How should we diagnose suspected deep-vein thrombosis?
From the 1Medical Care Research Unit and 2Health Economics and Decision Science, University of Sheffield, Sheffield, 3Department of Health Sciences, University of Leicester, Leicester, and 4Sheffield Vascular Institute, Northern General Hospital, Sheffield, UK
Address correspondence to Dr S. Goodacre, Medical Care Research Unit, University of Sheffield, Regent Court, 30 Regent Street, Sheffield, S1 4DA. email: s.goodacre{at}sheffield.ac.uk
Received 21 February 2006 and in revised form 21 March 2006
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Background: Many different approaches are used to diagnose suspected deep-vein thrombosis (DVT), but there has been little formal comparison of strategies.
Aim: To identify the most cost-effective strategy for the UK National Health Service (NHS).
Design: Systematic review, meta-analysis and cost-effectiveness analysis.
Methods: We identified 18 strategies and estimated the diagnostic performance of constituent tests by systematic review and meta-analysis. Outcomes of testing and treatment were estimated from published data or by an expert panel. Costs were estimated from NHS reference costs and published data. We built a decision-analysis model to estimate, for each strategy, the overall accuracy, costs, and outcomes (valued as quality-adjusted life-years, QALYs), compared to a 'no testing, no treatment' alternative. Probabilistic analysis estimated the net benefit of each strategy at varying thresholds for willingness to pay for health gain.
Results: At the thresholds for willingness to pay recommended by the National Institute for Clinical Excellence (£20 000–£30 000 per QALY), the optimal strategy was to discharge patients with a low or intermediate Wells score and negative D-dimer, limiting ultrasound to those with a high score or positive D-dimer. Strategies using radiological testing for all patients were only cost-effective at £40 000 per QALY or more.
Discussion: The optimal strategy for DVT diagnosis is to use ultrasound selectively in patients with a high clinical risk or positive D-dimer. Radiological testing for all patients does not appear to be a cost-effective use of health service resources.
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Introduction |
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Deep-vein thrombosis (DVT) is an important cause of morbidity and mortality, but most patients presenting with suggestive symptoms do not have DVT.1 Investigations range from the accurate but expensive (contrast venography) to the cheap but unreliable (clinical assessment). Recent studies suggest that algorithms combining simple diagnostic tests may provide an acceptable way of reducing the need for expensive, definitive tests, but these studies have not explicitly weighed the costs and benefits of different diagnostic approaches.2 Despite a wealth of published data, there is substantial variation between hospitals in their diagnostic approach to suspected DVT.3
Choosing an appropriate diagnostic strategy requires explicit consideration of the benefits, harms and costs of diagnosis (or misdiagnosis). The benefit of using accurate but expensive tests (in terms of correctly identifying and treating those with DVT) needs to be weighed against their additional costs. We also need to consider whether health service resources used diagnosing DVT could be better spent elsewhere, and to decide how much we are willing to pay, as a society, to achieve health gains. Only then can we determine what is likely to be an appropriate diagnostic strategy for suspected DVT.
We aimed to estimate the accuracy and cost-effectiveness of available diagnostic strategies for suspected DVT and identify a practical, cost-effective strategy that could be implemented throughout the National Health Service (NHS).
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Methods |
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We searched the literature to identify studies of diagnostic algorithms for suspected DVT that used widely available tests (i.e. Wells clinical score, D-dimer, ultrasound and venography)3 and reported follow-up of patients with negative results. Four further algorithms, each based on a single test with high sensitivity for proximal DVT (contrast venography, above-knee ultrasound, full-leg ultrasound, and ultrasound with repeat if negative), and a zero-option alternative (no testing or treatment), were also included.
We developed a decision analysis model to compare algorithms in a hypothetical cohort of 1000 out-patients with suspected DVT. Estimates of the sensitivity and specificity for each algorithm were applied to the population to determine the proportions of patients with and without DVT who would receive treatment. This then determined which patients would suffer events relating to DVT or treatment over the minimum treatment period of 3 months. We then estimated subsequent lifetime health outcomes, valued as discounted quality-adjusted life years (QALYs), and costs accrued by testing and treatment.
