How to differentiate COVID-19 pneumonia from influenza and bacterial pneumonia on CT?

Kanas Chan
28 Mar 2023

Arched bridge and/or vacuole signs on CT may support a diagnosis of COVID-19 pneumonia and assist in differentiation from influenza and bacterial pneumonia, a retrospective study by the Chinese University of Hong Kong has revealed.

Common CT features of COVID-19 pneumonia are nonspecific and could overlap with those of influenza or bacterial pneumonia. “COVID-19 pneumonia and influenza or bacterial pneumonia distinctly differ in terms of their disease course, temporal progression, and available therapeutics. Therefore, there is a need for early and accurate differentiation between these entities,” wrote the researchers. [Hong Kong Med J 2023;29:39-48]

According to previous reports, the arched bridge sign (ie, ground-glass opacities or consolidation with an arched margin outlining unaffected lung parenchyma) and vacuole sign (ie, focal oval or round lucent area [typically <5 mm] present within ground-glass opacities or consolidation sites) could be distinct CT features of COVID-19 pneumonia.  [AJR Am J Roentgenol 2020;214:1287-94; Quant Imaging Med Surg 2020;10:1551-8.]

arched bridge sign

To evaluate the diagnostic value of arched bridge and vacuole signs, CT images of 187 patients with pneumonia were independently reviewed by two radiologists in the study. The incidence of arched bridge and vacuole signs were compared between patients with COVID-19 pneumonia (n=66), those with influenza pneumonia (n=50), and those with bacterial pneumonia (n=71).

Patients with COVID-19 pneumonia were much more likely to exhibit the arched bridge sign and/or the vacuole sign compared with patients with influenza pneumonia or bacterial pneumonia (arched bridge sign: 63.6 percent vs 8.0 percent and 5.6 percent; both p<0.001) (vacuole sign: 21.2 percent vs 2.0 percent and 1.4 percent; both p≤0.005). As CT was performed mainly during admission (mean, 5.3 days after admission), the authors suggested that the arched bridge and vacuole signs generally appeared at an early stage of pneumonia.

“[Importantly], both signs were seen together in 16.7 percent of patients with COVID-19 pneumonia, but they did not occur together in patients with influenza or bacterial pneumonia,” highlighted the researchers. “Our findings suggest that the presence of both arched bridge and vacuole signs strongly supports a diagnosis of COVID-19 pneumonia.”

However, the arched bridge and vacuole signs are not prognostic biomarkers, as there were no significant differences in total hospitalization duration in COVID-19 pneumonia patients with or without these signs (both p>0.05).

Notably, the arched bridge and vacuole signs exhibited high specificity (93.4 percent and 98.4 percent, respectively) for identifying COVID-19 pneumonia, with moderate or low sensitivity (63.6 percent and 21.2 percent, respectively). They also showed high positive predictive values (84.0 percent and 87.5 percent, respectively) and high or moderate negative predictive values (82.5 percent and 69.6 percent, respectively).

“Although underlying pathophysiological mechanisms behind these signs remain unclear, the morphological appearances of arched bridge and vacuole signs may indicate different pathophysiological lung-sparing mechanisms that occur during infection-related pneumonia,” noted the researchers. “Identification of these signs in clinical practice may be useful in increasing suspicion or providing confirmatory evidence to support a diagnosis of COVID-19 pneumonia.”

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