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Original Research |
1 Department of Radiology, Erasmus MC, PO Box 2040, 3000 CA Rotterdam, The
Netherlands.
2 Department of Epidemiology and Biostatistics, Erasmus MC, Rotterdam, The
Netherlands.
3 Department of Radiology, Maastricht University Hospital and Cardiovascular
Research Institute Maastricht, Maastricht, The Netherlands.
4 Department of Vascular Surgery, Erasmus MC, Rotterdam, The Netherlands.
5 Department of Vascular Surgery, St. Catharina Hospital, Eindhoven, The
Netherlands.
6 Department of Radiology, St. Catharina Hospital, Eindhoven, The
Netherlands.
7 Department of Vascular Surgery, University Medical Centre, Nijmegen, The
Netherlands.
8 Department of Radiology, University Medical Centre, Nijmegen, The
Netherlands.
9 Department of Vascular Surgery, Maastricht University Hospital and
Cardiovascular Research Institute Maastricht, Maastricht, The
Netherlands.
10 Department of Health Policy and Management, Harvard School of Public Health,
Boston, MA.
Received January 22, 2007;
accepted after revision November 21, 2007.
Supported by a grant (nr. 945-01-039) from ZonMW, Netherlands Organization
for Health Research and Development and by a grant (nr. 904-66-091) from the
Netherlands Organization for Scientific Research (NWO).
Abstract
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MATERIALS AND METHODS. Of 984 patients assessed for eligibility, 514 patients with peripheral arterial disease were randomized to MR angiography (MRA) or duplex sonography in three hospitals and to MRA or CT angiography (CTA) in one hospital. The outcome measures included the clinical utility, functional patient outcomes, quality of life, and actual diagnostic and therapeutic costs related to the initial imaging test during 6 months of follow-up.
RESULTS. With adjustment for potentially predictive baseline variables, the learning curve, and hospital setting, a significantly higher confidence and less additional imaging were found for MRA and CTA compared with duplex sonography. No statistically significant differences were found in improvement in functional patient outcomes and quality of life among the groups. The total costs were significantly higher for MRA and duplex sonography than for CTA.
CONCLUSION. The results suggest that both CTA and MRA are clinically more useful than duplex sonography and that CTA leads to cost savings compared with both MRA and duplex sonography in the initial imaging evaluation of peripheral arterial disease.
Keywords: aorta • cost-effectiveness analysis • CT angiography • duplex sonography • MR angiography • peripheral arterial disease
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Diagnostic imaging is performed when PAD becomes lifestyle-limiting, and a revascularization procedure is considered. Noninvasive imaging tests, including duplex sonography, CT angiography (CTA), and MR angiography (MRA), are increasingly used for the initial evaluation of patients with PAD. Duplex sonography provides both anatomic and functional information about the arterial system and has been shown to be a reliable technique with fairly good sensitivity and specificity [2, 3]. However, duplex sonography is operator-dependent and does not provide a precise roadmap for planning treatment. Both MRA and CTA are relatively new noninvasive vascular imaging tests used in the diagnostic workup of PAD. Both techniques provide 3D images of the arterial system with high sensitivity and specificity [4–11]. Disadvantages of MRA include the higher investment cost for equipment, the small number of patients in whom the image is uninterpretable because of artifacts, and that some patients are claustrophobic or have a contraindication for MRI. The main disadvantages of CTA are the use of radiation, the use of potentially nephrotoxic iodinated contrast media, vessel wall calcifications that affect image interpretation, and the time-consuming 3D reconstruction techniques. The question arises as to which imaging test is preferred in the diagnostic workup of PAD.
To determine which noninvasive test is preferred as the initial imaging test in clinical practice, we need to take into account not only the diagnostic accuracy of each test, but also the related effects of diagnostic imaging tests on treatment planning and costs; and we need to show that improvement in functional capacity and quality of life are maintained with substitution of another diagnostic workup [12, 13]. For this purpose, we designed the Diagnostic Imaging of Peripheral Arterial Disease (DIPAD) randomized trial to compare outcomes after MRA versus the currently used test, which was either duplex sonography or CTA, as the initial imaging test in the diagnostic workup of patients with PAD. Primary outcomes evaluated were quality of life and costs. Secondary outcomes evaluated were clinical utility and functional patient outcomes. The analysis of the subgroup undergoing MRA versus CTA was reported separately [14], as was the analysis of the subgroup undergoing MRA versus duplex sonography [15]. Here we report the overall results of the three-way comparison.