Sensitivity and specificity
We undertook systematic literature review and meta-analysis of each diagnostic test used in the algorithms.4–7 Estimates from meta-analysis were applied to each algorithm to estimate overall sensitivity and specificity. Sensitivities for proximal and distal DVT were estimated separately. In estimating overall sensitivity and specificity, we assumed, based upon empirical data,5 that D-dimer specificity was dependent upon Wells score, while sensitivity was independent. In the absence of similar data for ultrasound, we assumed that the sensitivity and specificity of ultrasound were independent of both Wells score and D-dimer.
If the algorithm defined ultrasound as being above-knee only, we assumed that sensitivity for distal DVT was zero. Some algorithms recommend repeat ultrasound after 1 week if the initial scan is negative, based on the pathophysiological rationale that repeat scanning detects propagating distal DVT. On this basis, we assumed that repeat ultrasound results were entirely dependent upon initial ultrasound (i.e. that a false negative initial ultrasound for proximal DVT would remain false negative on repeat scanning) and that the results of repeat scanning only differed from initial scanning if the patient initially had a distal DVT that then propagated proximally. We assumed that contrast venography had perfect sensitivity and specificity, but would not be feasible in 10%, would cause DVT in 1%,8,9 and carried a 1:55 000 risk of fatal analphylaxis.10,11
Population characteristics
We estimated the prevalence of proximal DVT from a recent study,12 the additional proportion of distal DVT using data from our meta-analysis of ultrasound, and the mean age and sex distribution from the VERITY DVT registry.1
Probability of events
Anticoagulant treatment may lead to fatal haemorrhage, disabling intracranial haemorrhage, or other non-fatal haemorrhage. We estimated the probability of these events using a recent meta-analysis.13 Proximal DVT may lead to fatal pulmonary embolus (PE), non-fatal PE, or post-thrombotic syndrome. We estimated the probability of these events in treated patients using a recent meta-analysis14 and cohort study.15 We assumed that a distal DVT carried a 21% probability of propagating proximally,16 where it would then carry the same risks as proximal DVT.
Anticoagulant therapy has been the established treatment for DVT for over 40 years, so few data are available regarding the risks associated with untreated proximal DVT. To estimate the probability of fatal and non-fatal PE, we analysed studies that followed-up untreated patients after negative results from tests that do not have 100% sensitivity for DVT. We estimated the anticipated number of missed DVTs, given the estimated sensitivity of the tests used, and compared this to the actual occurrence of fatal or non-fatal PE to calculate the risks of these outcomes (full details available from the authors).7 An expert panel estimated the probability of developing post-thrombotic syndrome to be 
33% in untreated patients.
Valuation of outcomes
Individuals who died from an initial event were assigned zero QALYs. We assumed that initial event-free survival was followed by normal quality-adjusted life expectancy of 11.58 QALYs for an individual aged 60 years, based on interim life tables17 and estimates of age specific quality of life.18 We estimated QALYs for individuals who suffered non-fatal events by adjusting normal expected quality-adjusted, life expectancy using decrements from published data19 or expert panel estimates.
Valuation of costs
Clinical scoring was assumed to cost 5 min of consultant time. D-dimer assay costs were estimated using NHS Trust data.20 NHS reference costs were used to estimate ultrasound and venography costs, with a higher estimate being used for full-leg scanning.21 We used NHS reference costs for fatal and non-fatal PE. We valued post-thrombotic syndrome as a new vascular surgery out-patient visit plus two follow-up visits per annum21 and two extra general practitioner (GP) consultations per annum.22 We estimated treatment of proximal DVT using data from Boccalon et al.,23 followed by 3 months of warfarin therapy. We took drug costs from the 2004 BNF,24 and GP and nursing costs from Netten and Curtis.22 The cost of non-fatal, non-intracranial bleeding was based on NHS reference cost data for gastrointestinal bleeding,21 while fatal bleeding and non-fatal intracranial bleeding were based on data from Sandercock et al.25
Model analysis
The parameters used in the model are outlined in the Appendix (Tables 5–8
). The time horizon was the lifetime of the patient. We assumed a health and social services perspective, and applied a discount rate of 3.5% to all future costs and benefits. Costs are expressed in 2003/4 UK sterling values.
