westermark sign westermark sign in chest radiography, the westermark sign is a sign that represents a focus of oligemia (hypovolemia) (leading to collapse of vessel) seen distal to a pulmonary embolism (pe). while the chest x-ray is normal in the majority of pe cases, the westermark sign is seen in 2% of patients. westermark signdifferential diagnosishypovolemia 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 dilation of the pulmonary arteries proximal to the embolus and the collapse of the distal vasculature creating the appearance of a sharp cut off on chest radiography. sensitivity and specificity 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. put more simply, the westermark sign is seldom seen in pulmonary embolism. when visible on a chest x-ray, the positive predictive value is only 33%. that is, 33% of the time that westermark sign is seen on chest xray does a pulmonary embolism actually exist . etymology it is named after nils westermark, a swedish radiologist.