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Patients were excluded if they had contraindications for MRA (e.g., pacemaker, cerebral vessel clipping, or claustrophobia) or CTA (e.g., severe renal insufficiency or adverse reactions to iodinated contrast material), or if they needed an acute intervention at the time of randomization, or if they had a previous imaging workup indicating that revascularization was needed.
Study Design
This was an empirically based and pragmatic multicenter randomized
controlled trial evaluating the costs and effects of noninvasive diagnostic
imaging in patients with PAD. That is, we designed the trial to reflect
clinical practice as it can be implemented rather than creating a strictly
controlled, but probably unrealistic, experimental setting
[13]. The study protocol was
approved by the hospital institutional review boards of all participating
centers, and informed consent was obtained from all patients.
The study was performed following Good Clini cal Practice guidelines [18]. Data were analyzed and reported in accordance with the CONSORT (Consolidated Standards of Reporting Trials) guidelines [19]. Patients meeting all eligibility criteria were randomly assigned to undergo MRA or the currently used test, which was duplex sonography in three hospitals and CTA in one hospital, as the initial imaging test. The analysis of MRA versus CTA was reported separately [14], as was the analysis of MRA versus duplex sonography [15]. Here we report the overall results of the three-way comparison. Randomization was performed centrally and took place through the trial coordinating center by telephone. Stratified randomization was used, stratifying for center. A computer-generated list for the strategy assignment was used. Eligible patients were enrolled by one of several researchers who were all unaware of the randomization sequence. After randomization, patients and clinicians were not blinded for the imaging strategy because this would have been impractical and inconsistent with our pragmatic study design.
Imaging Techniques and Evaluation
Duplex sonography was performed by qualified, experienced vascular
technologists under supervision of either a radiologist or a vascular surgeon.
On the basis of patient history and findings at physical examination, the
referring vascular surgeons deter mined the extent of the duplex sonography
exami nation (aortoiliac, femoropopliteal, or crural). The procedure was
performed with 5- and 7.5-MHz transducers. The hemodynamic significance of
lesions was graded by peak systolic velocity ratios, calculated as the peak
systolic velocity in the stenosis divided by the peak systolic velocity in the
prestenotic or post stenotic region. The technologists graded stenosis on a
5-point ordinal scale and recorded the findings on a standardized reporting
sheet.
All MR examinations were performed on a 1.5-T imager. A body coil or a dedicated peripheral vascular phased-array coil was used for signal reception. In three hospitals the protocol included bolus-chase MRA with a single biphasic contrast material injection, automated table movement, and real-time bolus monitoring. In one hospital (37 patients) a multiinjection protocol was used. All patients received 40 mL of contrast agent (gadopentetate dimeglumine [Magnevist, Schering], 0.5 mmol/mL). In all hospitals a subtraction technique was used before maximum intensity projections (MIPs) were generated.
CTA was performed on a 16-MDCT scanner. A bolus-tracking technique was used with automated table movement and automated bolus detection. Before generating MIPs, a segmentation technique was used to remove bone structures and vessel wall calcifications.
Radiologists with extensive experience in interpreting MRA and CTA evaluated all MR and CT images for arterial stenosis or other disorder. All images were evaluated without knowledge of further workup.
Measurement of Quality of Life
Health-related quality of life was assessed using a self-administrated
questionnaire sent to all patients at the time of randomization and 2 weeks, 3
months, and 6 months after the initial imaging test. The questionnaires
contained the EuroQol-5D (EQ-5D), the Rating Scale (rating scale), the generic
Medical Outcomes Study 36-item Short Form Health Survey (SF-36), and the
disease-specific VascuQol.
The EQ-5D covers five health dimensions—mobility, self-care, usual activities, pain and dis comfort, and anxiety and depression—which give a total of 243 health states. Using a published population-based utility function, a single index score was calculated for each patient [20]. A value of 0 equals death and a value of 1 equals maximum health.
