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Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark at 5 years for a bariatric surgical service of 0.01% are based on the following sources of information:

  • epidemiological data on the prevalence of people with a body mass index (BMI) of 35 kg/m2 or more and comorbidities, and of people with a BMI of 40 kg/m2 or more
  • activity data to establish levels of bariatric surgery provided
  • current practice to identify people with a BMI of 35 kg/m2 or more and comorbidities who are registered with a general practice
  • expert clinical opinion of the topic-specific advisory group, based on experience in clinical practice and literature review.

Epidemiological data

Application of the estimated age-specific and sex-specific rates of BMI of 40 kg/m2 or more from the 2004 ‘Health survey for England' (HSE) to the English population suggests that around 680,000 people in England, or 1.4% of the population, have a BMI of 40 kg/m2 or more[1].

Analysis of data from the 2003 HSE suggests that, of those who have a BMI of 40 kg/m2 or more, 47,000 (95% confidence interval 25,000 to 70,000), or 7% of them, have a BMI of 50 kg/m2 or more.

This means that there are around 47,000 people in England who are eligible for bariatric surgery as first-line treatment for their obesity. For the remainder, surgery is considered only when other forms of medical management have been attempted but adequate, clinically beneficial weight loss has not been achieved or maintained.

Activity data

Activity data give an indication of the current rates at which bariatric surgery is being performed. These rates do not necessarily correlate with the rates that would be expected if NICE clinical guideline CG43 on obesity was uniformly followed.

Hospital Episode Statistics (HES) data indicate that the rate of bariatric surgery in England in 2004 was 7 per million population, with large regional variations[1]. Rates of bariatric surgery have been increasing, particularly since 2002 (see figure 1). This may be due in part to the publication at that time of a NICE technology appraisal on the use of surgery to aid weight reduction for people with severe obesity (replaced in 2006 by NICE clinical guideline CG43 on obesity).

However, the coding around bariatric surgery within HES is highly variable, and HES data may not provide the most accurate picture of the rate of bariatric surgery across England.

A survey of surgeons performing bariatric surgery in England in 2006 (survey by BariatricEdge, a division of Ethicon Endo Surgery: a Johnson & Johnson company, unpublished data) estimated that the average rate of bariatric surgery funded by the NHS was around 3 per 100,000 population. Within the private sector the estimated average rate of bariatric surgery was around 3.5 per 100,000 population, giving a total estimated average rate of 6.5 per 100,000 population.

In addition, the survey found large variations in the number of people receiving bariatric surgery between strategic health authority areas (see figure 2), and that rates did not follow regional differences in the expected numbers of those requiring surgery.

Current practice

Data from IMS Disease Analyzer, which holds patient data from a sample of GP practice systems, were extracted to determine the proportion of people with a BMI of between 35 and 39.9 kg/m2 with comorbidities who are in contact with GP services. The results of analysis suggest that around 0.8% of the English population, or 390,000 people, have a BMI of between 35 and 39.9 kg/m2 with a history of at least one of the following comorbidities: cardiomyopathy, coronary heart disease, hypertension, ischaemic attack, obstructive sleep apnoea, osteoarthritis, pulmonary hypertension, stroke and type 2 diabetes.

Expert clinical opinion

The consensus of the topic-specific advisory group was that:

  • Of the total population with a BMI of 35 kg/m2 or more with comorbidities, and those with a BMI of 40 kg/m2 or more, 90% to 95% are unlikely to achieve or maintain clinically beneficial weight loss through non-surgical means.
  • On the basis of current clinical opinion and published research it is anticipated that there would be an annual increase of severe obesity of 5% in the English population, and that the rate of increase would be the same in each BMI group.
  • Of the total population with a BMI of 35 kg/m2 or more with comorbidities, and those with a BMI of 40 kg/m2 or more, around 50% to 70% could be considered eligible for bariatric surgery. This is based on the proportion undertaking multicomponent specialist non-invasive weight management programmes including diet and exercise advice, and those making contact with health services.
  • Of those eligible for bariatric surgery, between 30% and 50% would take up surgery if offered it.
  • Of those eligible and willing to receive bariatric surgery, it would be possible to treat around 1.6% (around 4800 patients) per year, given appropriate future investment and optimal service capacity.
  • Rates of surgery should be expressed as a benchmark that may be achieved annually after a number of years - for example, 5 years - given the expected current unmet need in the population.

