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Objective:
After a thorough review of available literature, we found that knowledge of appropriate cutoffs for categorizing patients with respiratory illness according to serviceable chest x-ray (CXR) scores using the index CXR (i.e., the first CXR at admission) and the effect of comorbidities (apart from hypertension (HTN), diabetes mellitus (T2DM), and asthma on them is still lacking. In this study, we aim to bridge the unmet need for literature on CXR scores in the background of COVID-19 and invigorate its further use in low resource centers for oncoming waves of similar or novel respiratory illnesses using the zonal scoring system; the most simple and versatile scoring system developed during the pandemic.

Materials and Methods:
This retrospective cohort study uses clinical and imaging data from 751 COVID-19 RT-PCR + patients with appropriate index CXRs admitted to a tertiary and referral health care center in south India. Concordant CXR scores of 3 radiologists were reported, and interrater reliability was measured using kappa indices. Receiver operating characteristic curve analysis, along with Youden index calculation, were used to establish cutoff scores for the outcomes of interest: mild or severe disease, admission into ICU, and intubation. Categorical data on comorbidities and hospital stay were expressed using means and percentages, and chi-square or t-tests were used for comparison at an alpha level of 0.05. Unadjusted odds ratios for each outcome of interest versus CXR score and comorbidity were then calculated using binary logistic regression.

Results:
CXR findings included interstitial and alveolar infiltrates (46.07%), pleural effusions (7.05%), consolidation and fibrosis (4.43%), pneumothoraces (2.71%), and cardiomegaly (2.26%). Kappa coefficient of the 3 radiologists was calculated to be 0.82 (p < 0.05), showing excellent inter-rater reliability. The mean Index CXR score was 1.9280, with a standard deviation of 2.3426. A total of 17 patients had an ICXRS of 1 (2.26%), 55 (7.32%) - 2, 22(2.92%) - 3, 114 (15.15%) - 4, 29 (3.86%) - 5, and 107 (14.24) had a score of 6.Most patients had an index CXR Score of 0 (54.19%). The index cut-off score of =1 (sensitivity, specificity -82.95, 81.68) was established for mild disease, =4 for severe disease (85.71,83.99), =3 for ICU admission (86.90,71.91), and =4 for intubation (87.61,72.90). Hypertension, Type 2 diabetes mellitus, hypothyroidism, history of ischemic heart disease, and history of tuberculosis were independent risk factors for a high CXR index score, intubation, and ICU admission.

Conclusion:
The zonal scoring system, having excellent interrater reliability, can be used to train new residents, promoting an easy transition into core radiology. It can also, due to its high sensitivity and specificity, be used in low resource centers not only for categorization and prognosis of respiratory illnesses but also to maintain radiological and clinical records, track patient progression, and act as a measure of quality improvement in healthcare.