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Original Research |
1 Department of Radiology, Oregon Health Science University, 3183 SW Sam Jackson
Park Rd., Portland, OR 97239-3098.
2 Oregon Health Science University School of Medicine, Portland, OR.
3 Present address: Stanford University Medical Center, Stanford, CA.
4 Present address: Department of Diagnostic Radiology, Bridgeport Hospital and
Yale New Haven Health, Bridgeport, CT.
Received May 12, 2007;
accepted after revision February 26, 2008.
Address correspondence to M. M. Costantino
(costanti{at}ohsu.edu).
Abstract
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MATERIALS AND METHODS. We reviewed a total of 575 CT angiograms obtained to evaluate for PE at a large level 1 trauma teaching hospital from January 2004 through March 2005. Various clinical settings were used for 267 inpatient (46%), 258 emergency department (45%), and 50 outpatient (9%) studies. We excluded CTA performed for other reasons, repeated CTA, and patient records with incomplete clinical data. On the basis of chart review in which the investigators were blinded to final diagnoses, pretest probability of PE according to the Wells criteria was retrospectively assigned to each patient. D-dimer values, when obtained, also were reviewed. The diagnosis of PE was based on final CTA reports.
RESULTS. PE was diagnosed in 9.57% of 575 patients. Positivity rates by location were 32 (12%) of the 267 inpatients, 22 (8.5%) of the 258 emergency department patients, and one (2.0%) of the 50 outpatients. Three (< 1%) of the 575 patients had high probability of PE, even though 351 patients had gone directly to CTA. Of the other 572 patients, 158 (28%) had intermediate and 414 (72%) low probability of PE. In the high, intermediate, and low probability groups, two (67%), 24 (15%), and 29 (7%), respectively, of the patients had PE. A D-dimer assay was performed for 224 (39%) of the 575 patients. Thirty-nine (17%) of the 224 patients had normal results (< 0.5 µg/mL); 107 (48%), intermediate results (0.6–2.0 µg/mL); and 78 (35%), abnormal results (> 2.0 µg/mL). In the emergency department cohort, 151 (59%) of 258 patients underwent a D-dimer assay. Thirty-two (21%) of the 151 patients had normal results; 81 (54%), intermediate results; and 38 (25%), abnormal results. Only one patient with a normal D-dimer level and three patients with intermediate D-dimer levels had PE, the equivalent of 3% of each group. The number of CTA examinations ordered for patients with normal and intermediate D-dimer results was 146 (25% of the 575 total studies). Twenty-two (8%) of the 258 emergency department patients had PE, and clinical suspicion of PE was high for 11 (50%), intermediate for 10 (45%), and low for one (5%) of those patients.
CONCLUSION. Our data showed suboptimal use of the Wells criteria and subjective overestimation of the probability of PE before ordering of CTA. Although a definitive acceptable PE positivity rate for CTA has not been established, the 10% yield represents overuse of CTA as a screening rather than a diagnostic examination.
Keywords: CT angiography • D-dimer assay • emergency department • pulmonary embolism • radiation
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The perceived risks associated with a missed PE diagnosis with regard to morbidity and mortality leave treating physicians with the challenge of how best to approach patients with symptoms and signs that may be associated with PE. Clinical concern may be lagging behind the evolving ideas of PE. Newer articles [4] and pathology teaching show PE is not as severe a diagnosis as once thought. Although this concept is controversial, it is possible that the lungs are meant to act as a filtering device, preventing small emboli from traveling to end organs such as brain, kidneys, and bowel.
Because subsegmental emboli are being detected with high sensitivity, in some cases images show a physiologic process [5]. Instead of reporting CTA findings as either positive or negative for PE, perhaps radiologists should describe the degree of pulmonary arterial obstruction, which can be correlated with blood gas values to determine the clinical significance of small subsegmental emboli [6]. The widespread use of sensitive and specific 16- and 64-MDCT and increased detection of subsegmental emboli present an interesting challenge for treating physicians in deciding whether to manage incidental subsegmental PE in patients without symptoms or small subsegmental PE in patients with symptoms out of proportion to a small clot burden.
With the advent of helical CT, CTA has evolved into the new reference standard for diagnosis of PE. Two of the most widely used scoring systems are the Wells score [7] and the Geneva clinical probability [8]. Numerous studies [9–14] have yielded data ascribing a high sensitivity and specificity to CTA and showing acceptable clinical outcome among patients with normal findings on CTA. CTA is fast, easy, reliable, diagnostic, and widely available. These attributes have led to a significant increase in the number of patients undergoing imaging for PE since the late 1990s. At our institution, this marked increase in use of CTA seemed to be paired with a concomitant decrease in the overall positivity rate for PE in the scans obtained. The reason for this decrease in positivity may be that some physicians have begun using an overly ag gressive diagnostic strategy for patients at low risk. These physicians may be motivated by the ease and availability of CTA and by perceived fears surrounding a missed diagnosis and its medicolegal ramifications.
