Despite Early Data, CT Alone Not Appropriate for COVID-19 Diagnosis

April 21, 2020

A fourth-year medical student writes about the use of CT for diagnosing COVID-19.

Adam Sturts, MSIV

A myriad of scientific reports, news articles, and editorials have evoked both discussion and confusion for clinicians trying to make sense of coronavirus disease 2019 (COVID-19) screening and diagnostics.

The Centers for Disease Control and Prevention (CDC) recommends against the use of chest CT for the screening and diagnosis of COVID-19. Notwithstanding, the role of CT has been a subject of debate during the ongoing outbreak. The following perspectives and recommendations provide direction for frontline clinicians.

A new study, titled “Chest CT and Coronavirus Disease (COVID-19): A Critical Review of the Literature to Date” published in American Journal of Roentgenology, sought to determine the role of CT in screening and/or diagnosis of COVID-19 based on review of the most frequently cited studies reporting CT sensitivity and specificity. A group of 10 authors from 6 US institutions contributed to the “Clinical Perspective.”

In the study, which was led by Constantine Raptis, MD, of Washington University School of Medicine, authors highlighted the wide range of CT sensitivity values that have been reported. Some of these measurements may be over-estimated in the context of selection bias. For example, the authors noted in their review that it was often unclear why patients were selected for CT testing—this is crucial information for clinicians as clinical context affects sensitivity measurement.

“If a study cohort contains patients who are more likely to have a true-positive finding and less likely to have a false-negative finding, sensitivity will be overestimated,” authors noted.

Also contributing to sensitivity over-estimation was the use of CT as a binary test with an undefined and low threshold for positivity. As described in a previous article, the data was collected from sites of outbreak. Therefore, radiologists may have been more likely to make a diagnosis of COVID-19 resulting in high sensitivity measurement.

None of the studies reviewed by the authors reliably reported high specificity of CT in differentiating COVID-19 pneumonia from other diseases with similar CT findings; thus limiting the use of CT as a confirmatory diagnostic test.

Raptis suggested the aforementioned studies still hold value and reporting the CT features of COVID-19 remains “an important first step” in helping radiologists identify infected patients, but noted the danger of over-interpretation of certain results.

“Test performance and management issues arise when inappropriate and potentially overreaching conclusions regarding the diagnostic performance of CT for COVID-19 pneumonia are based on low-quality studies with biased cohorts, confounding variables, and faulty design characteristics.”

Based on their review, the authors concluded that the radiology literature on COVID-19 does not substantiate the use of CT as a diagnostic test for COVID-19.

This critical review supports the notion that low quality observational and hospital based series should not be used for population inference. These studies include only a small percentage of the globally aggregated cases and, because of that, the findings are not generalizable to larger populations living in different environments.

To provide guidance in the absence of quality evidence, the CDC, American College of Radiology, the Radiological Society of North America, the Society of Thoracic Radiology, and the American Society of Emergency Radiology have released recommendation statements to guide CT use.

Most recently, the Fleischner Society shared “The Role of Chest Imaging in Patient Management during the COVID-19 Pandemic: A Multinational Consensus Statement from the Fleischner Society.” The statement, published in CHEST and Radiology, provides context for the use of CXR and CT imaging in the management of COVID-19 based on three clinical scenarios.

Representing opinions and perspectives of radiology, pulmonology, intensive care, emergency medicine, laboratory medicine and infection control experts practicing in 10 countries, the guidance that it provides is broadly applicable and perhaps the most clinically useful thoracic imaging recommendations thus far.

The panel answered 14 key questions corresponding to eleven decision points within 3 clinically driven scenarios based on the anticipated value of the information that thoracic imaging would be expected to provide. The 3 clinically driven scenarios were defined by the severity of the patients’ presenting features, pre-test probability, and the presence of medical resource constraints. The scenarios are presented as easily interpretable decision pathways.

Results were aggregated, resulting in 5 main and 3 additional recommendations to guide the use of CXR and CT in the management of COVID-19.

Essential recommendations are listed below:

  1. Imaging is not indicated in patients with suspected COVID-19 and mild clinical features unless they are at risk for disease progression.
  2. Imaging is indicated in a patient with COVID-19 and worsening respiratory status.
  3. In a resource-constrained environment, imaging is indicated for medical triage of patients with suspected COVID-19 who present with moderate-severe clinical features and high pre-test probability of disease.

It is important to note that the diagnostic algorithm always begins with either RT-PCR or the point of care rapid COVID-19 test, which are both considered first-line tests for active infection.

Clinicians faced with the circumstances described in essential recommendation 3 should use imaging for triage—however, suspicion for other conditions must remain high.

The recommendations did not distinguish between CXR and CT. Clinical context and resource availability should guide modality selection. CXR appears to be poorly sensitive early in the disease course. Comparatively, CT has a better sensitivity at this stage. However, in a resource-constrained environment where CT access is limited, CXR may be preferred unless features of respiratory worsening warrant the use of CT.

The authors acknowledged that the statement represents opinion at a moment in time within an environment where the status of regional epidemics and the availability of critical resources can vary daily.

As an aside, I would like to thank the expert clinicians and researchers from around the world who are working overtime to synthesize vital guidance from the limited data that is currently available. I would also like to thank all of the healthcare workers risking their lives to preserve the health of others.

Adam Sturts, MSIV is a fourth-year medical student at Rowan University School of Osteopathic Medicine. This piece reflects the author’s views and not necessarily those of the publication.

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