The rating scale is an evaluation instrument and consists of one question in which the patient is asked to rate his or her current state of health on a scale from 0 to 100, where 0 represents death and 100, perfect health [21].
The SF-36 is a multiitem scale and covers eight health dimensions [22]. On the basis of a previous study, we determined that physical functioning, role functioning limitations due to physical prob lems, bodily pain, and general health were the relevant dimensions to describe the health status of PAD [23]. Each dimension is valued on a 100-point scale, in which 0 means death and 100 indicates maximum health.
The VascuQol is a disease-specific descriptive quality-of-life instrument especially for patients with PAD and contains five domains (activity, symptom, pain, emotion, and social functioning) [24]. These five domains give a total score, which is valued on a 7-point scale, in which 1 means poor quality of life and 7 indicates maximum health.
For each patient we compared the scores of the different quality-of-life measures at 2 weeks, 3 months, and 6 months of follow-up with the baseline score of that particular measure, which resulted in a mean improvement for each quality-of-life measure. We used standard rules for item recoding, treat ment of missing items, and scoring [20–22, 24].
Measurement of Costs
For the cost analysis, we collected information concerning all relevant
items of medical care (i.e., diagnostic and therapeutic) used by each patient
during the entire trial. The cost of diagnostic imaging included the initial
imaging test, all additional vascular imaging, and the associated hospital
admissions. The therapeutic cost included costs for percutaneous vascular
interventions (i.e., percutaneous angioplasty, stent placement, and
thrombolysis), vascular surgery (i.e., aortic bifurcation reconstruction,
bypass surgery, endarterectomy, and amputation), and associated hospital
admissions. Furthermore, we assessed the costs for outpatient visits during 6
months of follow-up. All costs were computed from the hospital perspective
according to the Dutch guidelines for cost calculations in health care
[25].
Diagnostic costs can be divided into directly and indirectly assignable costs. Directly assignable costs include personnel costs, material costs such as film, and equipment costs. Personnel costs were computed using the measured time spent on a diagnostic imaging test for each involved personnel category and the mean wage rates from our hospital. Social security of 37% of the wage was added in accordance with national guidelines. Costs of materials used in diagnostic procedures were based on cost prices and summed. The annuitized costs [26] of the radio logy equipment and the annual equipment servicing costs were summed and divided by the proportion of the total available room time (80% of a 40-hour workweek) [25, 26]. Costs were discounted at a rate of 3% per annum [27].
Indirectly assignable costs include costs of supporting departments, the
facility space costs, and overhead costs. Information on costs of supporting
departments was obtained from records of our financial and economics
department. The facility space costs were computed for the involved radiology
rooms by multiplying the surface space with the facility space costs of
204 per square millimeter per year. The overhead costs for MRA, duplex
sono graphy, and CTA were estimated to be 15% of directly assignable costs
[25].
The costs of percutaneous vascular interventions were measured and calculated in a similar fashion. We obtained unit costs of surgery from another study with a comparable study domain and setting to calculate an overall cost per patient per surgical procedure [28]. For a limited number of surgical procedures performed in our study, the unit costs were not available from that article and we had to estimate these costs using the published values as a starting point (oral communication, van Sambeek MRHM). The number of days of hospital admission and the number of outpatient visits were collected, and the associated costs were calculated using national estimates of hospital ad mission, ICU admission, and outpatient visits [25]. All costs were reported in euros for the year 2002.
Measurement of Clinical Utility
We assessed the therapeutic confidence of vasc ular radiologists and
surgeons during the weekly vascular conference at which the findings of the
initial imaging test were discussed, and each clinician was asked to rate his
or her individual confidence in making a well-founded therapeutic choice on a
10-point rating scale. The attendance at the weekly conference varied, but
each case was rated by at least one interventional radiologist and one
vascular surgeon. To adjust for variability in using the rating scale, we
normalized scores from each physician
[29].
Furthermore, we measured the recommendations for additional imaging (duplex sonography, digital subtraction angiography, MRA, or CTA) during the vascular conference. Physicians were free to choose the imaging technique that they thought was necessary and the most helpful. Any additional vascular imaging test performed within 60 days after the initial test was noted. In addition, all addi tional vascular imaging tests performed during 6 months of follow-up were collected.
