Published 24 June 2009, doi:10.1136/bmj.b2185
Cite this as: BMJ 2009;338:b2185

Editorials

Screening for abdominal aortic aneurysm

Remains effective and is likely to be cost effective in the UK, but data need to be monitored

A national NHS programme exists in England that offers screening for abdominal aortic aneurysm in men aged 65, with the aim of reducing deaths from aneurysm rupture.¹ The key evidence underpinning the formulation of this policy was the results of the multicentre aneurysm screening study (MASS), funded by the Medical Research Council.² More than 67 000 men aged 65-74 were recruited from four centres in the UK for this study and were randomised to either receive or not receive an invitation to screening. The results after 4 years’ follow-up showed a 42% relative risk reduction in mortality related to abdominal aortic aneurysm (absolute risk reduction from 0.33% to 0.19%). An economic evaluation indicated that the investment in screening was close to being acceptably cost effective, even in the short term.³ The evidence of clinical effectiveness was reinforced by a subsequent Cochrane Review—which estimated a 40% relative risk reduction (absolute risk reduction from 0.27% to 0.16%)—and then by results from MASS after 7 years’ follow-up.^{4 5} A formal long term economic model built on the MASS data indicated that abdominal aortic aneurysm screening was likely to be extremely cost effective.⁶ Taken together, these findings provided an unusually solid evidence base for a screening policy.

Two linked studies (doi:10.1136/bmj.b2307; doi:10.1136/bmj.b2243) further add to the evidence base. ^{7 8} In the first study, Thompson and colleagues provide 10 year follow-up results from MASS, in which the relative risk reduction in mortality related to abdominal aortic aneurysm is maintained.⁷ They recalculate the original cost effectiveness estimates to account for this extended period of observation and now indicate an incremental cost effectiveness ratio of £7600 (8800, $12 500; 95% CI £5100 to 13 000 (5900 to 15 000, $8350 to 21 300) per life year gained, much lower than the £20 000-30 000 threshold used by the NICE. On the other hand, the second linked study by Ehlers and colleagues presents a lifetime modelling study of a screening programme for abdominal aortic aneurysm and concludes that screening is not cost effective, with a mean cost per quality adjusted life year of £43 485.⁸ What is the reason for this discrepancy, particularly given that the modelling paper uses the previously published cost effectiveness evidence from the MASS?

From the evidence in the papers as presented here, it is hard to identify the key reason for the differing findings or, more likely, the most important of multiple contributory reasons. Importantly, the two papers agree that screening significantly reduces the number of deaths related to abdominal aortic aneurysm in the long term, although the Danish model suggests that there may be a small increase in net deaths in the short term. Ehlers and colleagues also argue, as have others,⁹ that a model that draws on all available data is inherently superior to a model based around one trial, albeit the biggest. Ehlers and colleagues have directly modelled the effects of the current policy of screening 65 year olds, whereas the MASS results reflect a sample group screened at age 65-74.

The Danish authors do recognise, however, that there are other important differences between their long term cost effectiveness model and cost effectiveness estimates based directly on MASS, including their use of Danish data on long term mortality after elective and emergency surgery. Possibly the key difference is the cost data used. Ehlers and colleagues used estimates of Danish costs that differed in important ways from the estimates originally in MASS—the costs of screening are higher in Denmark compared with the UK and the differential cost between elective and emergency surgery is substantially lower. Both these factors would make screening seem less cost effective. This is just one difference between the models, however, and cost effectiveness ratios are known to be relatively unstable where the average difference in effect is small (at 10 years, an average additional survival of just 4.8 days per man invited to screening according to the MASS data).

Understanding fully the differences between modelling studies usually requires access to and exploration of the models—for example, by substituting alternative data and assumptions, and, most importantly in this case, incorporating into each the new 10 year MASS results. The need to explore what happens when parameter estimates and model structures are changed is why the NICE insists on access to a transparent working version of models when appraising new technologies. A compromise for journals would be to insist that much more detailed explanations of the models that underpin articles are made available on the internet.

So without such additional information and analyses, what should we conclude about the cost effectiveness of screening for abdominal aortic aneurysm? The 10 year results from MASS confirm that screening is effective in the long term, and there is no indication of the survival benefit being eroded. It would also seem reasonable to infer that a UK screening programme will be acceptably cost effective, providing that effectiveness can be maintained across the country and that the cost estimates remain relevant. These caveats emphasise that screening costs need to be controlled while quality is maintained, and that changes in the management of elective or emergency repair of aneurysms might have a significant impact. For example, as indicated in one of the sensitivity analyses in the Danish paper, the introduction of more costly endovascular repair might reduce the cost effectiveness of screening.⁹ The accumulated evidence suggests that a national screening programme in the UK is appropriate and likely to be cost effective, but its costs and outcomes need to be carefully monitored and the data need to be regularly re-analysed to ensure that both the effectiveness and cost effectiveness remain acceptable in the context of changing practice.

Cite this as: BMJ 2009;338:b2185

¹ Health Economics Research Group, Brunel University, Uxbridge UB8 3PH

Research, doi: 10.1136/bmj.b2307; research, doi: 10.1136/bmj.b2243

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Competing interests: MB was a co-investigator on the MASS trial, lead author of the original MASS cost effectiveness paper, and was involved in some of the subsequent studies.

Provenance and peer review: Commissioned; not externally peer reviewed.

References

1. National Screening Committee. National Screening Committee policy—abdominal aortic aneurysm screening. February 2009. http://www.library.nhs.uk/screening/ViewResource.aspx?resID=60457 (accessed 27 May 2009).
2. Multicentre Aneurysm Screening Study Group. The Multicentre Aneurysm Screening Study (MASS) into the effect of abdominal aortic aneurysm screening on mortality in men: a randomised controlled trial. Lancet 2002;360:1531-9.[CrossRef][Web of Science][Medline]
3. Multicentre Aneurysm Screening Study Group. Multicentre aneurysm screening study (MASS): cost effectiveness analysis of screening for abdominal aortic aneurysms based on four year results from randomised controlled trial. BMJ 2002;325:1135-8.[Abstract/Free Full Text]
4. Cosford PA, Leng GC. Screening for abdominal aortic aneurysm. Cochrane Database Syst Rev 2007;(2):CD002945. doi:1002/14651858.CD002945.pub2.
5. Kim LG, Scott RAP, Ashton HA, Thompson SG. A sustained mortality benefit from screening for abdominal aortic aneurysm. Ann Intern Med 2007;146:699-706.[Abstract/Free Full Text]
6. Kim LG, Thompson SG, Briggs AH, Buxton MJ, Campbell HE. How cost-effective is screening for abdominal aortic aneurysms? J Med Screening 2007;14:46-52.[Abstract/Free Full Text]
7. Thompson SG, Ashton HA, Gao L, Scott RAP. Screening men aged 65 or more for abdominal aortic aneurysm: 10 year mortality and cost effectiveness results from the Multicentre Aneurysm Screening Study. BMJ 2009;338:b2307.[Abstract/Free Full Text]
8. Ehlers L, Overvad K, Sørensen J, Christensen S, Bech M, Kjølby M. Analysis of cost effectiveness of screening Danish men aged 65 for abdominal aortic aneurysm. BMJ 2009;338:b2243.[Abstract/Free Full Text]
9. Sculpher MJ, Claxton K, Drummond M, McCabe C. Whither trial-based economic evaluation for health care decision making? Health Econ 2006;15:677-87.[CrossRef][Web of Science][Medline]

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