With the rapid spread of the COVID-19 pandemic, the development of a standardized score with fixed criteria is essential to improve the consistency of the interpretations of chest CT images among radiologists [13, 14]. Regarding the complexity and time-consuming application of the current scores in clinical practice, we hypothesized a simple score that is a simplification of a well-known current chest CT score. This simple score would be useful and convenient for evaluating the severity of lung involvement in COVID-19. A comparison of the criterion-fixed score systems and text-free scales is needed. In this study, we compared the simple and current chest CT scores concerning their validity, reliability, and survival outcomes for evaluating the severity of lung involvement in COVID-19. Moreover, our study showed that the simple score that takes only a few minutes to administer is a valid and reliable score for evaluating the severity of lung involvement in COVID-19.
Our study compared the validity of the two scores in detecting the severity of lung involvement in COVID-19 and found that no statistically significant differences were observed between the two scores. Both scores had shown great value in the evaluation of the severity of lung involvement in COVID-19 with statistically comparable sensitivity and specificity. We found that a simple score cutoff value of > 3 predicted death with a sensitivity of 81.8% and specificity of 96.3% and a current score cutoff value of > 12 predicted death with a sensitivity of 86.4% and specificity of 93.7%. Our results were comparable with those of previous studies [7,8,9,10, 15] about the correlation of the current scores with the clinical severity or patient survival. Yang et al.  using the CT-SS revealed a sensitivity of 83.3% and specificity of 94% with a cutoff value of 19.5. Kunwei Li et al.  using the TSS revealed a sensitivity of 82.6% and specificity of 100% with a cutoff value of 7.5. Kunhua Li et al.  using the current chest CT score revealed a sensitivity of 80% and specificity of 82.8% with a cutoff value of 7. Francone et al.  using a semi-quantitative CT-SS proposed by Pan et al.  found that a CT-SS of ≥18 was associated with increased mortality risk.
Interreader reliability is essential for assessing any new scoring system. The results of this study showed higher IRA in the evaluation of the severity of lung involvement in COVID-19 when using the simple score compared with the current score. The IRA between two experienced readers for the simple score was good (k = 0.645) and for the current score was moderate (k = 0.458). This better reliability of the simple score compared with the current score could be attributed to the inherent simplifications of this score. Moreover, the more regions assessed in the current score may increase the variability. Additionally, good IRA may be due to the higher experience of readers in our study. However, this is potentially affecting the validity of the simple score. Thus, further studies on the validity of this score when applied by less experienced radiologists are needed. Previous studies [7, 8] reported excellent IRA for the current score. Yang et al.  reported excellent IRA for the CT-SS with an intraclass correlation coefficient (ICC) of 0.925. Kunwei Li et al.  reported good repeatability for the TSS with an ICC of 0.976.
Our survival curve analysis demonstrated that both scores had comparable high values in predicting survival outcomes in patients with COVID-19 with no statistically significant difference (p = 0.146). Similar findings were reported by previous studies [15, 16], which found a positive correlation between the extent of CT lung involvement and short-term mortality.
In the present study, we found that the most common CT findings were the peripheral distribution, multifocal affection, GGO, consolidation, and crazy-paving pattern, which were more frequent in the death group. These findings are similar to that of previous studies [9, 16]. Death in patients with mild scores may be attributed to causes other than pneumonia, e.g., pulmonary embolism that will not be depicted in non-contrast CT .
Our study had some notable limitations. First, it was a single-center experience with a retrospective design, so patient selection biases could have been present. Second, from the consecutive patient cohort diagnosed with COVID-19 in our institution, we included only the subgroup of patients who underwent CT within 12 h after admission. Hence, to confirm the true validity of the simple chest CT score, the score probably has to be assessed prospectively in all patients with COVID-19 at the time of presentation. However, because not all patients with COVID-19 need to have a CT examination, the methodology used in our study indicates, in some respects, current clinical practice. Third, all CT images were obtained within 12 h after admission regardless of the onset of symptoms. Fourth, we did not consider the impact of comorbidity factors on CT severity. Finally, no long-term follow-up data were available.