Conclusions

On the basis of the epidemiological data and other information outlined above, it is concluded that a benchmark at 5 years for bariatric surgery is 10 per 100,000 population or 0.01% of the population.

This is based on the following assumptions:

  • the population with a BMI of or 35 kg/m2 or more with comorbidities, or 40 kg/m2 or more, is 2.22% or around 1,070,000 people
  • around 60% of the above group would be considered eligible for bariatric surgery (mid point of the estimates provided by the topic-specific advisory group)
  • 40% of those eligible would take up surgery if it was offered (mid point of the estimates provided by the topic-specific advisory group), resulting in 0.53% of the population or around 257,000 people who are currently eligible and willing to take up surgery
  • the annual growth rate of the eligible population is 5%
  • around 1.6% of the population eligible and willing could be treated each year, which is around 4800 people a year, given appropriate investment in services over the next 5 years.

Therefore, in 5 years time, a population benchmark of 0.01% or 10 per 100,000 population is considered appropriate. This is a more than threefold increase compared with the estimated levels of annual bariatric surgery currently commissioned within the NHS in England, and equates to around 4800 people a year.

Sensitivity analysis

Owing to the uncertainty of the estimates regarding eligibility for and take up of bariatric surgery, sensitivity analysis has been carried out. These analyses take into account the upper and lower estimates of eligibility and take up provided by members of the topic-specific advisory group, as well as the expected growth rates in obesity and anticipated growth rates in surgical procedures to meet a benchmark at 5 years of 0.01% of the population (tables 1-3). These are also based on the consensus of the topic-specific advisory group that it may be possible to treat around 4800 patients per year, in 5 years' time, given appropriate future investment and optimal service capacity.

Table 1 Results of sensitivity analysis using a figure of 60% for people eligible for bariatric surgery and 40% for the eligible people taking up the service

Year Expected prevalence in the English population of people eligible and willing to take up surgery per year Proportion of the prevalent eligible and willing population receiving surgery per year Number of people in England receiving surgery per year Proportion of people in England receiving surgery per year
1 0.53% 0.57% 1472 0.003%
2 0.55% 0.86% 2312 0.005%
3 0.58% 1.12% 3151 0.006%
4 0.60% 1.37% 3991 0.008%
5 0.62% 1.60% 4830 0.010%
Figures presented above have been rounded after calculation.

Table 2 Results of sensitivity analysis using a figure of 70% for people eligible for bariatric surgery and 50% for the eligible people taking up the service

Year Expected prevalence in the English population of people eligible and willing to take up surgery per year Proportion of the prevalent eligible and willing population receiving surgery per year Number of people in England receiving surgery per year Proportion of people in England receiving surgery per year
1 0.77% 0.39% 1472 0.003%
2 0.81% 0.59% 2312 0.005%
3 0.84% 0.77% 3151 0.006%
4 0.88% 0.93% 3991 0.008%
5 0.91% 1.09% 4830 0.010%
Figures presented above have been rounded after calculation.

Table 3 Results of sensitivity analysis using a figure of 50% for people eligible for bariatric surgery and 30% for the eligible people taking up the service

Year Expected prevalence in the English population of people eligible and willing to take up surgery per year Proportion of the prevalent eligible and willing population receiving surgery per year Number of people in England receiving surgery Proportion of people in England receiving surgery
1 0.33% 0.92% 1472 0.003%
2 0.35% 1.37% 2312 0.005%
3 0.36% 1.81% 3151 0.006%
4 0.37% 2.22% 3991 0.008%
5 0.38% 2.61% 4830 0.010%
Figures presented above have been rounded after calculation.

In this sensitivity analysis, the base population has remained the same and the annual rate of increase in the eligible and willing population has been taken to be the same as the annual rate of increase in severe obesity in the population - that is, 5%.

There are other factors that are likely to influence variation around the benchmark, but eligibility, uptake and service capacity are thought to be the factors with the greatest sensitivity.

Use the bariatric surgical service commissioning and benchmarking tool to determine the level of service that might be needed locally and to calculate the cost of commissioning the service using the indicative benchmark and/or your own local data.

References

  1. Ells LJ, Macknight N, Williamson JR (2007) Obesity surgery in England: an examination of the Health Episode Statistics 1996-2005. Obesity Surgery 17:400-5

This page was last updated: 02 March 2012

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Copyright 2014 National Institute for Health and Care Excellence. All rights reserved.