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American Journal of Respiratory and Critical Care Medicine Vol 167. pp. 39-44, (2003)
© 2003 American Thoracic Society

Original Article

Cost-Effectiveness Analysis of Diagnostic Strategies for Suspected Pulmonary Embolism Including Helical Computed Tomography

Arnaud Perrier, Mathieu R. Nendaz, François P. Sarasin, Nigel Howarth and Henri Bounameaux

Medical Clinic 1; Division of Medical and Surgical Emergencies; Division of Angiology and Hemostasis; Department of Internal Medicine, Division of Radiodiagnosis; and Department of Radiology, Geneva University Hospital, Geneva, Switzerland

Correspondence and requests for reprints should be addressed to Arnaud Perrier, M.D., Medical Clinic 1, Geneva University Hospital, 24, rue Micheli-du-Crest, CH-1211 Geneva 14, Switzerland. E-mail arnaud.perrier{at}medecine.unige.ch


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We performed a formal decision analysis to evaluate the cost-effectiveness of various strategies for pulmonary embolism, including helical computed tomography (CT), and determined the most cost-effective schemes for each clinical probability of pulmonary embolism. Other tests included D-dimer (DD), lower limb venous ultrasound (US), ventilation–perfusion (/) scan, and angiography. Outcome measures were 3-month survival and costs per patient managed. Baseline sensitivity of CT was 70%, corresponding to the performance of single-detector CT, and that figure was raised in sensitivity analysis to account for the expected higher sensitivity of newer multidetector CT scanners. All strategies were compared with a reference strategy, namely the / scan in all patients followed when nondiagnostic by an angiogram. For low clinical probability patients, the most cost-effective strategy was DD, US, and / scan, patients with a nondiagnostic / scan being left untreated. Replacing / scan by CT was also cost-effective. For intermediate and high clinical probability patients, a fourth test must be added, either CT or angiography in patients with nondiagnostic / scan, or angiography in patients with a negative helical CT. When using sensitivity figures above 85% (in the multidetector range), DD, US, and CT became the most cost-effective strategy for all clinical probability categories. Helical CT as a single test was not cost-effective. In summary, including helical CT in diagnostic strategies for pulmonary embolism is cost-effective provided that it is combined with DD and US. In contrast, helical CT as a single test is not cost-effective.

Key Words: pulmonary embolism • fibrin fibrinogen degradation products • ultrasonography, doppler, duplex • tomography scanners, x-ray computed • cost-effectiveness analysis


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Pulmonary embolism remains a diagnostic challenge. Until recently, a ventilation–perfusion (/) lung scan was the cornerstone of the diagnostic workup, a normal or near-normal scintigram excluding pulmonary embolism (1, 2), and a high-probability lung scan considered sufficient to establish the disease (3). However, lung scan is diagnostic only in 30 to 40% of patients, and pulmonary angiography, although still the definitive diagnostic standard for pulmonary embolism, is costly and invasive (4). Hence, numerous strategies for pulmonary embolism, including noninvasive instruments, have been proposed (5–7). Clinical assessment allows stratifying patients into three categories corresponding to an increasing prevalence of pulmonary embolism and individualizing the diagnostic workup according to the clinical probability category (1, 3, 5). Lower limb venous compression ultrasonography is noninvasive and highly specific for deep-vein thrombosis (8) and reveals a deep-vein thrombosis in 30 to 50% of patients with pulmonary embolism (9, 10). Plasma D-dimer (DD) measurement is increasingly used to exclude pulmonary embolism, and enzyme-linked immunosorbent assays (11, 12) and some automated turbidimetric assays (13, 14) have a sensitivity of 97 to 99% for acute pulmonary embolism. Finally, helical computed tomography (CT) is already widely used to diagnose pulmonary embolism (15, 16).

