[1] While the chest x-ray is normal in the majority of PE cases,[2] the Westermark sign is seen in 2% of patients.
[3] Essentially, this is a plain X-ray version of a filling defect as seen on computed tomography pulmonary arteriogram.
The sign results from a combination of: The Westermark sign, like Hampton's hump (a wedge shaped, pleural based consolidation associated with pulmonary infarction), has a low sensitivity (11%) and high specificity (92%) for the diagnosis of pulmonary embolism.
[4] Put more simply, the Westermark sign is seldom seen in pulmonary embolism.
That is, 33% of the time that Westermark sign is seen on Chest XRay does a pulmonary embolism actually exist [1].