The Role of CT in the Diagnosis of COVID-19
A state of constant flux
After the first cases of disease due to the SARS coronavirus 2 (CoV-2) in China became known, a polymerase chain reaction (PCR) test for CoV-2 was quickly developed; however, testing capacities were limited, with long lag times to test results. This led to frequent nosocomial transmissions of CoV-2 between patients, as well as from patients to hospital staff, which contributed significantly to the spread of the infection (1). In Germany, the first large number of infected persons was reported after a carnival event in the municipality of Gangelt. The surrounding hospitals, and in particular the Aachen University Hospital (Universitätsklinikum Aachen), were the first to be confronted with a large number of suspected cases of COVID-19 (COVID, coronavirus disease).
In their large and carefully conducted prospective systematic study, Schulze-Hagen et al. have now shown that, in comparison to PCR, chest computed tomography (CT) can achieve both high sensitivity and high specificity for a primary diagnosis of COVID-19 pneumonia in symptomatic patients (2).
The high specificity of CT for COVID-19 in this study as compared to the first results from China (3) is remarkable. Note that these good results could also be due to the relatively high prevalence of the disease in the early phase of the pandemic at the Gangelt hotspot. Thus, while the test’s sensitivity should also remain very high in other scenarios, its specificity could be significantly lower if the disease prevalence is low.
Clearly distinguishable changes in the lungs
The typical radiological changes in COVID-19 are prominent, mostly round ground glass opacities, with a late-stage prominent “crazy paving” pattern and lung consolidation, with bilateral, multifocal, peripheral, and posterior distribution. It is important to note here that these lung changes differ significantly from those caused by pulmonary bacterial or fungal infections. Typical findings in COVID-19–positive patients also clearly differ from manifestations caused by other viral pathogens. The Thoracic Imaging Section of the German Radiological Society describe these changes in detail in their recommendations and advise using a structured diagnostic approach (4).
Underlying the unusual CT morphology of CoV-2 are pathophysiological and immunological processes that have not previously been observed for any other pathogen. Even though it is a respiratory infection, the respiratory epithelium is not significantly damaged in the early stage of the disease. Instead, capillaritis has been observed, with subsequent capillary leak and fluid leakage into the pulmonary interstitium; this can be either due to infiltration of endothelial cells in the area of the pulmonary capillaries caused by the virus itself (subsequent to viremia), or to secondary damage to the endothelium caused by the immune response (5). The various stages can also be morphologically detected using CT.
Use for specific issues
Especially in advanced stages of COVID-19, the results of PCR from a nose or throat swab can be negative (6, 7). The study by Schulze-Hagen et al. now shows that, in this case, CT was able to rapidly confirm a clinically-founded suspected diagnosis of COVID-19, which could be later confirmed by PCR using deep airway samples or by antibody tests. Due to the convincing results, the authors envision using CT in parallel to the PCR test and refer to its use as a primary test in the event of shortage of PCR testing capacities.
In Germany, sufficient testing capacities for CoV-2 are now available. A delay in diagnosis due to unavailable tests or to long lag times for test results is therefore increasingly unlikely. Thus, routine use of CT as an additional method for diagnosis of COVID-19 in addition to swab/PCR tests does not seem to make sense. Rather, CT can be used in a targeted manner to address specific issues.
- Clinically urgent suspicion of COVID-19 but a negative PCR diagnosis. CT can be used to confirm suspicion, with the resulting consequences for protective measures and treatment.
- Clinical symptoms that match COVID-19 with a risk constellation (age, comorbidities) (9), and PCR results that are not yet available or are negative. CT can be used as a supplement to determine a differential diagnosis and to exclude COVID pneumonia.
- Suspected pulmonary vascular complications from COVID-19, and especially pulmonary embolism (10).
The role of CT in the different scenarios of the COVID-19 pandemic is dynamic and subject to constant change, and should thus be adapted to the respective regional requirements and available resources. While CT testing was an instrument of primary diagnosis in the initial phase of the pandemic, it now takes its place alongside medical history, clinical examination, and diagnostic virology. Of note, performing CT on patients with COVID-19 must be associated with an expected added value in the further management of the patient, due to the additional risk of infection for the hospital staff and the radiation exposure of the patient.
Conflict of interest statement
The authors declare that no conflict of interest exists.
Translated from the original German by Veronica A. Raker.
Prof. Dr. med. Tobias Welte
Department of Respiratory Medicine
Hannover Medical School (MHH)
30625 Hannover, Germany
Cite this as:
Kauczor HU, Welte T: The role of CT in the diagnosis of COVID-19—a state of constant flux. Dtsch Arztebl Int 2020; 117: 387–8. DOI: 10.3238/arztebl.2020.0387
Translational Lung Research Center Heidelberg, German Center for Lung Research (DZL): Prof. Dr. med. Hans-Ulrich Kauczor
German Center for Lung Research (DZL), Hannover Medical School (MHH): Prof. Dr. med. Tobias Welte
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