Cost-effectiveness analysis is ideally suited to compare schemes combining those diagnostic tools, and several have been published previously (17–20). Although they evaluated numerous strategies, they did not incorporate the patient's clinical probability of pulmonary embolism and adopted optimistic figures for the performance of helical CT (17, 18). Indeed, baseline sensitivity of CT was 89% (18) and 95% (17), whereas it was only 70% in two recent and more methodologically robust series from the literature (21, 22). Therefore, we studied the cost-effectiveness of incorporating single-detector helical CT in the diagnostic approach of pulmonary embolism, either as a single test, or as a substitute for / lung scan or pulmonary angiography in a sequential diagnostic strategy, including DD and ultrasound (US). Moreover, we evaluated the most cost-effective strategy for the three levels of clinical probability of pulmonary embolism. Finally, we assessed whether using multidetector CT, which should have a higher sensitivity, might modify the ranking of the strategies in sensitivity analysis.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Decision Model
We used a decision analysis software (Decision Analysis by TreeAge, version 3.0.5; TreeAge Software, Inc., Williamston, MA) to create a model representing alternative strategies for diagnosing pulmonary embolism. The decision model is represented in Figure 1 .



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  		Figure 1. Structure of the decision tree. Angio = pulmonary angiography; DVT = deep-vein thrombosis; neg = negative; NonDx = nondiagnostic; normal-near N = normal or near normal; PE = pulmonary embolism; pos = positive; Rx = treatment.

 
Assumptions.
The following assumptions were made:
  1. Patients with a normal DD level (below 500 µg/L as determined by an enzyme-linked immunosorbent assay) were left untreated, whereas those with an abnormal DD level (>= 500 µg/L) proceeded to other tests.
  2. Patients in whom lower limb venous US showed a deep-vein thrombosis were treated without further testing, whereas those with a normal US proceeded to further tests.
  3. In strategies including / lung scan, patients with a high-probability lung scan were treated, and those with a normal or near-normal lung scan were left untreated.
  4. All tests were conditionally independent.
  5. To facilitate the results' interpretation, we selected / scan ± angiography (discussed later here) as the reference strategy. Arbitrarily, we considered any strategy with a less than 0.5% difference in 3-month survival compared with the reference strategy of acceptable effectiveness.
  6. All effective strategies that were cost-saving compared with the reference strategy (/ scan ± angiography) were considered cost-effective.

Strategies.
The following strategies were evaluated (Figure 1):

  1. / scan ± angiography: All patients undergo a lung scan. Patients with a nondiagnostic lung scan undergo pulmonary angiography.
  2. CT: All patients undergo a helical CT as a single test and are treated according to the CT scan result.
  3. US ± CT: Lower limb venous US is the initial test. Patients with a normal US undergo a helical CT and are treated according to the CT scan result.
  4. DD ± US ± CT: DD is the initial test in all patients, followed by US in patients with an abnormal DD level. Those with a normal US undergo a helical CT and are treated according to the CT scan result.
  5. DD ± US ± CT ± angiography: This strategy is similar to DD ± US ± CT except that patients with a negative CT are submitted to a pulmonary angiogram and are treated according to the angiogram result.
  6. DD ± US ± / scan: DD is the initial test in all patients, followed by US in patients with an abnormal DD level. Those with a normal US undergo a / lung scan. Patients with a nondiagnostic / lung scan are left untreated.
  7. DD ± US ± / scan ± angiography: This strategy is similar to the DD ± US ± / scan, but patients with a nondiagnostic / scan undergo a pulmonary angiogram and are treated according to the angiogram result.
  8. DD ± US ± / scan ± CT: This strategy is similar to DD ± US ± / scan ± angiography in which helical CT replaces pulmonary angiography.

Because patients are managed differently according to the clinical probability of pulmonary embolism in most recent algorithms (1, 5, 23), all analyses were performed for three levels of prevalence of pulmonary embolism corresponding to a low, intermediate, and high clinical probability of pulmonary embolism.

Summary of Data Used for Analysis
Tables 1 and 2 summarize the baseline probabilities, utilities and costs used in the analysis, and the range of values tested in sensitivity analysis. Those data were extracted from a literature search of the Medline database completed by a manual search of the references cited in the retrieved articles. The data are presented and discussed in more detail in the online data supplement.


