Radiologists decry overuse of CTA in suspected PE

Evidence is mounting that CT is overused for ruling out pulmonary embolism, exposing patients unnecessarily to radiation and hiking up healthcare costs. Acute pulmonary embolism is the third leading cause of cardiovascular death. Mortality rates are as high as 30% if a diagnosis is missed and the condition is left untreated, whereas mortality rates with treatment range from 2% to 10%. Not surprisingly, CT angiography has become a routine part of working up patients with chest pain.

But CT angiograms expose patients to radiation, which is of particular concern in young people and women, whose breasts receive a relatively high organ dose during the study. Research presented at the 2007 RSNA meeting suggests that PE is not very common and that ordering physicians should use clinical criteria and serum D-dimer tests more rigorously to triage patients for CT.

In one retrospective study, engineer Mark Mamlouk and colleagues from the University of Arizona found a relatively low 9.8% positive PE rate among 1022 emergency room patients and 981 inpatients scanned over an 18-month period at a 700-bed hospital. Inpatients were twice as likely to have a positive result compared with ER patients: 13.5% versus 6.4%.

Researchers found that risk factors were closely correlated with CT findings. In the 1806 negative studies, 62% of patients had no risk factors for PE. Of 197 positive results, however, only 15 (8%) had no risk factors. The risk factor most commonly associated with a positive pulmonary embolism result was immobilization. The serum D-dimer test had a sensitivity of 92.4% and negative predictive value of 98.7%.

"With no risk factors and a negative D-dimer test, it is extremely unlikely to have a positive CTA. This study suggests the frequency of ordering CT can be markedly reduced, with cost savings and reduced radiation exposure," Mamlouk said.

To ensure appropriateness, hospitals can ask clinicians to submit a checklist noting risk factors with the order for a CTA exam.

In another study from Rhode Island Hospital, radiologist Dr. Michael Corwin and colleagues retrospectively evaluated CT utilization patterns and the negative predictive value of D-dimer tests in a busy emergency room to test the effectiveness of appropriateness protocols for CT in suspected PE cases. They evaluated records for 5344 patients who had a D-dimer or CT from January 2003 to October 2005.

Guidelines for assessing risk for PE were based on the well-established Wells clinical criteria. According to the protocols, those with higher risk should go directly to CT. Those with low probability have a D-dimer test, and if this yields a negative result, the patient does not need further workup. Those who are at low risk but test positive should then go on to CT.

In the study group, there was high suspicion for PE in 30% of patients based on clinical factors, and in 70% suspicion was low. Of patients in the low suspicion group, 39% tested positive and 61% tested negative on the D-dimer test.

Though protocols indicate that all of those in the high suspicion group should go straight to CT, only 58% actually had this imaging study. It's possible that an alternative diagnosis was made in some of these cases, Corwin said. In those who had a negative D-dimer test and no risk factors, 7% had a CTA, against the direction of the protocols.

The prevalence of PE on CT was 6% overall and 9% in the high-suspicion group. In those with low suspicion and a positive D-dimer test, only 2% tested positive for PE on CT.

"Clinicians had a low-threshold evaluation of patients with suspected PE, and this results in a high number of false-positive exams," Corwin said.

Of 166 patients with a negative D-dimer result who then went on to have CT against protocols, only one tested positive for PE. Researchers calculated that in this study, the D-dimer test had a sensitivity of 95% and negative predictive value of 99%.