Why COVID-19 Causes False Positive CTPA Results for Pulmonary Embolism
COVID-19 pneumonia creates imaging patterns on CTPA that mimic pulmonary embolism, leading to false positives because the bilateral ground-glass opacities, consolidations, and peripheral wedge-shaped lesions characteristic of COVID-19 can be misinterpreted as perfusion defects or infarcts, particularly when the pretest probability of COVID-19 is high and community prevalence is elevated. 1, 2
The Core Problem: Overlapping Imaging Features
The fundamental issue is that COVID-19 pneumonia produces CT findings that closely resemble PE-related changes:
Wedge-shaped peripheral lesions with pleural base: COVID-19 characteristically produces wedge-like opacities with a pleural base, which is also the classic appearance of pulmonary infarction from PE 1
Bilateral peripheral distribution: Both COVID-19 pneumonia and PE can present with bilateral peripheral predominant abnormalities, making differentiation challenging 1, 2
Ground-glass opacities and consolidation: These patterns are typical for COVID-19 but can also be seen in PE-related hemorrhage or infarction 1
The Pretest Probability Problem
When community prevalence of COVID-19 is high, the positive predictive value of chest CT for PE drops dramatically, creating a large number of false-positive results. 2
In one prospective study of 612 patients, chest CT had a false-positive rate of 7.2% for COVID-19 diagnosis, with 74.3% of false-positive cases ultimately diagnosed with other pulmonary infections 3
The specificity of CT for COVID-19 was only 76.4%, meaning nearly 1 in 4 patients without COVID-19 had CT findings suspicious for the disease 3
The Confounding Factor: Actual PE in COVID-19
Paradoxically, COVID-19 patients have a genuinely elevated risk of PE (5.6% overall prevalence, 20% in clinically suspected cases), which complicates interpretation further. 4
In hospitalized COVID-19 patients at moderate-to-high risk by Wells score, PE prevalence reached 76% in one Italian series 5
Critically, 87% of PE in COVID-19 patients occurs within lung parenchyma already affected by COVID-19 pneumonia, making it nearly impossible to distinguish PE from COVID-19 parenchymal changes on non-contrast imaging 6
Specific Imaging Pitfalls
Several COVID-19 features create false-positive interpretations:
Crazy-paving pattern and interlobular thickening: These can be mistaken for mosaic perfusion patterns seen in chronic PE 1
Adjacent pleural thickening: This accompanies COVID-19 consolidations but can suggest pleural-based infarction 1
Linear opacities: COVID-19 produces linear opacities that may be confused with Hampton's hump or other PE-related findings 1
Clinical Decision Framework to Avoid False Positives
To minimize false positives, CTPA should only be performed when there are specific clinical indicators beyond typical COVID-19 presentation:
Perform CTPA when D-dimer levels are markedly elevated beyond what is expected for COVID-19 severity (PE patients had D-dimer levels 5.1 times higher than controls) 4
Consider CTPA when oxygen saturation is disproportionately low compared to the extent of parenchymal disease on non-contrast CT 6
Order CTPA when CT severity score shows extensive consolidation (>50% lung involvement) with clinical deterioration, as this correlates with higher PE risk 4, 6
Avoid CTPA in early-stage COVID-19 (first 0-4 days) when 56% of patients have normal CT, as PE is less likely and parenchymal changes haven't fully developed 1
Key Timing Considerations
The stage of COVID-19 infection affects false-positive rates:
Early phase (0-4 days): 56% have normal CT, making any abnormality more suspicious for alternative pathology 1
Intermediate phase (5-13 days): 76% show bilateral involvement with evolving consolidations, peak period for imaging overlap with PE 1
Advanced phase (>14 days): 88% show bilateral disease with crazy-paving and reverse halo signs that can mimic chronic PE 1
Practical Approach to Reduce False Positives
Combine CT findings with clinical and laboratory data rather than relying on imaging alone:
Require at least two of the following before ordering CTPA: Wells score ≥4, D-dimer >5x upper limit of normal for COVID-19 severity, oxygen saturation <90% despite appropriate supplementation, or clinical deterioration despite treatment 4, 6
When CTPA shows filling defects in areas of dense COVID-19 consolidation, consider that 87% of true PE in COVID-19 occurs in affected parenchyma, but correlation with clinical trajectory is essential 6
In resource-constrained settings, chest radiography should be used first, reserving CTPA for patients with specific features suggesting PE rather than COVID-19 progression alone 1, 7