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  		TABLE 1. Test characteristics: values (baseline and range used in sensitivity analyses) used in the decision model

 

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  		TABLE 2. Outcomes: values (baseline and range used in sensitivity analyses) used in the decision model

 
Outcome Measures
We selected the 3-month quality-adjusted expected survival as the main outcome measure for effectiveness. For example, untreated patients free of pulmonary embolism had a 100% 3-month expected survival, whereas those with treated pulmonary embolism had an expected survival of 100% minus the mortality associated with treated pulmonary embolism (8%) minus that of a 3-month course of anticoagulants (0.2%), that is, 91.8%. A quality-adjustment factor used in the previously published literature was used to take into account the consequences of permanent sequelae after a major bleed (24) (Table 2). The strategy / scan ± angiography, in which all patients with a nondiagnostic lung scan undergo an angiogram (3), was selected as the reference strategy in terms of effectiveness. The proportion of angiograms required in each strategy was also calculated. Sensitivity analyses were performed to test the stability of the results over a wide range of clinically relevant values (Tables 1 and 2).

Costs
We considered the direct costs, that is, those involving the health care system. Indirect costs such as the loss of earnings were not considered. Cost data were actual costs, not charges, extracted from the database of our hospital, a 1,300-bed primary to tertiary-care urban facility, during the fiscal year 1996. All of these costs are expressed in United States dollars. Mean costs of treatment per patient with pulmonary embolism in our institution are $5,982 and include costs of tests, hospital stay, inpatient and outpatient treatment and monitoring, and costs of major bleeding complications. The costs of individual tests vary widely between countries, and costs are fixed values in each location. Therefore, rather than evaluating random cost variations in sensitivity analysis, we calculated the expenses associated with each strategy for three sets of costs: those from our center and recent cost estimates from Canada (18) and the United States (20) (Table 3) .


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  		TABLE 3. Costs of tests and treatment in various locations (in usd)

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Baseline Analyses
Results are presented independently for each clinical probability of pulmonary embolism (low, intermediate, and high) (Figure 2) .



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  		Figure 2. Three-month survival and average costs per patient yielded by diagnostic strategies for patients with low clinical probability (A), intermediate clinical probability (B), and high clinical probability (C) of pulmonary embolism. The solid line corresponds to the effectiveness of the reference strategy (/ scan ± angiography), and the dotted line represents an arbitrary 0.5% difference in 3-month survival with the reference strategy within which a strategy is considered as acceptable. The data used to plot the graphs are available on the online data supplement.

 
Low clinical probability of pulmonary embolism.
In patients with a low clinical probability (10%), Figure 2A shows that all strategies were reasonably effective, considering that the 3-month survival remained within a range of 0.5% mortality (see ASSUMPTIONS) compared with the reference strategy (/ scan ± angiography, $1,728). The cheapest strategy was DD ± US ± / scan ($845), followed by DD ± US ± CT ($1,230). All other strategies required the performance of a fourth test in the sequence (CT or angiography) without yielding a significant survival benefit. CT as a single test was the least effective strategy (0.34% difference in 3-month survival with the reference strategy) and was less cost saving ($1,503). In summary, for patients with a low clinical probability of pulmonary embolism, the most cost-effective strategy was DD ± US ± / scan, followed by DD ± US ± CT.

Intermediate clinical probability of pulmonary embolism.
When clinical probability of pulmonary embolism was intermediate (Figure 2B), the most cost-effective strategy was DD ± US ± / scan ± CT, in which a nondiagnostic / scan is followed by helical CT ($2,674). That strategy allowed us to forego pulmonary angiography in all patients. DD ± US ± / scan ± angiography was also cost saving compared with the reference strategy ($2,832 versus $3,164, respectively) and required an angiogram in 29% of patients. Of note, DD ± US ± CT ± angiography ($3,214) was not cost saving compared with the reference strategy and required an angiogram in 35% of patients. All other strategies were associated with a higher 3-month mortality in the intermediate clinical probability subgroup. CT as a single test was clearly associated with the highest mortality. In summary, the most cost-effective strategies for intermediate clinical probability patients were DD ± US ± / scan ± CT, followed closely by DD ± US ± / scan ± angiography. DD ± US ± CT ± angiography was not cost saving.