Measurement of Functional Patient Outcomes
The brachial, dorsal pedal, and posterior tibial arterial systolic
pressures were assessed using a blood pressure cuff and continuous-wave
Doppler sonography, both before starting and immediately after completion of
the treadmill test, to determine resting and postexercise ABIs. To calculate
the ABI, the highest ankle pressure was divided by the highest brachial
pressure. A treadmill test, based on a standard constant-load protocol, was
performed to assess the maximum walking dis tance. The patients walked until
they had to stop because of leg pain or until they reached the time limit.
Both ABI and maximum walking distance were measured at baseline and after 6
months of follow-up.
Furthermore, we assessed the change in clinical status during 6 months of follow-up. For this purpose, we used the criteria for reporting significant change in clinical status according to Rutherford et al. [30]. These criteria are a combination of standard clinical categories with objective ABIs.
Improvement in ABI and change in clinical status during the trial period were assessed for the treated leg only. If both legs or neither leg was treated, we selected the leg with the most severe symptoms at baseline. In case a patient had the same symptoms of both legs at baseline, we selected a leg at random.
Statistical Analyses
For each moment in time, we calculated the response rate of the
quality-of-life questionnaires. Furthermore, we entered 20% of both the
quality-of-life data and the data of the case record form twice in the
database to calculate the entry error.
The intention of the study was to show cost savings for the diagnostic
workup while quality of life and other patient outcomes are not detrimentally
affected. The sample size calculation was therefore primarily based on
quality-of-life outcomes between the new strategy with MRA versus the
currently used workup strategy. At the same time, we ensured that the sample
size would be sufficient to show diagnostic cost differences. With adequate
treatment, approximately 40–50% of patients could be expected to have
substantial improvement of their symptoms after 6 months as measured by the
physical functioning and pain attributes of the SF-36
[23]. A percentage difference
of 10–15% would be considered clinically relevant. A sample size of 488
would be required to avoid missing a percentage difference of at least 15% if
45% of patients improve using an 
value of 0.05, a power of 0.90, and a
two-tailed test. We verified that this sample size would also be sufficient to
show a difference in diagnostic costs between any two strategies of 200
(SD, 350) and a difference in SF-36 scores of 10 points (SD, 15 points)
with an value of 0.05, a power of 0.90, and a two-tailed test.
The results were analyzed according to the intention-to(-diagnose-and)-treat principle. For con tinuous variables, statistical significance of differ ences among the three groups was evaluated using analysis of variance. The statistical significance of differences in dichotomous variables among the three groups was assessed using the chi-square test. We determined the statistical significance of differ ences in improvement in primary and secondary outcomes among the three groups with multivariable and logistic regression. Differences in improvement among the three groups are presented with adjustment for predictive baseline characteristics, learning curve of physicians (i.e., increasing experi ence with interpretation of new imaging techniques over time), and hospital setting. Based on previous studies [31] and on clinical experience, we assumed that severity of disease (critical ischemia vs clau dication), renal disease (i.e., renal insufficiency and renal transplantation), cerebrovascular disease, cardiac disease, and diabetes mellitus at baseline were potentially predictive of the outcomes. To adjust for the learning curve of the physicians, we included the rank order of the initial imaging tests in the re gression analysis. We expressed the rank order by ranking the dates when the initial imaging tests were performed. To analyze the improvement in quality of life, ABI, and maximum walking distance during follow-up, we also adjusted for the baseline scores of these outcome measures. A one-way sensitivity analy sis was performed for the diagnostic costs by exploring the effect on the outcome of using alternative plausible estimates (50% and 200%, re spectively) of the investment costs of radiology equipment.
For all outcome measures, we used mean imputation for missing values. A p value of 0.01 was considered statistically significant for the quality-of-life outcomes and the costs because multiple measures were tested in these groups of outcomes. For other tests, a significance level of 0.05 was used. Calculations were performed with SPSS version 11.0 (SPSS) for Windows (Microsoft).
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Quality of Life
The response rate of the quality-of-life questionnaires was 99% at
baseline, 93% at 2 weeks, 89% at 3 months, and 89% at 6 months of follow-up.
The improvement in all quality-of-life measures from baseline to 2 weeks, 3
months, and 6 months of follow-up was not statistically significant among the
groups (Table 2). However, a
consistent difference was seen in one direction among all (except one)
quality-of-life measures of slightly more improvement in the CTA group
compared with the MRA group.
