Podcast: How accurate is chest imaging for diagnosing COVID-19?

Our programme of rapid reviews for COVID-19 covers both the diagnosis and treatment of the disease and the reviews are being updated as new evidence becomes available. Among these is a review on imaging tests for diagnosing the condition, and we asked the lead author of the March 2021 update, Nayaar Islam from The Ottawa Hospital Research Institute in Canada, to describe the latest findings in this podcast.

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Monaz: Hello, I'm Monaz Mehta, editor in the Cochrane Editorial and Methods department. Our programme of rapid reviews for COVID-19 covers both the diagnosis and treatment of the disease and the reviews are being updated as new evidence becomes available. Among these is a review on imaging tests for diagnosing the condition, and we asked the lead author of the March 2021 update, Nayaar Islam from The Ottawa Hospital Research Institute in Canada, to describe the latest findings in this podcast.

Nayaar: People suspected of having COVID-19 need to know quickly whether they are infected, so that they can receive appropriate treatment, self-isolate and inform close contacts. However, accurately diagnosing COVID-19 is challenging, and the value of imaging tests, such as chest CT, X-ray and ultrasound, remains undefined. In the November 2020 version of our review, the evidence showed that chest CT may have limited capability to differentiate COVID-19 from other causes of lung infection. Because the research on this topic is rapidly evolving, we updated our review to further investigate these findings.
Our most recent search allowed us to review 51 studies in people suspected of having COVID-19 that were published up to the end of September 2020. In these studies, people had the imaging test of interest, as well as the current best available test to diagnose an infection, the RT-PCR test.
Most of the studies tested chest CT scans and the results of these 41 studies show that the scans gave a correct diagnosis of COVID-19 in 88% of people who had the disease, and an incorrect diagnosis of COVID-19 in 20% of people who didn’t. The number of incorrect diagnoses in people without the disease has notably decreased compared to our earlier findings, meaning that chest CT is better at differentiating COVID-19 from other causes of lung infection than previously thought.
Nine studies tested chest X-rays, including one that also tested chest CT scans. Their results showed that chest X-ray gave a correct diagnosis of COVID-19 in 81% of people with the disease and an incorrect diagnosis of COVID-19 in 29% of people without it. This means that chest X-ray performs moderately well in identifying COVID-19, but has limited capability in differentiating it from other causes of lung infection.
Five studies evaluated ultrasound of the lungs, including two that also tested chest CT scans and one that also tested chest X-ray. We found that ultrasound of the lungs correctly diagnosed COVID-19 in 86% of people with the disease, and incorrectly diagnosed COVID-19 in 45% of people without it. This means that ultrasound performs well in identifying COVID-19 but cannot distinguish it from other causes of lung infection.
Looking at all the studies we reviewed, there were notable differences across their results and most had a high or unclear risk of bias. However, we believe that one important conclusion from this update is that CT scans perform well in identifying COVID-19, but still have limited capability in differentiating COVID-19 from other causes of lung infection. 
In summary, the diagnostic accuracy of chest CT scans is better now than in our previous review. This may be because of the stricter study inclusion criteria we used this time, because radiologists reading the scans now use better‐developed definitions of a positive CT result, or because the recent studies in our updated review have likely built on knowledge from previous experience and research. We will continue updating this review as more evidence becomes available.

Monaz: If you would like to read more about the current evidence, and watch for future updates of this review as the evidence moves forward, it’s available free to view at Cochrane Library dot com. Just go to the website and type in a search for ‘thoracic imaging and COVID-19’ to find it.

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