Clinicians can reduce false positive CT angiogram diagnosis of pulmonary embolus by using the appropriate clinical decision rules to establish pretest probability.
From EMC podcasts #113 and #114, here is the decision rule that they recommend using :
Suggested diagnostic decision tool algorithm for pulmonary embolism.
There are a number of decision rules that are used as objective aids in the work up of PE. Wells and PERC (Pulmonary Embolism Rule out Criteria) are the two most commonly utilized tools in North American EDs. It is important to understand how the prevalence of PE in your population impacts decision making. Simply put, the prevalence of a disease can be considered the pre-test probability of the patient ruling in for that disease. The maximum suggested prevalence for PE in order to use the PERC rule is 7%. In other words, if there is a high prevalence of PE in your population, PERC may not be applicable.
The PROPER trial out of France, where the prevalence of PE is low, showed that gestalt performed similarly to PERC in terms of 3-month PE rate, but PERC resulted in an 8% decrease in unnecessary CT scanning, and a 40-minute decrease in ED stay . While studies have suggested that physician gestalt may be as accurate as these decision tools [11,12], there is an argument to be made that even seasoned docs should take the time to calculate these scores because even they can have a tendency to overestimate pretest probability at times.
An Algorithmic Approach
Once you have decided to test for PE, our experts suggest starting with Wells to get an idea of the pre-test probability.
1. If <2, use PERC
2. If 2-4, send D-dimer
3. If >4, consider a CTPA
It seems to me that the methodology that they used to determine their figures on the percentages of false positive diagnosis of was flawed.
Of the 937 CT Angiograms for PE performed by the study period, a panel of three subspecialist chest radiologists reviewed the 174 cases that the general radiologists diagnosed as positive. The negative studies were not reviewed so the review panel was not reading the scans blind as would a working radiologist. And they were designating themselves as the gold standard.
So I’m not sure how much their results on false positive PE can be applied to decision making.
That said, the article discusses what the clinician evaluating the patient can do to help the radiologist minimize the false positive diagnosis of pulmonary embolus. And they argue that the way to accomplish this is to use validated decision rules to determine the pretest probability of PE. And then only order the CTA if the pretest probability via the decision rule exceeds a certain percentage.
So here are some excerpts from the article, Overdiagnosis of Pulmonary Embolism by Pulmonary CT Angiography:
Because the clinical presentation [of pulmonary embolus] is often nonspecific and can be mimicked by a range of other conditions, in routine practice, pulmonary CT angiography (CTA) is often used as the imaging method of choice for further
investigation . Pulmonary CTA has been shown to be highly sensitive and specific when pretest clinical diagnostic tools are used  but surprisingly inaccurate in patients with
low pretest probability, with false-positive rates as high as 42% . Unfortunately, adherence to referral guidelines for pulmonary CTA has repeatedly been shown to be low [4,
5]. Nonetheless, many clinicians will initiate anticoagulation therapy on the basis of a positive result, regardless of pretest probability , even in isolated subsegmental PE .
The risk of hemorrhage related to anticoagulation
therapy is potentially significant. A large meta-analysis in 2003  found a 7% annual risk of major bleeding and a 0.4% incidence of bleeding-related fatality in patients treated with oral anticoagulation therapy for venous thromboembolism for longer than 3 months. The practical implications of
long-term anticoagulation therapy for the patient are also potentially significant, requiring frequent attendance to their medical practitioners for blood tests, consequent time off from work, potential adverse drug interactions with other medications, adjustments to travel and lifestyle, implications for future dental and medical procedures, and possible
negative effects on life insurance status.