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Costs
The mean unit cost of the individual imaging tests was 104 (SD,
38) for all duplex sonography tests, 472 (133) for all MRA
examinations, and 163 (18) for all CTA examinations performed
during the trial. For the additional vascular imaging tests, the mean unit
cost for all diagnostic digital subtraction angiography examinations
(including hospital stay) was 1,207 (542). The total diagnostic
costs per patient were 206 higher in the duplex sonography group than in
the CTA group, which was not a statistically significant difference when
considering the multiple comparisons that we performed (p = 0.04).
However, the total diagnostic costs per patient were significantly higher in
the MRA group than in the duplex sonography group (difference, 138 [95%
CI, 31–245]; p = 0.01) and CTA group (difference,
344 [182–506]; p < 0.001;
Table 3). This increase in
diagnostic costs was not caused by more costs for additional imaging but by
the higher unit costs of the initial imaging test in the MRA group
(Table 3).
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With one-way sensitivity analysis, only the difference in total diagnostic costs between the MRA and the duplex sonography groups was sensitive to variation of the investment costs for radiology equipment (50% and 200% of the baseline estimate, respectively) (Table 4). If the investment costs of MR equipment were 50% of the baseline estimate, the difference in total diagnostic costs between the MRA and the duplex sonography groups was no longer statistically significant, but the difference between the MRA and CTA groups was still significant. Note that the difference in diagnostic costs between the CTA and the duplex sonography groups also changed with varying the investment costs of MR equipment. This is explained by the additional MRA examinations in the duplex sonography group. The costs for percutaneous interventions and surgical procedures were not statistically significant among the groups when considering the multiple comparisons that we performed (Table 3). The costs for outpatient visits were comparable among the groups (Table 3). The total costs, including diagnostic, therapeutic, and outpatient visit costs, were significantly lower in the CTA group than in the MRA group (p = 0.001) and compared with the duplex sonography group (p = 0.01). The total costs were comparable between the MRA and the duplex sonography groups (p = 0.6; Table 3).
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Therapeutic Confidence and Additional Imaging
The mean therapeutic confidence for vascular radiologists and surgeons in
making a therapeutic choice on a 10-point rating scale was 8.1 (SD, 1.4) for
MRA, 8.0 (1.1) for CTA, and 7.5 (1.7) for duplex sonography. The therapeutic
confidence was significantly higher for MRA than for duplex sonography
(difference, 0.8 [95% CI, 0.5–1.1]; p < 0.001) and for CTA
compared with duplex sonography (difference 1.0 [0.4–1.5]; p
< 0.001). Within 60 days after the initial imaging test, on average, more
additional vascular imaging tests per patient were performed in the duplex
sonography group than in the MRA group (0.23 vs 0.08 more imaging tests;
p < 0.001) and compared with the CTA group (0.23 vs 0.06 more
imaging tests; p = 0.01). During the total follow-up of 6 months,
this difference was 0.19 more imaging tests per patient for duplex sonography
compared with MRA (0.42 vs 0.23 more imaging tests; p = 0.001) and
0.22 for duplex sonography compared with CTA (0.42 vs 0.20 more imaging tests;
p = 0.03). No significant differences were seen in the confidence or
the number of additional vascular imaging tests between the MRA group and the
CTA group.
Ankle–Brachial Index, Maximum Walking Distance, and Clinical Status
The difference in improvement in ABI, maxi mum walking distance, and
clinical status from baseline to 6 months' follow-up was not statistically
significant among the groups (Table
5). For ABI, maximum walking distance, and clinical status, there
was a consistent difference in one direction of slightly more improvement in
the CTA group compared with the MRA group and a preponderance of slightly more
improvement in the CTA group compared with the duplex sonography group.
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The mean diagnostic costs were significantly higher in the MRA group than in the duplex sonography and the CTA groups, which is explained by the higher costs of the initial imaging test. MRA is more expensive than duplex sonography and CTA because of higher investment costs, construction costs, costs for the contrast agent, and personnel costs. A reduction in the investment costs of MR equipment would make the difference in total diagnostic costs of MRA compared with the duplex sonography strategy insignificant, but the CTA strategy would still be cost-saving compared with MRA. Furthermore, the average costs for treatment in the CTA group were significantly lower than in either of the two other groups, possibly because a more efficient treatment plan could be made on the basis of the CT images, although we do not have a definitive explanation.
