Artery screening weighed-up

Monday June 29 2009

Aneurysm screening is "set to save thousands of lives”, The Guardian has reported. The newspaper said that nearly 2,000 lives could be saved each year by a new screening programme for elderly men, which will look for weaknesses in the aorta, the body’s biggest artery. This national screening programme for abdominal aortic aneurysms (a potentially fatal swelling of the aorta) is being rolled out across England.

Two important studies, published in the British Medical Journal, have assessed the effectiveness and cost-effectiveness of the national screening programme. The programme was established based on the results of one of these studies: the Multicentre Aneurysm Screening Study. In a long-term follow-up of this pilot, a reduced rate of death from aneurysms was seen after 10 years. An economic analysis demonstrated that the programme is cost-effective. In other words, the benefits that the screening programme achieved are worth the cost.

A second, Danish study was based on a mathematical model of a hypothetical screening programme. It concluded that, based on UK thresholds of cost-effectiveness, ultrasound screening for abdominal aortic aneurysm is unlikely to be cost-effective.

Both studies acknowledged that a screening programme would reduce the number of deaths from aneurysms. Their discrepancies in terms of the cost-effectiveness results may be due to the costs used in the Danish model.

Where did the story come from?

The British Medical Journal has published two separate studies which assessed the clinical and financial effectiveness of abdominal aortic aneurysm screening programmes.

The first is a publication of the long-term results of a study conducted in the UK by Dr SG Thompson and colleagues on behalf of the Multicentre Aneurysm Screening Study Group (MASS). The MASS study, which began in 1997, was funded by the Medical Research Council.

In a second study, Danish researchers Lars Ehlers and colleagues from Aarhus University and other academic institutions across Denmark carried out a mathematical modelling exercise to determine the cost-effectiveness of screening men aged over 65 for aortic abdominal aneurysm. This study was funded by the Centre for Public Health in the Central Demark Region.

What kind of scientific study was this?

The aorta is a major blood vessel which originates in the heart and branches through the chest, abdomen and into the legs. At any point along its length, the vessel can balloon and dilate due to structural weakness. This is called an aortic aneurysm and, if it ruptures, can prove fatal.

The national aortic aneurysm screening programme, which has recently been rolled out across England, is largely based on the evidence from the MASS study. This study demonstrated a mortality benefit of ultrasound screening in men aged 65 to 74 in the UK. The longer-term results and an analysis of the cost-effectiveness of screening were presented in the most recent publication from this ongoing research.

The MASS study began in 1997 and enrolled 67,770 men, aged 65 to 74. The men were randomised to enter a control group or to receive an invitation for screening for abdominal aortic aneurysm. Early results showed that, of the 33,883 men invited, 27,204 (80%) attended screening. 1,344 aneurysms of 3cm or larger were detected. Men with aneurysms larger than 5.5cm were referred for surgery. Men with aneurysms of 3.0-4.4cm were re-scanned every year, while those with aneurysms 4.5 to 5.4cm were re-scanned every 3 months. Surgery was also offered if the aneurysm increased in size by 1cm or more in a single year, or if there were associated symptoms.

The men were followed up for an average of 10 years. The researches collected information from the UK Office for National Statistics on any aneurysm ruptures, deaths within 30 days of surgery for aortic aneurysm or deaths recorded as being due to a ruptured aortic aneurysm, abdominal aneurysm with or without mention of rupture or ruptured thoracoabdominal aneurysm.

The researchers used statistical methods to compare the deaths due to any cause and deaths due to abdominal aortic aneurysm between the screened and unscreened groups. The cost-effectiveness of the screening programme was also established by working out how much treatment cost in relation to the mortality benefits seen.

In the Danish study, researchers used a mathematical model to forecast the cost and benefits of screening a hypothetical cohort of men aged 65. The researchers used a hypothetical screening programme consisting of a mobile ultrasound team in a community setting.