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A mathematical model was used to estimate the expected additional costs and QALYs accrued by each algorithm, compared to no testing. The model was analysed probabilistically. Probability distributions were assigned to parameters used in the model, and Monte Carlo simulation was used to sample randomly from those distributions, the model being recalculated for each simulation. A number of one-way sensitivity analyses were performed in addition to the probabilistic sensitivity analysis outlined above (full details available from the authors). The results were expressed as a net benefit (additional QALYs multiplied by
, with the additional costs subtracted, where is the threshold willingness to pay per QALY). The optimal strategy is the one with the greatest mean net benefit. Thresholds for willingness to pay of £10 000, £20 000 and £30 000 per QALY were used, based on guidance from the National Institute for Clinical Excellence (NICE).27 |
Results |
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We identified 14 studies of algorithms combining Wells score, D-dimer, ultrasound or venography that followed-up patients with negative results (Table 1). Rates of thromboembolism during follow-up of patients testing negative were low and are thus likely to be acceptable for clinical practice. One study evaluated two algorithms in a randomized trial,40 three of the algorithms could be interpreted in two ways,33,40 and several of the studies evaluated similar algorithms.28–32,40,41 So although there were a total of 14 algorithms, these do not correspond exactly to the 14 studies. We labelled the 'no testing, no treatment' strategy as strategy 0, the four single-test strategies as 1 to 4, and the published algorithms as 5 to 18. All the strategies are described in Table 2.
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Table 3 shows the proportion of patients who will receive treatment, according to whether they have proximal DVT, distal DVT that propagates proximally, distal DVT that does not propagate, or no DVT. A perfect strategy would treat all patients with proximal DVT or distal DVT that propagates proximally, but none of the other two groups. All the strategies appear to detect and treat >90% of patients with proximal DVT, thus explaining the low rates of thromboembolism reported in the studies in Table 1.
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Table 4 shows the costs and QALYs accrued by each strategy, and the net benefit, assuming willingness to pay £10 000, £20 000 and £30 000 per QALY. If we are willing to pay £10 000 per QALY then strategy 16 will have the highest mean net benefit, whereas if we are willing to pay £20 000 or £30 000 per QALY, strategy 9 will have the highest mean net benefit.
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Figure 1 shows the cost-effectiveness acceptability curves. These plot the probability that an algorithm will be the most cost-effective at each value for willingness to pay, from zero to £100 000 per QALY. Up to the £30 000 threshold, algorithms 16, 9 and 13 are most likely to be optimal; for thresholds of £40 000 to £70 000 per QALY, algorithm 5 is most likely to be optimal; and for thresholds of £80 000 to £100 000 per QALY, a strategy of venography for all is most likely to be optimal. The algorithms are shown in Figure 2.
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Discussion |
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Guidance from the National Institute for Clinical Excellence (NICE)27 suggests that the £20 000 per QALY threshold should be used to determine whether an intervention is cost-effective in the National Health Service (NHS). A higher threshold of £30 000 per QALY may be used if additional factors are considered in determining cost-effectiveness, while thresholds >£30 000 per QALY should only be used if there are strong additional factors. In our analysis, algorithm 16 was the most cost-effective strategy at the £10 000 per QALY threshold, while algorithm 9 was most cost-effective at the £20 000 and £30 000 per QALY thresholds. These algorithms are thus the most appropriate strategies for DVT diagnosis in the NHS.
Algorithms 9 and 16 both use a negative D-dimer to rule out DVT in low- and intermediate-risk patients, and use above-knee ultrasound in those with a positive D-dimer or high clinical score. They differ in the use of repeat ultrasound scanning. All patients receive a repeat scan in algorithm 9, whereas only those with a high Wells score and positive D-dimer receive repeat scanning in algorithm 16. Strategies that provide radiological testing (ultrasound or venography) for all patients are only likely to be cost-effective if we are willing to pay £40 000 per QALY or more. Algorithm 5, which uses ultrasound on all patients and venography selectively, is most likely to be optimal for thresholds from £40 000 to £70 000 per QALY, while algorithm 1 (venography for all patients) is most likely to be optimal if we are willing to pay £80 000 per QALY. However, these values all exceed the NICE recommended threshold, so it appears that diagnostic strategies based upon radiological testing for all patients are unlikely to represent a cost-effective use of resources.