High clinical probability of pulmonary embolism.
In patients with a high clinical probability of pulmonary embolism (Figure 2C), DD ± US ± / scan ± CT remained the cheapest strategy ($4,308) but was of marginal effectiveness (difference in 3-month survival with the reference strategy, 0.47%). DD ± US ± / scan ± angiography, requiring an angiogram in 25% of patients, was also cost-effective ($4,598 versus $4,866 for the reference strategy). DD ± US ± CT ± angiography was effective, but it was as expensive ($4,867) as the reference strategy while requiring an angiogram in 25% of patients.

Sensitivity Analyses
We performed sensitivity analyses on all variables of the model for each clinical probability of pulmonary embolism (low, intermediate, and high). Overall, the ranking of strategies was not affected by any variation within the range of values described in Table 1. However, some of the variables had an impact on the strategies that were marginally effective in the baseline analysis.

Diagnostic performance of helical CT.
Lowering the sensitivity of CT to 62%, the lower value of the 95% confidence interval found in our previous study using single-detector CT (21), did not affect the ranking of strategies in the low clinical probability of pulmonary embolism category. Nevertheless, to account for the expected higher sensitivity of multidetector CT, we also performed the analysis using higher sensitivity values for helical CT. Angiography was no longer necessary after CT and US (DD ± US ± CT strategy) for a CT sensitivity above 76% in intermediate clinical probability patients and above 85% in the high clinical probability subgroup. The corresponding figures for the CT alone strategy were 87% and 92%. Figure 3 shows these values in intermediate clinical probability patients. A lower specificity of CT had little impact on effectiveness but entailed higher costs due to a higher proportion of patients treated unnecessarily, whatever the sensitivity of helical CT.



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  		Figure 3. Influence of the sensitivity of helical CT on the 3-month mortality in patients with an intermediate clinical probability of pulmonary embolism. Results are expressed as the difference in mortality with the reference strategy (/ scan ± angiography). The dotted line represents an arbitrary 0.5% difference in 3-month survival with the reference strategy (/ scan ± angiography) within which a strategy is considered acceptable.

 
Prevalence of pulmonary embolism in clinical probability categories.
Although the CT alone strategy seemed acceptable when the prevalence of pulmonary embolism in the low clinical probability subgroup was 10% (effectiveness 98.7%), its effectiveness dropped to 98.0% (0.57% less than the reference strategy) when that prevalence reached 15%. In patients with an intermediate clinical probability of pulmonary embolism, US ± CT and DD ± US ± CT, which were significantly less effective in the baseline analysis, became marginally acceptable (difference in effectiveness 0.40% with reference strategy) when the prevalence of pulmonary embolism in that category dropped to the lower limit of 25%. Finally, in the high clinical probability category, the DD ± US ± / scan ± CT strategy, which was only marginally effective in the baseline analysis, became associated with a more than 0.5% 3-month mortality difference with the reference strategy when the prevalence of pulmonary embolism became 70% or higher in that subgroup.

Diagnostic performance of / scan.
The analysis was performed using the Canadian (1) and the Prospective Investigation On Pulmonary Embolism Diagnosis (PIOPED) (3) values of the performance of / scan diagnostic scans (Table 1) for each clinical probability category. In the low- and intermediate-probability subgroups, using the Canadian figures, in which the proportion of diagnostic / scans is higher, decreased costs without significantly affecting their effectiveness. The strategy that was most sensitive to variations of the performance of / scan was DD ± US ± / scan ± CT, which became of similar effectiveness to the reference strategy in patients with a high clinical probability (difference in effectiveness, 0.3%) when using the Canadian figures (1). Conversely, using the PIOPED values (3) for / scan performance rendered that strategy unsafe (0.66% difference in effectiveness with the reference strategy in high-probability patients).

