How accurate is chest imaging for diagnosing COVID-19?

Why is this question important?

People with suspected COVID-19 need to know quickly whether they are infected, so they can receive appropriate treatment, self-isolate, and inform close contacts.

Currently, a formal diagnosis of COVID-19 requires a laboratory test (RT-PCR) of nose and throat samples. RT-PCR requires specialist equipment and takes at least 24 hours to produce a result. It is not completely accurate, and may require a second RT-PCR or a different test to confirm diagnosis.

COVID-19 is a respiratory disease. Clinicians may use chest imaging to diagnose people who have COVID-19 symptoms, while awaiting RT-PCR results or when RT-PCR results are negative, and the person has COVID-19 symptoms.

What did we want to find out?

We wanted to know whether chest imaging is accurate enough to diagnose COVID-19 in people with suspected infection. This is the second update of this review; in it, we included studies in people with suspected COVID-19 only; we excluded studies in people with confirmed COVID-19.

The evidence is up to date to 30 September 2020.

What are chest imaging tests?

X-rays or scans produce an image of the organs and structures in the chest.

- X-rays (radiography) use radiation to produce a 2-D image. Usually done in hospitals, using fixed equipment by a radiographer, they can also be done on portable machines.

- Computed tomography (CT) scans use a computer to merge 2-D X-ray images and convert them to a 3-D image. They require highly specialised equipment and are done in hospital by a specialist radiographer.

- Ultrasound scans use high-frequency sound waves to produce an image. They can be done in hospitals or other healthcare settings, such as a doctor’s office.

What did we do?

We searched for studies that assessed the accuracy of chest imaging to diagnose COVID-19 in people of any age with suspected COVID-19. Studies could be of any design, except for case control studies, and could take place anywhere.

What did we find?

We found 51 studies with 19,775 participants. Of these people, 10,155 (51%) had a final diagnosis of COVID‐19. Forty-seven studies confirmed COVID-19 infection using RT-PCR alone. Four studies used RT-PCR with another test.

Forty-seven studies evaluated one imaging technique each, and four studies evaluated two imaging techniques each.

Chest CT was evaluated by 41 studies (16,133 participants, 8110 (50%) confirmed COVID-19 cases), chest X‐ray by nine studies (3694 participants, 2111 (57%) confirmed COVID-19 cases), and ultrasound by five studies (446 participants, 211 (47%) confirmed COVID-19 cases). Thirty-three studies were conducted in Europe, 13 in Asia, three in North America and two in South America. Twenty-six studies included only adults, 21 included both adults and children, one included only children, one included participants aged 70 years and older, and two studies did not report participants' ages. Two studies included hospital inpatients and 32 included hospital outpatients. The setting was unclear in the remaining 17 studies.

Where four or more studies evaluated a particular type of chest imaging, we pooled their results and analysed them together.

Chest CT

Pooled results showed that chest CT correctly diagnosed COVID-19 in 87.9% of people who had COVID-19. However, it incorrectly identified COVID-19 in 20% of people who did not have COVID-19.

Chest X-ray

Pooled results showed that chest X-ray correctly diagnosed COVID-19 in 80.6% of people who had COVID-19. However, it incorrectly identified COVID-19 in 28.5% of people who did not have COVID-19.

Lung ultrasound

Pooled results showed that lung ultrasound correctly diagnosed COVID-19 in 86.4% of people with COVID-19. However, it incorrectly diagnosed COVID-19 in 45% of people who did not have COVID-19.

How reliable are the results?

The studies differed from each other and used different methods to report their results. Few studies evaluated chest X-ray and chest ultrasound; and very few studies directly compared one type of imaging test with another. Therefore, we cannot draw confident conclusions based on results from studies in this review.

What does this mean?

The evidence suggests that chest CT is better at ruling out COVID‐19 infection than distinguishing it from other respiratory problems. So, its usefulness may be limited to excluding COVID‐19 infection rather than distinguishing it from other causes of lung infection.

We plan to update this review as more evidence becomes available. Future studies should predefine what a positive test is, and compare different types of imaging tests on similar groups of people.

Authors' conclusions: 

Our findings indicate that chest CT is sensitive and moderately specific for the diagnosis of COVID‐19. Chest X-ray is moderately sensitive and moderately specific for the diagnosis of COVID-19. Ultrasound is sensitive but not specific for the diagnosis of COVID-19. Thus, chest CT and ultrasound may have more utility for excluding COVID-19 than for differentiating SARS‐CoV‐2 infection from other causes of respiratory illness.

Future diagnostic accuracy studies should pre‐define positive imaging findings, include direct comparisons of the various modalities of interest in the same participant population, and implement improved reporting practices.