Several scoring systems and algorithms have been developed to better assign pretest PE risk categories and facilitate management decisions. Implementation of these algo rithms in clinical practice, however, is seemingly inconsistent, and CTA may be increasingly used more as a screening study in the care of patients with suspected cardio thoracic disease than as a more appropriate later step of diagnostic evaluation in the presence of truly intermediate to high pretest probability of PE. Problems with this approach arise with respect to the increasing costs of unnecessary imaging studies and the potential carcinogenic and atherosclerotic effects of ionizing radiation on radiosensitive tissues, such as thyroid, breast, and coronary arteries, that often are not considered or deemed relevant to acute care.
The purpose of this study was to evaluate the use of CTA in the evaluation of PE at our institution, to determine the positivity rate, and to correlate this rate with the use of D-dimer assay and pretest probability based on established diagnostic algorithms. We performed a retrospective review of a total of 575 pulmonary CTA examinations performed to rule out PE within a 14-month period with correlation of retrospective application of the Wells criteria, clinical presentation, and D-dimer results.
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CTA results were categorized as positive or negative for PE. Patient demographics, including age, sex, and location (inpatient, outpatient, emergency department), were recorded. Monoclonal antibody D-dimer results were categorized as normal (0–0.5 µg/mL), intermediate (0.6–2.0 µg/mL), or abnormal (> 2.1 µg/mL) according to our standard laboratory assay (Liatest, Stago). At our institution, clinicians interpret an intermediate result as indeterminate; therefore, these patients undergo CTA. Equipment for the D-dimer assay was readily available in the emergency department, and results were reported from the laboratory within approximately 20 minutes.
A single observer blinded to final diagnosis used the clinical information from the electronic medical record to retrospectively assign the Wells criteria. The following data points were obtained: clinical signs and symptoms of deep venous thrombosis, heart rate greater than 100 beats/min, surgery or immobilization for longer than 3 days in the previous 4 weeks, previous PE or deep venous thrombosis, hemoptysis, therapy for malignant dis ease in the previous 6 months, or pal liative care. Seven patients had D-dimer values documented in the medical record, but the D-dimer results were excluded from the study for the following reasons, all in one case each: hemolyzed sample, sample in which a lipemic plasma substance rendered the sample nondiagnostic, D-dimer ordered 3, 5, or 6 days before CTA, D-dimer ordered at an outside hospital that used an assay different from ours, and fibrinogen measurement ordered. All seven of these patients underwent CTA. Two of the seven had CTA findings of PE: the patient who underwent fibrinogen measurement and the patient for whom D-dimer assay was ordered 6 days before CTA.
CTA was preformed on 16- and 64-MDCT scanners (Brilliance, Philips Healthcare). After a 20-mL timing bolus over the main pulmonary artery, 100 mL of contrast medium, either isoosmolar (iodixanol, Visipaque, GE Healthcare) or nonionic low osmolar (iohexol, Omnipaque 300, GE Healthcare) was administered IV. Caudocranial 1-mm helical images were acquired from the thoracic inlet to the adrenal glands at 140 kV, 120 mAs, and 2-mm axial reconstructions. Coronal reconstructions were available for review at the discretion of the interpreting radiologist. Fellowship-trained board-certified thoracic radiologists interpreted the CTA images. All studies were interpreted on a PACS workstation. The diagnosis of PE was based on final CTA reports. There was no additional review of images during our study.
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Among the 575 patients, only one patient with a normal D-dimer value and three patients with an intermediate D-dimer value had PE, the equivalent of 2% (3/146) of all of the patients with normal and intermediate D-dimer values. The number of CTA examinations ordered for patients with low and intermediate D-dimer values accounted for 146 (25%) of the total 575 studies ordered. In the emergency department, 22 (8%) of the 258 patients had PE. Clinical suspicion of PE was high for 11 (50%), intermediate for 10 (45%), and low for one (5%) of those 22 patients.
Among the 351 (61%) of the 575 patients for whom D-dimer was not measured, 36 (10%) had PE. Among the 351 patients who did not undergo D-dimer assay, 240 (68%), 109 (31%), and two (< 1%) had low, intermediate and high, respectively, clinical probability of PE. Our emergency department purports to routinely use the Wells criteria in the evaluation of patients with suspected PE. Therefore, we assume that all patients who did not undergo D-dimer assay but had PE were designated as having high clinical probability of PE by the ordering clinician, because this is the cohort who goes straight to CTA in evaluation for PE.
