing and concluded that cough, dyspnoea, mild chest pain, and WBC counts of 4–10×109/L were associated with viral infection. However, the discriminant factors are inconsistent with those of our study because our study was conducted on inpatients. Chinese scholars from Hong Kong found that a history of living in nursing homes and an increase in WBC counts were significantly correlated with bacterial infections in older adults [20]. Spanish scholars found that acute onset, older age, and increased or decreased WBC counts are independent predictors of bacterial pneumonia [21]. Canadian scholars analysed the clinical characteristics of hospitalised adult patients with CAP and found that, compared with patients with bacterial infection, patients with viral infection were older, had more complicated heart disease, and often weaker [22]. The prediction models are diverse and are related to the study sample size, patient type, and region. In this study, the prediction model can only provide a preliminary diagnostic direction for adult patients hospitalised with CAP in our region. The identification of specific pathogens is still based on etiological test results, and the determination of the specificity and sensitivity of the model requires further research.