Read the full abstract...
Background: 

The respiratory illness caused by SARS‐CoV‐2 infection continues to present diagnostic challenges. Our 2020 edition of this review showed thoracic (chest) imaging to be sensitive and moderately specific in the diagnosis of coronavirus disease 2019 (COVID‐19). In this update, we include new relevant studies, and have removed studies with case-control designs, and those not intended to be diagnostic test accuracy studies.

Objectives: 

To evaluate the diagnostic accuracy of thoracic imaging (computed tomography (CT), X‐ray and ultrasound) in people with suspected COVID‐19.

Search strategy: 

We searched the COVID‐19 Living Evidence Database from the University of Bern, the Cochrane COVID‐19 Study Register, The Stephen B. Thacker CDC Library, and repositories of COVID‐19 publications through to 30 September 2020. We did not apply any language restrictions.

Selection criteria: 

We included studies of all designs, except for case-control, that recruited participants of any age group suspected to have COVID‐19 and that reported estimates of test accuracy or provided data from which we could compute estimates.

Data collection and analysis: 

The review authors independently and in duplicate screened articles, extracted data and assessed risk of bias and applicability concerns using the QUADAS‐2 domain‐list. We presented the results of estimated sensitivity and specificity using paired forest plots, and we summarised pooled estimates in tables. We used a bivariate meta‐analysis model where appropriate. We presented the uncertainty of accuracy estimates using 95% confidence intervals (CIs).

Main results: 

We included 51 studies with 19,775 participants suspected of having COVID‐19, of whom 10,155 (51%) had a final diagnosis of COVID‐19. Forty-seven studies evaluated one imaging modality each, and four studies evaluated two imaging modalities each. All studies used RT‐PCR as the reference standard for the diagnosis of COVID‐19, with 47 studies using only RT‐PCR and four studies using a combination of RT‐PCR and other criteria (such as clinical signs, imaging tests, positive contacts, and follow-up phone calls) as the reference standard.

Studies were conducted in Europe (33), Asia (13), North America (3) and South America (2); including only adults (26), all ages (21), children only (1), adults over 70 years (1), and unclear (2); in inpatients (2), outpatients (32), and setting unclear (17).

Risk of bias was high or unclear in thirty-two (63%) studies with respect to participant selection, 40 (78%) studies with respect to reference standard, 30 (59%) studies with respect to index test, and 24 (47%) studies with respect to participant flow.

For chest CT (41 studies, 16,133 participants, 8110 (50%) cases), the sensitivity ranged from 56.3% to 100%, and specificity ranged from 25.4% to 97.4%. The pooled sensitivity of chest CT was 87.9% (95% CI 84.6 to 90.6) and the pooled specificity was 80.0% (95% CI 74.9 to 84.3). There was no statistical evidence indicating that reference standard conduct and definition for index test positivity were sources of heterogeneity for CT studies.

Nine chest CT studies (2807 participants, 1139 (41%) cases) used the COVID-19 Reporting and Data System (CO-RADS) scoring system, which has five thresholds to define index test positivity. At a CO-RADS threshold of 5 (7 studies), the sensitivity ranged from 41.5% to 77.9% and the pooled sensitivity was 67.0% (95% CI 56.4 to 76.2); the specificity ranged from 83.5% to 96.2%; and the pooled specificity was 91.3% (95% CI 87.6 to 94.0). At a CO-RADS threshold of 4 (7 studies), the sensitivity ranged from 56.3% to 92.9% and the pooled sensitivity was 83.5% (95% CI 74.4 to 89.7); the specificity ranged from 77.2% to 90.4% and the pooled specificity was 83.6% (95% CI 80.5 to 86.4).

For chest X‐ray (9 studies, 3694 participants, 2111 (57%) cases) the sensitivity ranged from 51.9% to 94.4% and specificity ranged from 40.4% to 88.9%. The pooled sensitivity of chest X-ray was 80.6% (95% CI 69.1 to 88.6) and the pooled specificity was 71.5% (95% CI 59.8 to 80.8).

For ultrasound of the lungs (5 studies, 446 participants, 211 (47%) cases) the sensitivity ranged from 68.2% to 96.8% and specificity ranged from 21.3% to 78.9%. The pooled sensitivity of ultrasound was 86.4% (95% CI 72.7 to 93.9) and the pooled specificity was 54.6% (95% CI 35.3 to 72.6).

Based on an indirect comparison using all included studies, chest CT had a higher specificity than ultrasound. For indirect comparisons of chest CT and chest X-ray, or chest X-ray and ultrasound, the data did not show differences in specificity or sensitivity.

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