2.3. CT scan

A CT scan of the chest is a detailed specific chest X-ray that is taken from different angles followed by the final cross-sectional image, which helps in examining the abnormalities in the lungs and inside the chest to further diagnose the cause of the abnormalities (^{Whiting et al., 2015}). A CT scan of the chest is a painless, non-invasive routine test for the diagnosis of pneumonia and other respiratory diseases performed by radiologists that can provide fast results and is easy to administer. As with the other methods of SARS-CoV-2 detection, the involvement of lungs is dependent on number of the days that have passed after the onset of infection. A related study showed that in the first two days of manifesting symptoms, 56% of the CTs of patients were found to be normal, and the maximum observable abnormalities were around 10 d after manifesting symptoms (^{Bernheim et al., 2020}).

Small-scale (^{Huang et al., 2020; Pakdemirli et al., 2020; Xie et al., 2020}) and larger-scale studies (^{Chung et al., 2020}) have shown that PCR-based methods have limited sensitivity and that chest CTs can reveal abnormalities in almost all COVID-19 patients. This was found to be the case even for asymptomatic individuals or patients with initial negative RT-PCR but clinical symptoms, thus showing that CT scans have higher sensitivity compared with RT-PCR (^{Ai et al., 2020; Fang et al., 2020; Xie et al., 2020}). CT, however, has disadvantages such as low specificity as the features from COVID-19 patients are similar to those of other viral cases of pneumonia (^{Ai et al., 2020}). To show the high sensitive value of chest CT as a diagnostic test, some research groups have compared the results from the chest CT scan with RT-PCR.

A study (^{Fang et al., 2020}) that compared the sensitivity of chest CT with RT-PCR reported that chest CT had a sensitivity of 98% for COVID-19 while RT-PCR sensitivity was 71% when 51 patients were tested within 3 d. The study also suggested chest CT could be used as an early diagnostic method of respiratory diseases such as COVID-19, while RT-PCR could maintain its position as a standard of reference.

In a similar study on the sensitivity of RT-PCR and chest CT, a comparison of the results from chest CT and the initial and serial RT-PCR was performed (^{Ai et al., 2020}). The results indicated that 59% of the patients had positive RT-PCR, while the positive rate for chest CT was 88%. Also, 75% of the patients who were diagnosed negative by RT-PCR had abnormalities in their chest CT and had positive chest CT for COVID-19. The results from serial RT-PCR indicated that the average time between initial negative RT-PCR to a positive one is 5.1 ± 1.5 d. The false-negative results of initial RT-PCR were in line with the results from other reported studies (^{Chung et al., 2020; Xie et al., 2020}). These false-negative results would aid the further spread of the virus, which would not be desirable. In conclusion, the results highlighted that chest CT is more sensitive compared with initial RT-PCR, and that, in epidemic areas, it could be considered as the primary diagnosis method for COVID-19.