We found that the therapeutic confidence for MRA and CTA was higher than for duplex sonography. A probable explanation is that both MRA and CTA provide a precise roadmap for planning treatment, whereas duplex sonography provides interpreted data on a schematic drawing. Probably as a result of the lower confidence in duplex sonography, physicians requested additional vascular imaging tests more frequently in the duplex sonography group than in the MRA and CTA groups.
A cohort study has traditionally been used for the evaluation of new diagnostic imaging tests by performing both the new test and the reference test in all patients to determine the sensitivity and specificity. For duplex sonography, a sensitivity of 88% and a specificity of 95% have been reported [3]. For both MRA and CTA, a sensitivity between 91% and 98% and a specificity between 92% and 99% have been reported [4–11]. However, these results are difficult to translate into a meaningful clinical decision with respect to which diagnostic strategy should actually be implemented. A decision about the usefulness of a diagnostic strategy requires either a decision analysis or a randomized controlled trial [12]. Although randomized controlled trials are not frequently used to evaluate diagnostic tests, we found our pragmatic randomized trial to be both feasible and inexpensive [13, 32–35].
We acknowledge several limitations of our study. Although eligible patients were randomized between MRA and duplex sonography in three hospitals and between MRA and CTA in one hospital, we also compared the duplex sonography group with the CTA group. CTA is a relatively new test and was performed in only one hospital. We think that the comparison between duplex sonography and CTA is valid because both groups were randomized in one study with the same inclusion and exclusion criteria. Furthermore, the baseline characteristics were comparable between these two groups and we adjusted for predictive variables. In addition, duplex sonography is an operator-dependent test performed by technologists and does not provide a roadmap of the arteries, whereas MRA and CTA do. We chose a pragmatic approach in which the tests were compared as they are performed in clinical practice. Optimizing duplex sonography and the transfer of information from the examination to the clinician may make duplex sonography more useful than we have been able to show.
Another limitation of the study was that although patients were randomized, there were differences between the MRA group and the other two groups at baseline in EQ-5D and the dimension general health of SF-36. To calculate the difference in improvement of quality of life, we adjusted for the baseline quality-of-life scores. Minor differences existed among the groups at baseline in diabetes mellitus, cardiac disease, renal disease, and critical ischemia. These baseline differences were not statistically significant, but we thought it would be prudent to adjust for predictive baseline variables that may lead to differences in outcomes [36, 37]. Therefore, adjustment for potentially predictive variables in a multivariable or logistic regression was used to correct the estimates of the outcomes for any imbalance that by chance may have occurred among the randomized groups. Furthermore, imaging techniques were different among the hospitals. For this reason, we adjusted for hospital setting in the regression analysis.
A possible limitation was that patients and physicians were not blinded for group allocation. At the same time, the goal of our study was to evaluate the outcomes of the diagnostic tests as they are used in routine clinical practice. Patients could not be blinded, and blinding of the treating physicians—for example, by transferring the diagnostic information to a schematic drawing—would have introduced an artificial step that could have affected diagnostic interpretation and therapeutic planning. Furthermore, although sche matic drawings are used in routine clinical practice as adjuncts, they are not used solely when the imaging test provides a good roadmap.
Finally, the costs were calculated from a hospital perspective rather than a societal perspective. There is international consensus that an economic evaluation should be performed from the societal perspective [27]. A societal perspective implies that not only the costs in the health care sector, but also the direct (i.e., patient costs) and indirect costs (i.e., costs of production losses) outside the health care sector must be included in the cost analysis [27]. In our study, we chose the hospital perspective because other studies that assessed the costs related to the management of PAD showed that patient costs were low in both the Dutch and U.S. settings [38, 39]. Furthermore, in the setting of PAD, the costs of production losses are negligible because most patients are retired [40].
In conclusion, our results suggest that both MRA and CTA are clinically more useful than duplex sonography, and that CTA leads to cost savings compared with both MRA and duplex sonography in the initial imaging evaluation of patients with PAD.
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