Radiologists decry overuse of CTA in suspected pulmonary embolism

November 27, 2007

Evidence is mounting that CT is overused for ruling out pulmonary embolism, exposing patients unnecessarily to radiation and hiking up healthcare costs, according to research presented Tuesday morning.

Evidence is mounting that CT is overused for ruling out pulmonary embolism, exposing patients unnecessarily to radiation and hiking up healthcare costs, according to research presented Tuesday morning.

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 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 from the University of Arizona, researchers analyzed the use of CTA for suspected PE in 2003 patients: 1022 emergency room patients and 981 inpatients. The scans were performed over an 18-month period at a 700-bed hospital.

In the overall patient population, only 197 of the 2003 CTAs were positive, which equates to a relatively low 9.8% positive rate, said Dr. Mark Mamlouk, who presented study results. Inpatients were twice as likely to have a positive result (13.5%) compared with emergency room referrals, which yielded only a 6.4% positive rate.

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, 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, researchers retrospectively evaluated CT utilization patterns and negative predictive value of D-dimer tests in a busy emergency room to test the effectiveness of appropriateness protocols for CT in suspected pulmonary embolism 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, said Dr. Michael Corwin, who presented the study results.

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%.

In a third study, researchers from the VA Maryland Health Care System found a relatively low number of positive results on CT pulmonary angiograms for suspicion of PE and questioned appropriateness of study orders.

Of 189 consecutive exams performed, only 20 were positive for PE (10.6%). The review also showed that CT results were not having the expected effect on patient management. Some patients who were on anticoagulation therapy and subsequently tested negative on CT nevertheless remained on the therapy. Conversely, others who tested positive on CT did not receive anticoagulation therapy.

It's important to note, however, that detection of incidental findings on CT, such as pneumonia and liver abscess, altered patient management in 20% to 30% of cases, said Dr. Amy Musk, who presented results.