Costs.
Absolute costs per patient were different in the various countries, consistently highest in the United States and lowest in Canada. However, the cost-effectiveness ranking of strategies was unaffected. For instance, in patients with a low clinical probability, the costs of the DD ± US ± / scan strategy were $845 in Geneva, $2,431 in the United States, and $325 in Canada, but it remained the most cost-effective strategy in all countries.

Other sensitivity analyses.
All other variables, including increasing mortality of major bleeding, morbidity and mortality of tests, and mortality of pulmonary embolism, had no significant influence on the results in the range tested.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This analysis confirms the crucial role of clinical assessment in choosing the most cost-effective strategy for suspected pulmonary embolism. In patients with a low clinical probability of pulmonary embolism, in which the prevalence of the disease is only approximately 10%, the most cost-saving strategy was DD followed when abnormal by US and by a / scan in the absence of a deep vein thrombosis, patients with a nondiagnostic / scan being left untreated (DD ± US ± / scan; Figure 1A). Alternatively, CT after DD and US (DD ± US ± CT) was also a cost-saving strategy. Angiography or CT in case of a nondiagnostic / scan or angiography in patients with a negative helical CT was not necessary in that low-risk group. CT alone, although safe in low-probability patients, was expensive and therefore was only marginally cost-effective.

In patients with an intermediate or high clinical probability of pulmonary embolism, further testing was required in patients with a nondiagnostic / scan or a negative single-detector helical CT. The most cost-effective option in all countries considered was performing a / scan followed by a CT when the / scan was nondiagnostic (DD ± US ± / scan ± CT; Figures 1B and 1C). It was both the cheapest option and the only entirely noninvasive one. However, it entails the availability of both nuclear medicine and CT scan technology. Replacing CT by pulmonary angiography in that scheme (DD ± US ± / scan ± angiography) was slightly less cost-saving but was effective. Finally, performing a CT as the third test instead of a / scan and an angiogram in patients with a negative CT (DD ± US ± CT ± angiography) was safe but not cost-saving compared with the reference strategy, although it reduces the requirement for angiography. CT alone was associated with a higher mortality in intermediate- and high-probability patients and was therefore not an effective option.

Sensitivity analyses showed that the sensitivity of CT was crucial in selecting the appropriate strategy. Adopting sensitivity figures corresponding to the expected performance of multidetector CT (above 85%) would make DD ± US ± CT the most cost-effective strategy in patients with an intermediate or high clinical probability and allow foregoing angiography in all clinical probability subgroups. Indeed, it would be cheaper than strategies based on / scan because the latter require an angiogram in patients with a nondiagnostic / scan, in contrast with those who have a negative multidetector CT. In summary, strategies based on / scan were more cost saving than those resorting to single-detector helical CT, but multidetector CT-based strategies were clearly the most cost-effective.

Our analysis has several limitations. First, the assumption that the tests used in the diagnostic strategies are conditionally independent is probably inaccurate. However, numeric data are lacking to include that in the model and we do not believe that it significantly biased our results. Second, it is always difficult to set the effectiveness threshold below which a strategy ceases to be clinically recommendable. We selected a rather conservative threshold, considering that any strategy yielding a mortality difference greater than 5 per 1,000 patients compared with the reference strategy should be considered unsafe for clinical use. This can of course be debated, but our data should allow the reader to form her or his own opinion. We did not include in the model the strategy by Wells and colleagues (1), which relies on lung scan in every patient and serial lower limb venous compression ultrasonography. Indeed, although well validated, it is not widely used because of the necessity to follow-up a significant proportion of the patients during 2 weeks to perform the three repeat USs required by that algorithm. Finally, for lack of data, we were unable to incorporate in our analysis an important feature of helical CT, namely its ability to detect other diseases in patients suspected of pulmonary embolism that ipso facto rule out venous thromboembolism, such as for instance pneumonia. Also, we did not account for the fact that small segmental or subsegmental emboli, which are more likely to be missed by helical CT, are probably of little clinical consequence provided that there is no associated deep-vein thrombosis. Hence, we may have underestimated the cost-effectiveness of helical CT in this analysis, and CT-based strategies may well be as cost-effective as / scan-based schemes even using single-detector CT. It was also impossible to account for different levels of local expertise with CT and / scan. The diagnostic yield may be greater when using the method one prefers and knows best. Hence, the clinician might want to consider several elements in selecting the most appropriate strategy in a particular center, such as the available technology, the local expertise, and the relative costs of tests.