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Our data show wavering adherence to the clinical guidelines. Thirty-one patients who on retrospective application of Wells criteria were assigned low clinical probability and on presentation had a normal D-dimer result underwent CT. Thirty of these patients had CTA findings negative for PE. Eight intermediate-probability patients also had normal D-dimer results and underwent CTA, all with negative results for PE. A total of 351 (61%) of the CTA studies were performed on patients who had low or intermediate clinical probability of PE and went directly to CTA with no D-dimer assay ordered. If the algorithm had been followed by clinicians, these patients would have to be assumed to have high clinical probability; however, only 36 (10%) of the 351 had CTA findings positive for PE. In comparison, Wells et al. [7] found that approximately 66.7% of high-probability patients had positive CTA findings for PE. Interestingly, in our study the rates of positive PE findings among patients retrospectively designated as having low, intermediate, and high prob ability of PE were 7%, 15%, and 67%. This finding mir rors the incidence of PE in each of these groups according to the study by Wells et al. [7] (positive rate, 3.6%, 20.5%, and 66.7%). We believe this finding lends validation to our retrospective assignment of clinical probability.
Our data indicate the incidence of PE among patients with intermediate D-dimer levels is extremely low. The incidence of PE among patients with low or intermediate clinical probability and normal or intermediate D-dimer values was 2.7%, or four cases among 146 patients. This finding supports the hypothesis that an intermediate D-dimer concentration is a negative predictor of the presence of PE.
The positivity rate for PE from our data was 10%. Although a definitive acceptable positivity range for CTA in the evaluation of acute PE has yet to be established, we believe a positivity rate of 10% is on the low side, especially in consideration of the large number of studies preformed on patients with low or intermediate probability of PE, both with normal D-dimer levels and for whom a D-dimer assay was not ordered. The not negligible risk associated with CTA, predominately radiogenic cancer, must be considered.
CT pulmonary angiography has been reported [15] to result in a minimum radiation dose of 2.0 rad (20 mGy) to the breasts of an average-size woman. This dose approximates 10–25 two-view mammograms and as many as 100–400 chest radiographs [16–19]. Data from the fifth Biological Effects of Ionizing Radiation report [20] show an increase of four new cases of cancer per 10,000 persons exposed to 1 rad (10 mGy) of radiation, or eight cases of radiogenic cancer per 10,000 CT examinations. These numbers are of particular importance to women younger than 35 years, who are at the most risk from radiation and who, along with pediatric patients, should avoid unnecessary exposure.
With rigorous implementation of the Wells criteria and appropriate use of the D-dimer assay, the total number of CTA examinations performed for PE could be significantly reduced. In our study, the number of CTA examinations could have been decreased at least 25% if CTA had not been performed on patients with low or intermediate probability of PE who had normal D-dimer results. The reduction likely would have been greater if D-dimer had been measured for the 351 (61%) patients who actually had low or intermediate probability but went directly to CTA.
The strengths of this study include the large sample size, consistent application of the Wells criteria by a single reviewer, and consistency in performance of the D-dimer assay for all patients. In addition, that all CTA images were read by one of two board-certified fellowship-trained thoracic radiologists decreased interobserver variability. The major weakness of this study was retrospective assignment of clinical probability by persons other than the physician evaluating the patient. However, the positivity rate for PE among the three groups is close to the positivity rate reported for each of the three groups in the study by Wells et al. [7], lending validity.
In conclusion, CTA is fast, diagnostic, and widely available for evaluation of acute PE. The Wells criteria are used for clinical stratification of patients and guidance of evaluation for PE. Our data show suboptimal use of the Wells criteria and subjective overestimation of PE probability before CTA is ordered. The finding of normal D-dimer values is not deterring unnecessary CTA. A paradigm shift has occurred whereby clinical tools are supplanted by laboratory values and imaging that, although noninvasive, are not without cost or risk. Modern algorithms are changing; less emphasis is being placed on clinical presentation and signs and symptoms and more on imaging and laboratory results, decreasing risk to the physician. Although a definitive acceptable PE positivity rate for CTA has not been established, we believe that the rate of 10% represents overuse of CTA as a screening rather than diagnostic test. This practice equates with ineffective resource utilization, increased health care costs, and unnecessary radiation and contrast exposure. On the basis of our data, a prospective study is being implemented at our institution whereby our physician order entry system will require any physician who orders CTA to complete an electronic calculation of the Wells score and Geneva clinical probability. Results of further large-scale investigations of PE positivity rates may help define an acceptable positivity rate.
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