The authors offer a few specific suggestions relevant to radiologists to improve image quality and account for technical issues, but their primary complaint was simply this: we scanned too many patients who did not have a PE. After subtracting the patients with false-positives, yield in this study was 129 of 937, or 13.7 percent, falling at the low end of most published performance characteristics. This prompted another recommendation: the best way to improve yields is to refer patients for scan only when they have a higher pretest likelihood of disease.
Explore This IssueACEP Now: Vol 34 – No 11 – November 2015
Referring appropriate patients for CT is, unfortunately, something we do terribly in the United States. A comparison of populations of patients evaluated for PE in several observational studies, with 3,174 patients in Europe and 7,940 patients in the United States, showed patients were reliably higher risk in European populations.4 Interestingly, this was most exaggerated in the clinical gestalt of treating clinicians: in Europe only a third of patients were thought to be low risk, while in the United States these totaled nearly two-thirds. The net effect in this study was an overall yield for PE of 28.1 percent in Europe compared with 7.1 percent in the United States. The PEs diagnosed in the United States were also generally less severe as stratified by the Pulmonary Embolism Severity Index, and PE-related deaths were likewise lower. The concise summary: we’re performing astounding numbers of negative CTs and finding less significant disease, and it’s almost certain our already-low numbers of positive results are further diluted by false-positives.
Using a validated diagnostic strategy, grounded in sound risk assessment, can reduce excessive testing. The fantastic Ali Raja, MD, leads a team that recently published updated American College of Physicians clinical guidelines for the evaluation of patients with suspected acute PE.5 These guidelines include most of the same strategies espoused in ACEP’s prior guideline but now updated to include age-adjusted D-dimer.6 The age-adjusted threshold, age × 10 ng/mL added to the generic 500 ng/mL in patients older than age 50, has been validated in multiple studies. Most recently, a review of a large cohort of Kaiser Permanente patients revealed a small handful of additional missed PEs, but the corresponding decrease in radiation exposure and contrast-induced nephropathy provided a net benefit.7 These authors did not account for the likelihood of false-positive CTs in patients with low pretest probability, and it is reasonable to suggest their study overstates the excess misses while understating the harm reduction.