In summary, the present decision analysis showed that algorithms combining clinical assessment, DD and lower limb US to either / scan or single-detector helical CT were highly cost-effective provided that an angiogram is performed on patients with an intermediate or high clinical probability of pulmonary embolism. Alternatively, in those patients, angiography could be replaced by CT when using / scan as the third test in the sequence. Using multidetector CT combined with DD and US might allow foregoing angiography in all clinical probability subgroups. In contrast, single-detector helical CT as a single test was not cost-effective in the diagnosis of pulmonary embolism because of its low sensitivity.


   FOOTNOTES
 
Supported by grant 32-52798.97 from the Swiss National Research Foundation.

This article has an online supplement, which is available from this issue's table of contents online at www.atsjournals.org

Received in original form June 27, 2001; accepted in final form October 22, 2002


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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D-Dimer Assay to Exclude Pulmonary Embolism in High-Risk Oncologic Population: Correlation with CT Pulmonary Angiography in an Urgent Care Setting
Radiology, June 1, 2008; 247(3): 854 - 861.
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 ANN INTERN MEDHome page
G. Le Gal, M. Righini, P.-M. Roy, O. Sanchez, D. Aujesky, H. Bounameaux, and A. Perrier
Prediction of Pulmonary Embolism in the Emergency Department: The Revised Geneva Score
Ann Intern Med, February 7, 2006; 144(3): 165 - 171.
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 RadioGraphicsHome page
B. Ghaye, A. Ghuysen, P.-J. Bruyere, V. D'Orio, and R. F. Dondelinger
Can CT Pulmonary Angiography Allow Assessment of Severity and Prognosis in Patients Presenting with Pulmonary Embolism? What the Radiologist Needs to Know
RadioGraphics, January 1, 2006; 26(1): 23 - 39.
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 Am. J. Roentgenol.Home page
S. Patel and E. A. Kazerooni
Helical CT for the Evaluation of Acute Pulmonary Embolism
Am. J. Roentgenol., July 1, 2005; 185(1): 135 - 149.
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M. K Gould
Review: negative ELISA results are best for excluding a diagnosis of deep venous thrombosis or pulmonary embolism
Evid. Based Med., November 1, 2004; 9(6): 186 - 186.
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 Am. J. Respir. Crit. Care Med.Home page
M. J. Tobin
Chronic Obstructive Pulmonary Disease, Pollution, Pulmonary Vascular Disease, Transplantation, Pleural Disease, and Lung Cancer in AJRCCM 2003
Am. J. Respir. Crit. Care Med., January 15, 2004; 169(2): 301 - 313.
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 Am. J. Respir. Crit. Care Med.Home page
R. J. Lipchik, L. R. Goodman, A. Perrier, M. Nendaz, F. Sarasin, H. Bounameaux, and N. Howarth
Cost-effectiveness of Computed Tomography in Diagnosis of Pulmonary Embolism
Am. J. Respir. Crit. Care Med., January 1, 2004; 169(1): 129 - 130.
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 NEJMHome page
P. F. Fedullo and V. F. Tapson
The Evaluation of Suspected Pulmonary Embolism
N. Engl. J. Med., September 25, 2003; 349(13): 1247 - 1256.
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 JNMHome page
H. F. Corbus, M. A. Meignan, and E. Itti
Tomographic Evaluation for a 3-Dimensional Organ
J. Nucl. Med., September 1, 2003; 44(9): 1543 - 1543.
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 BMJHome page
K. Janata
Managing pulmonary embolism
BMJ, June 19, 2003; 326(7403): 1341 - 1342.
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