Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark for new referrals to a service for the diagnosis and management of rheumatoid arthritis (RA) in adults of around 0.1% or 100 per 100,000 people aged 18 years and older, per year, and the benchmark rate for ongoing management within such a service of around 0.8% or 800 per 100,000 people aged 18 years and older, per year, are based on the following source(s) of information:

• epidemiological data on the incidence and prevalence of RA
• current practice on detection of RA
• expert clinical opinion of the Topic-specific Advisory Group, based on experience in clinical practice and literature review.

For the purpose of this commissioning guide the adult population has been defined as people aged 18 years and older. Approximately 80% of the population in England is aged 18 years and older. Ongoing management is used within this commissioning guide to describe care for people with established RA after an estimated 1 year of monthly monitoring until active disease is stabilised.

Epidemiological data

Estimates of the population-wide incidence and prevalence of RA vary.

The incidence of RA in people aged over 18 years has been estimated to be around 0.036% in women and 0.015% in men^[1], equivalent to around 10,000 people over the age of 18 years per year in England. The population prevalence of RA has been estimated to be 0.8% (for England, around 315,000 people over the age of 18 years).

The 2009 National Audit Office (NAO) report Services for people with rheumatoid arthritis suggested that the incidence and prevalence of RA in people over the age of 16 years may be somewhat higher. This difference is likely to be due to coding issues and commissioners may wish to review current provision and the expected number of people with RA in their local population.

Findings from the NAO report Services for people with rheumatoid arthritis also indicated that the majority of PCTs had not compared the current prevalence of people with RA in their population against the expected number of cases.

Current practice

IMS Disease Analyser is a database that holds data from a sample of GP practice systems. Data were extracted from the database to assess the incidence and prevalence of diagnosed RA. Based on these data, the incidence of diagnosed RA, that is the average detection rate of new cases, is between 0.019% and 0.025% (19 to 25 per 100,000 population). This is in line with previous estimates ^[1] used within the costing report for NICE clinical guideline CG79 Rheumatoid arthritis.

The data extracted from the database suggests the prevalence of diagnosed RA is between 0.49% and 0.63% (490 to 630 per 100,000 population). This range is slightly below previous estimates ^[2].

Expert clinical opinion

The consensus opinion of the Topic-specific Advisory Group was that, based on current referral patterns, around 25% of people referred to a rheumatology service for investigation and diagnosis of early inflammatory arthritis would go on to be diagnosed with RA. That is, around 3 out of every 4 people referred to a rheumatology service would not go on to be diagnosed with RA.

At presentation the diagnosis of RA may not be readily apparent and referral to specialist services may also include people with suspected persistent synovitis or RA, or early inflammatory arthritis, who will require care or further investigation ^[3]. Commissioners should be aware that the principles of commissioning a service for people with RA can also be applied to services for people with early inflammatory arthritis.

Conclusions

Based on the epidemiological data and other information outlined above, it is concluded that the benchmark rate for new referrals to a service for the diagnosis and management of RA in adults is around 0.1% or 100 per 100,000 people aged 18 years and older, per year, of whom around 25 would go on to be diagnosed with RA. The benchmark rate for ongoing management within such a service is around 0.8% or 800 per 100,000 people aged 18 years and older. This is based on the following assumptions:

• the incidence of RA in people aged over 18 years is around 0.036% in women and 0.015% in men, equivalent to around 0.026% across the whole population aged over 18 years in England. This is equivalent to around 26 per 100,000 population aged over 18 years
• the Topic-specific Advisory Group suggested that around 25% of people referred to a rheumatology service would go on to be diagnosed with RA
• inflating the incidence figure for the 75% of people that are referred to the service but do not go on to be diagnosed with RA gives a figure of around 100 per 100,000 population aged over 18 years
• the prevalence of rheumatoid arthritis in the population aged over 18 years is around 0.8%. This is equivalent to around 800 per 100,000 population aged over 18 years.

Therefore the population benchmarks for new referrals and ongoing management are estimated to be 100 (of whom around 25 would go on to be diagnosed with RA), and 800 per 100,000 people aged 18 years and older, per year.

Use the service for the diagnosis and management of rheumatoid arthritis in adults 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. Symmons D, Barrett E, Bankhead C et al. (1994) The incidence of rheumatoid arthritis in the United Kingdom: results from the Norfolk Arthritis Register. British Journal of Rheumatology 33: 735-9.
2. Symmons D, Turner G, Webb R et al. (2002) The prevalence of rheumatoid arthritis in the United Kingdom: new estimates for a new century. Rheumatology 41: 793-800.
3. National Collaborating Centre for Chronic Conditions (2009) Rheumatoid arthritis. National clinical guideline for management and treatment in adults. London: Royal College of Physicians.