In the vast ocean of medicine, few diagnostic dilemmas descend so quickly into madness as does pulmonary embolism (PE). In the classical teaching, PE remains one of a handful of life-threatening diagnoses considered in the context of chest pain or shortness of breath. The proliferation of advanced imaging technology has also dramatically eased evaluation for PE, leading to an explosion of testing. Sadly, the cumulative effect of such expanded testing appears to be a pervasive preponderance of negative studies and low-yield, but costly, utilization.

And, frankly, it’s even worse than we’ve acknowledged.

The vast majority of PEs are diagnosed using one test, the computed tomography (CT) pulmonary angiogram. This test gained widespread acceptance with the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) studies, demonstrating adequate sensitivity for PE compared with conventional angiography.¹ Sensitivity is a valuable test attribute for a disease believed to have a high case-fatality rate. However, as technology has improved, CT has begun detecting smaller and smaller clots. By assigning the same clinical significance across the disease severity spectrum, it becomes unclear whether this improved sensitivity benefits our patients and whether our test specificity is adequate for our current strategy.

The problem is twofold, and two specialties are complicit in this predicament: radiology and emergency medicine. In radiology, the subsegmental PE is the culprit. As vessel size decreases, the quality of opacification and contrast capture diminishes. This results in consistent ambiguity regarding the presence of a flow-limiting lesion.

For example, a group of authors in Pennsylvania reviewed 415 images from their institution judged diagnostic for PE, focusing mostly on segmental and subsegmental PE.² Using five radiologists, four of whom were subspecialty trained in thoracic radiology, each image was individually re-reviewed. Based on their sample of 192 images read initially as segmental PE, a majority of authors could not agree on a positive finding in 5.7 percent of cases. For subsegmental PE, at least one reviewer dissented in 60 percent of cases. When compared with the original community radiologist’s official read, the consensus was a false-positive rate of 3.6 percent for segmental PE and 15 percent for subsegmental PE.

A second radiology department, this time in Ireland, reviewed 174 CTs reported positive for PE.³ Three subspecialty-trained thoracic radiologists subsequently reviewed each of the studies read initially by one of 15 general radiologists. In this study, 45 (25.9 percent) cases were judged erroneously reported positive, including 26.8 percent segmental and 59.4 percent subsegmental. The authors reported the most common causes of diagnostic error were technical image-acquisition artifacts underappreciated by the general radiologists.