Clinical Question: How accurate is multi-detector computed tomography (CT) for pulmonary embolism (PE)?

Setting: Emergency department

Study Design: Diagnostic test evaluation

Synopsis: CT technology continues to evolve, moving from single-slice CT to four- or 16-slice multidetector scans. In this study, 824 patients with suspected PE underwent a standard clinical evaluation using the Wells clinical decision rule, computed tomographic angiography (CTA), CTA combined with venous-phase imaging (CTA-CTV), ventilation perfusion scanning, venous compression ultrasonography of the legs, and pulmonary digital subtraction angiography, if necessary. Patients were drawn from a group of 7,284 patients with suspected PE, but large numbers were excluded because they could not complete testing within 36 hours, had abnormal renal function, declined to participate, were using anticoagulants, or were otherwise unable to complete the protocol.

The mean age of participants was 51 years; 65 percent were white; and 62 percent were women. Defined by the composite reference standard (high-probability ventilation perfusion scan, abnormal digital subtraction angiography result, or abnormal venous ultrasonography and nondiagnostic ventilation perfusion scan), 192 (23 percent) had a PE. Among those who had PE ruled out using this reference standard, only two had a likely PE during the six-month follow-up. Also, 51 had CTA that was of insufficient quality, and 87 had a CTA-CTV of poor quality and were excluded from the analysis. The CTA was 83 percent sensitive and 96 percent specific (positive likelihood ratio [LR+] = 19.6; negative likelihood ratio [LR–] = 0.18), and the CTA-CTV was 90 percent sensitive and 95 percent specific (LR+ = 16.5; LR– = 0.11).

It is important to note that the predictive value of the tests depended on the clinical assessment. The Wells rule was used to stratify patients as high, intermediate, or low risk. The positive predictive value of CTA and CTA-CTV was 96 percent, but the negative predictive value was only 60 to 82 percent. For patients with a low clinical probability, the positive predictive value was 57 to 58 percent, whereas the negative predictive value was 96 to 97 percent. Values for positive and negative predictive value in intermedate probability patients were between 89 and 92 percent.

Bottom Line: Patients with high or intermediate probability of PE and an abnormal result on CTA or CTA-CTV are highly likely to have a PE. Those with low or intermediate probability and a negative CTA or CTA-CTV result are unlikely to have a PE. All other patients—those with discordant findings between the clinical examination and CTA or CTA-CTV—need further testing or close clinical follow-up to confirm or exclude the diagnosis. Clinical evaluation using a validated decision rule remains an important part of the evaluation. (Level of Evidence: 2b)