The hypothetical population model took into account such factors as aneurysm prevalence and rates of screening attendance, which were based on estimates derived from a systematic review of relevant literature. For example, studies suggest that 4% of men over 65 have an abdominal aortic aneurysm larger than 3cm and that 77% of men invited will take up screening. Research also demonstrates the risk of rupture, aneurysm growth rate and the 30-day mortality for elective (non-emergency) and emergency surgery.

Estimates of long-term mortality figures after surgery for aneurysm were derived from a study of the Danish Vascular Registry. Costs of screening and surgery were taken from the literature and converted to 2007 pounds.

What were the results of the study?

Long-term follow-up of the men enrolled in the MASS study showed a mortality benefit in those invited for screening. Of the men invited for screening, 0.46% died due to abdominal aortic aneurysm, compared with 0.87% of men in the control group. These figures show that deaths were rare, but the group who received screening were 48% (1 - [0.46/0.87]) less likely to die from an abdominal aortic aneurysm.

As expected, more elective operations took place in the screened group than in the control group, which experienced more emergency surgeries than the screened group. However, there were no differences in mortality between the screened and unscreened groups receiving elective and emergency surgery.

The English study concluded that screening was cost-effective, at a cost of £9,400 per quality-adjusted life year (QALY) gained as a result. This figure is much lower than the £20,000 to £30,000 threshold used by the National Institute for Health and Clinical Excellence, which establishes guidelines for the use of treatments by the NHS. The Danish study estimated that abdominal aortic screening for men over 65 would cost £43,485 per QALY gained.

What interpretations did the researchers draw from these results?

In the MASS study, researchers concluded that the benefits of screening men aged 65 to 74 for abdominal aortic aneurysm are maintained for a period of up to 10 years and that the programme remains cost-effective over time. They say that to maximise the benefit from a screening programme, emphasis should be on achieving a high initial rate of attendance and good adherence to clinical follow-up, preventing delays in surgery and maintaining low operative mortality after surgery.

The Danish researchers concluded that, on the basis of acceptable thresholds of cost-effectiveness (£30,000 per QALY) in the UK, screening for aortic abdominal aneurysm is unlikely to be cost-effective. They say that further research is needed on long-term quality-of-life outcomes and costs.

What does the NHS Knowledge Service make of this study?

Both these studies are of interest given the introduction of England’s aortic abdominal aneurysm screening programme. They both highlight some important issues connected to this new programme and screening programmes in general.

The researchers of both studies highlight the possible limitations of their own work. The Danish researchers say that, while the type of model they used to estimate cost-effectiveness has strengths over and above the use of economic evaluations in primary studies, they were limited by having to rely on a combination of data from studies in different countries. Also, they only focused on men aged 65 at the start of screening.

The differences in the age groups of men in the MASS study and those in the Danish model may be one reason for the different conclusions about cost-effectiveness. They are also likely to have included different costs for elective and emergency surgery as MASS was UK-based and the Danish model used data from the Danish health system. Screening will also be sensitive to other costs, which can vary between countries.

The MASS researchers say that the cost-effectiveness of screening will improve over time because the main costs of the programme (screening and elective surgery for large aneurysms) occur early on. The Danish researchers point out that costlier elective surgery may increase the overall costs of screening.

An editorial that accompanies the publication of these two studies points out that the costs of screening in Denmark are higher than in the UK. Martin Buxton, the Professor of Health Economics who wrote this editorial, says that it is difficult to fully explain these different cost-effectiveness results without either access to more information about the Danish model or the chance to evaluate the inclusion of the new 10-year follow-up effectiveness data from the MASS study in the Danish model.

His conclusion is based on his appraisal of the available data and seems a sensible bottom line for policy makers: “The accumulated evidence suggests that a national screening programme in the UK is appropriate and likely to be cost-effective,” but with the proviso that “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.”

He says 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.”

Analysis by Bazian
Edited by NHS Choices