This study is a diagnostic test using a cross-sectional method with a retrospective approach. The research data were carried out from March 2020 to April 2021 based on patient medical records and chest X-rays at Doctor Kariadi General Hospital Semarang. The research subjects were male and female patients aged 18 years or older who were tested positive for COVID-19 and received inpatient services at the Doctor Kariadi General Hospital Semarang. The minimum sample size of this study was determined based on the prospective test formula to get the relative risk index. The formula np = 2 (Q1/P1 + Q2/P2)/(ln (1−e)2), with a significance level of 0.05, CI 95%, and power of 80%, according to the incidence of death due to COVID-19 in Indonesia of 4.1% (on September 8, 2020) to get a minimum sample size of 80.3 samples.

The inclusion criteria in this study were COVID-19 inpatients and received serial chest X-ray examination services at least 2 times in 1 hospitalization period. Exclusion criteria for study subjects were the presence of comorbid diseases or a history of previous chest X-ray images that could resemble COVID-19 pneumonia or could obscure the assessment of pulmonary lesions, including a history of pulmonary tuberculosis, history of lung malignancy or lung metastases, pulmonary oedema, history of interstitial lung disease, pleural effusion, pleural mass, or chest wall mass. Although more than 5000 sample candidates have been obtained, there were only a little less than 200 samples that met the inclusion and exclusion criteria. Moreover, many samples did not have a complete data set.

The CURB-65 score

The CURB-65 score was assessed based on the initial general condition and vital signs when the patient was admitted, as well as the creatinine value of the first blood examination. The CURB-65 assessment is based on the following factors: level of consciousness (confusion), blood urea (uremic), respiratory rate (respiratory), blood pressure (blood pressure), and age (≥ 65). Consciousness is assessed whether the patient has decreased consciousness or not. Blood urea nitrogen levels are declared to be valuable if they are more than 20 mg/dl. The respiratory rate was assessed if it was more than 30 breaths per minute. Blood pressure is declared at risk if the systolic blood pressure is less than 90 mmHg or the diastolic blood pressure is less than 60 mmHg. The risk factor for age is worth if it is more than equal to 65 years. Each criterion obtained is worth 1, so the total score ranges from 0 to 5 [5,6,7] (Fig. 1).

Fig. 1
figure 1

Representative radiologic severity index score serial for chest X-ray. RSI scores are labelled within each panel. Panels A–C show chest X-ray images from an individual patient with parainfluenza virus-associated lower respiratory infections from the previous study in America, in order of increasing severity [4]

Chest X-ray radiologic severity index

This study uses a posteroanterior or anteroposterior projection chest X-ray. The entire chest X-ray sample was assessed using the RSI method. The RSI scoring system uses two main variables: the pattern of spread and the volumetric density of the lesion. In the RSI assessment, the spread of the lesion was assessed based on the location of the lesion in the lung fields, where the right and left lung fields were divided into three parts, respectively; the upper zone (up to the carina), the middle zone (below the carina to the upper border of the inferior pulmonary vein), and the lower zone (below the upper border of the inferior pulmonary vein). An inferior pulmonary vein can be identified by a linear image of the right hilum pointing to the periphery, crossing the pulmonary artery [8]. The lesion pattern was divided into 3 categories; 1 for normal lungs, 2 for ground-glass opacity images, and 3 for consolidation images. The volumetric area is divided into 5 broad categories; 0%, 1 for 1–24% lesion area, 2 for 25–49% lesion area, 3 for 50–74% lesion area, and 4 for 75–100% lesion area. The RSI was obtained by adding up the multiplication of the value of the lesion pattern and its volumetric area in each zone, with a total value between 0–72. The chest X-ray RSI assessment of the study is shown in Fig. 2. The independent variable of this study was the factors that formed the values of RSI and CURB-65. The dependent variable in this study was the RSI chest X-ray, the CURB-65 score, and the incidence of death in hospitalized patients with COVID-19 pneumonia [4].

Fig. 2
figure 2

Radiologic severity index assessment in COVID-19 pneumonia patient. This image shows serial chest X-ray RSI values in same patient with COVID-19 pneumonia. A RSI values on initial chest X-rays at 16 March 2020 and B RSI values on serial chest x-rays at 21 March 2020. There is an increase in RSI value at 5-day interval

Statistical analysis

A receiver operating characteristic (ROC) curve was used to determine the cut-off value with high sensitivity and specificity in the research. Furthermore, an analysis of the area under the curve (AUC) was carried out to measure the accuracy of the diagnostic test. Cross tabulation of the cut-off values of RSI and CURB-65 was analysed using the chi-square method with the results of a significant relationship analysis if p value < 0.05 and continued with the analysis of prevalence ratio. Hypotheses testing was continued by using the logistic regression method to obtain a multivariable relationship between CURB-65 and RSI as a predictor of mortality.

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