Currently, retrospective studies investigating various aspects of COVID-19 are being conducted. The infectivity of SARS-CoV-2 is high, the source of the pathogen is unknown, there is no specific treatment method, and the mortality rate is high [15]; therefore, the early diagnosis of COVID-19 is a challenge. A correct and timely diagnosis is of great significance for treatment and prevention of dissemination. Due to delayed laboratory results and the limited sensitivity of the tests, the controversy regarding how to obtain an early clinical diagnosis remains [16]. Thoracic imaging plays an important role in early diagnosis. We believe that in epidemic areas, diagnosis with imaging characteristics is the key to distinguish COVID-19 from other CAPs.
The data were similar to most clinical observations [17-19]. COVID-19 group mostly comprised young adults as follows: 129 patients (78.2%) were aged 20–60 years, while only 18 patients (15.3%) were aged 20–60 years in CAP group. This distribution does not comply with the general spread of infectious diseases, i.e., children and elderly populations are often the susceptible groups. Young adults typically have the strongest resistance to infectious diseases, and the incidence rate should be low. Among the 72 314 cases noted by the Chinese Center for Disease Control and Prevention, the incidence rate in China among those aged under 10 years was less than 1% [20]. The reason for the high incidence rate in young adults cannot exclude the possibility of more virus contact and infection opportunities due to more social interactions. Notably, further studies investigating the epidemiology and pathogenesis of this disease are needed. Fever was the prominent clinical manifestation in both groups, but the increase of temperature in COVID-19 group was more obvious than that in CAP group. During the early stage of the disease (within seven days), the total number of leukocytes in the observed patients with COVID-19 was normal, low (< 4 × 109/L), or showed a decreasing trend, which is similar to that reported in the literature [21] and may be caused by a decrease in lymphocytes during the first few days after infection. The symptoms of muscle soreness and fatigue in COVID-19 group were significantly more prevalent than those in CAP group, while the symptoms of expectoration in CAP group were significantly more prevalent than those in COVID-19 group, which may be related to the fact that CAP is prone to producing more purulent secretion due to bacterial infection [22]. The results showed that the incidences of heart disease, hypertension, chronic lung disease and diabetes in CAP group were significantly higher than those in COVID-19 group.
Our study shows that COVID-19 chest CT reveals ground-glass shadows of different sizes, and the most typical was round or quasi circular. This shadow appeared in 32.1% (53 cases) of the patients, which is similar to other studies [23,24]. In addition, the incidence of a single lesion in COVID-19 group was significantly higher than that in CAP group, which may be related to the early stage of SARS-CoV-2 infection. With the progression of the disease, the GGO in some patients expanded and gradually developed into ground-glass shadows with multiple patches, large areas of fusion or large and small patches coexisting, but the density change was relatively small; this finding is in contrast to the rapid consolidation observed as CAP progresses. This important feature of COVID-19 has not been reported. Notably, in some patients with mild CAP, the lung lesions did not expand until absorption. Another 32.7% (54 cases) of patients with COVID-19 showed fine reticular shadows overlapping with the ground-glass shadow, which may be related to alveolar oedema in the lung lesion area and slight thickening of the alveolar septum with infiltration of monocytes, lymph and plasma cells [25].
In CAP group, new exudative inflammatory lesions were often accompanied by obvious fibrous components or texture aggregation and cord adhesion in other parts of the lung, which manifested as bronchitis or bronchial thickening and blurring of the outer edge, suggesting that these patients had previously experienced the process of lung inflammation and that there were some remaining chronic inflammation or later changes. This finding is quite different from the acute onset, rapid progress and multiple manifestations of lung inflammation observed with COVID-19. Therefore, this study suggests that the lung findings of fibrous cord and bronchial wall thickening can be used to exclude COVID-19. In addition, a certain proportion of emphysema, pulmonary bullae, reticular or honeycomb changes under the pleura, "mosaic" signs and/or small cavity shadows in the consolidation area were observed in CAP group. In conclusion, the diversity of lung lesions and the coexistence of new and old lesions in CAP group are helpful in distinguishing CAP from COVID-19.
Six patients with COVID-19 underwent chest CT re-examinations approximately one week after the onset of the disease. The lesions showed "wandering" characteristics, which may indicate heterogeneity in the pathological changes in different lung areas; thus, early changes and changes in the organizing phase of diffuse alveolar injury can appear in different segments simultaneously, which occurs more commonly in young patients. This finding is a new finding for the diagnosis of COVID-19. However, due to the small number of patients in this group, whether these findings have a good diagnostic specificity remains to be further confirmed.
This study found that a positive correlation exists among age, the size of the lesion and total number of lung segments involved in COVID-19 group. The correlation coefficient between age and the size of the lesion was 0.522 (P < 0.001), and the correlation coefficient between age and the total number of lung segments involved was 0.531 (P < 0.001). In COVID-19 group, the elderly patients had a significantly wider range of lesions in the first visit than the young patients, and the number of involved segments in COVID-19 group was significantly higher than that in CAP group, which may be due to related reasons, such as a weak constitution and many basic diseases among elderly patients. The subsequent follow-up of COVID-19 group showed that the patients aged over 60 years had many changes remaining in the lung, and the most serious manifestation was extensive fibrous cord shadows in the lung. In this group, 32 patients (19.4%) were aged over 60 years, and the prognosis of these patients was poor. The 2- to 3-month follow-up showed that most remaining changes in the lung could be absorbed, but the absorption was slow.
This study has several limitations. First, we retrospectively analysed the imaging data of the two groups of patients. Our screening strategy has selection bias. Currently, a more balanced and large-scale prospective study involving similar patients is still needed. In this study, the different situations of the disease in each centre and the different experiences of the radiologists had some influence on the diagnosis of the disease. In addition, some patients had positive results on the new crown pneumonia nucleic acid test. However, during the winter and spring seasons, these patients often have viral and bacterial pneumonia or other diseases, which may cause some interference with the performance of CT in evaluating new crown pneumonia.