A recent review of studies evaluating strategies that discharge patients with a low or intermediate Wells score and negative D-dimer concluded that this approach is ‘safe’.2 However, this conclusion is based upon a subjective judgement about whether a low probability of missed thromboembolism is acceptable, and thus considered ‘safe’. Our analysis has explicitly weighed the costs and benefits of alternative strategies to show that this approach is cost-effective unless we are willing to pay £40 000 per QALY or more. One previous study used decision analysis to evaluate diagnostic testing for DVT,42 comparing four strategies, incorporating combinations of clinical risk scoring, D-dimer and ultrasound, to a no treatment alternative. They estimated that the cheapest strategy (combining clinical risk scoring and D-dimer with a single ultrasound) was also the most cost-effective. This strategy was the same as algorithm 13 in our analysis and, consistent with the previous study, we found algorithm 13 to be highly cost-effective. However, this analysis only evaluated four strategies and did not make explicit the value judgement involved in deciding whether a strategy was cost-effective. By presenting our results as cost-effectiveness acceptability curves, we have shown how judgements regarding cost-effectiveness depend upon willingness to pay for health gain. Other cost-effectiveness analyses have focussed upon the cost-effectiveness of one particular technology and are less easily comparable.43–45
Our analysis has some limitations. Few data are available to determine how ultrasound results correlate with Wells score or D-dimer, so we had to assume that ultrasound was independent of these tests. One study has suggested that ultrasound performs better in those with a high Wells score.46 If this is so, then our assumption will favour strategies that use ultrasound in patients with a low score. This means that we may have under-estimated the cost-effectiveness of algorithms 9 and 16, but over-estimated the cost-effectiveness of algorithm 5. No data are available to determine whether D-dimer and ultrasound interact, but as these tests have a different pathophysiological basis, an assumption of independence is not unreasonable. Rates of thromboembolism among patients with negative tests reported in follow-up studies of algorithms combining Wells score, D-dimer and ultrasound (Table 1) are compatible with our estimates of overall sensitivity for the algorithms.
We only included algorithms that had been evaluated by management studies involving follow-up of patients with negative tests. There are numerous potential combinations of tests that could be used to diagnose DVT, but we felt that theoretical algorithms are unlikely to be widely adopted without empirical data showing how they work in practice. We also did not include algorithms that involved plethysmography in our analysis.47,48 This test is not currently available in many hospitals,3 does not appear to have adequate sensitivity or specificity to be used as a single test, and very little is known about how it interacts with other tests.7 However, algorithms using plethysmography may offer a cost-effective alternative to the strategies examined here.7
Our model does not allow us to determine the potential impact of the strategy upon selection of patients for testing, and whether this influences cost-effectiveness. For example, a D-dimer based strategy (such as algorithm 9) may be used in a wider group of patients than a strategy requiring radiological testing for all. There is very little empirical data on whether patient selection is influenced by the diagnostic tests used. Future research is needed to evaluate this possibility and determine whether it has consequences for cost-effectiveness. Finally, this analysis applies principally to out-patients with a suspected first DVT. Our findings may not apply to certain patient groups, such as in-patients developing symptoms of DVT, patients with suspected recurrent DVT, pregnant patients, intravenous drug abusers or those with prolonged symptoms.
Conclusion
Diagnostic strategies for DVT that involve radiological testing for all patients are unlikely to be cost-effective at currently recommended thresholds of willingness to pay. We recommend widespread adoption throughout the NHS of a diagnostic strategy that uses Wells score and D-dimer to exclude DVT in low- and intermediate-risk patients.
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Appendix: Mean value, probability distribution and source of parameters used in the model (Tables 5–8 )
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Steve Goodacre, UK Department of Health. This article is covered by Crown Copyright as a part of the funding contract between the University of Sheffield (Principal Investigator: Steve Goodacre) and the UK Department of Health.
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Acknowledgements |
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We thank Kathryn Paulucy for clerical assistance, Angie Ryan for help with literature searches, Tom Locker, Andy Webster, Francis Morris, Suzanne Mason and Edwin van Beek for help with literature reviews, and the members of the expert panel.
The NHS Health Technology Assessment R&D Programme funded this project (reference number 02/03/01). The views and opinions expressed therein are those of the authors and do not necessarily reflect those of the UK Department of Health. SG had full access to all the data in the study and had final responsibility for the decision to submit for publication.
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