Diagnostic accuracy of X-ray versus CT in COVID-19: a propensity-matched database study

Abstract

Objectives: To identify the diagnostic accuracy of common imaging modalities, chest X-ray (CXR) and CT, for diagnosis of COVID-19 in the general emergency population in the UK and to find the association between imaging features and outcomes in these patients.

Design: Retrospective analysis of electronic patient records.

Setting: Tertiary academic health science centre and designated centre for high consequence infectious diseases in London, UK.

Participants: 1198 patients who attended the emergency department with paired reverse transcriptase PCR (RT-PCR) swabs for SARS-CoV-2 and CXR between 16 March and 16 April 2020.

Main outcome measures: Sensitivity and specificity of CXR and CT for diagnosis of COVID-19 using the British Society of Thoracic Imaging reporting templates. Reference standard was any RT-PCR positive naso-oropharyngeal swab within 30 days of attendance. ORs of CXR in association with vital signs, laboratory values and 30-day outcomes were calculated.

Results: Sensitivity and specificity of CXR for COVID-19 diagnosis were 0.56 (95% CI 0.51 to 0.60) and 0.60 (95% CI 0.54 to 0.65), respectively. For CT scans, these were 0.85 (95% CI 0.79 to 0.90) and 0.50 (95% CI 0.41 to 0.60), respectively. This gave a statistically significant mean increase in sensitivity with CT of 29% (95% CI 19% to 38%, p<0.0001) compared with CXR. Specificity was not significantly different between the two modalities.CXR findings were not statistically significantly or clinically meaningfully associated with vital signs, laboratory parameters or 30-day outcomes.

Conclusions: CT has substantially improved diagnostic performance over CXR in COVID-19. CT should be strongly considered in the initial assessment for suspected COVID-19. This gives potential for increased sensitivity and considerably faster turnaround time, where capacity allows and balanced against excess radiation exposure risk.

Keywords: COVID-19; Chest imaging; accident & emergency medicine; diagnostic radiology; general medicine (see Internal Medicine).

Figure 1

Inclusion and exclusion of patients during study period with test results. RT-PCR, reverse transcriptase PCR.

Figure 2

Forest plot of ORs of variables associated with reverse transcriptase PCR positivity for SARS-CoV-2 following multiple imputation, propensity score matching and binomial logistic regression. *Significant difference at the <0.05 level; **significant difference at the <0.001 level. ED, emergency department; ITU, intensive treatment unit.

Figure 3

Forest plot of ORs of variables associated with classical chest X-ray features COVID-19 following propensity score matching and binomial logistic regression. *Significant difference at the <0.05 level; **significant difference at the <0.001 level. ED, emergency department; ITU, intensive treatment unit; RT-PCR, reverse transcriptase PCR.