Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark of 0.03% per year for new referrals into a service for the accurate diagnosis of the epilepsies are based on the following sources of information:

• epidemiological data on the prevalence/incidence of epilepsy
• current practice on detection rates of epilepsy
• 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 15 years and older. This definition is also used in the tool and has been chosen because of the availability of population data at general practice level within certain age bands.

Epidemiological data

Epilepsy is the most common chronic disabling neurological condition in the UK. The age-standardised prevalence of epilepsy in the UK is estimated to be 7.5 per 1000 population.

The incidence of epilepsy is estimated to be about 50 per 100,000 population per annum. Incidence is high in the child population, decreases in the adult population and rises again in the population of older people.

Current practice

IMS Disease Analyzer is a database that holds data from a sample of GP practice systems. Data based on the Quality and Outcomes Framework (QOF) epilepsy diagnosis codes were extracted to assess the prevalence and incidence of diagnosed epilepsy.

Analysis of the data suggests that the prevalence of diagnosed epilepsy in people aged 15 years and older is 1.15%. This estimate is higher than the unadjusted QOF prevalence for England for 2006/2007 for epilepsy of 0.6%. Characteristics of the QOF epilepsy disease register may explain this difference. First, the QOF register excludes individuals with a diagnosis of epilepsy who are not receiving drug treatment. Second, although the QOF register excludes individuals aged under 18 years, the prevalence is calculated with a denominator of all registered individuals. This means that the reported QOF prevalence is lower than the actual value. Commissioners should also be aware that the prevalence of diagnosed epilepsy is likely to include some degree of misdiagnosis, which has been estimated to be between 20-31% in the UK^[1].

Figure 1 shows the estimated prevalence of diagnosed epilepsy in England by age and sex in 2006/07.

The diagnosed incidence of epilepsy, that is, the average detection rate of new cases in a year, based on data extracted from IMS Disease Analyzer is estimated to be 0.02% of the population aged 15 years and older (95% confidence interval 0.017 to 0.024) .

The quality of data used in the analysis of diagnosed epilepsy relies on the information recorded in patient medical records. Some element of misdiagnosis in a proportion of new cases cannot be ruled out.

Expert clinical opinion

The consensus opinion of the topic-specific advisory group was that not all individuals who are referred with suspected epilepsy are subsequently diagnosed with epilepsy. The proportion of people who are referred and subsequently diagnosed with epilepsy is likely to be highly variable, and depends on a number of factors. However, based on clinical practice and literature review it is estimated that, on average, around 65-75% of people referred for investigation following a seizure are likely to be subsequently diagnosed with epilepsy.

There is a proportion of people who have a diagnosis of epilepsy (prevalent cases) who may benefit from having a review of the diagnosis (both for confirmation of diagnosis and of the type of epilepsy). However, the proportion of people who may benefit from this review is subject to a high degree of uncertainty and local variation. Commissioners will need to consider the diagnostic review of these individuals when planning service capacity.

Conclusions

Based on the epidemiological data and other information outlined above, it is concluded that the benchmark for new referrals to a service for the accurate diagnosis of the epilepsies is 0.03%. This is based on the following assumptions:

• the incidence of diagnosed epilepsy is estimated to be 0.02%
• around 70% of people referred for suspected epilepsy are likely to have the diagnosis confirmed
• adjusting the 0.02% to account for individuals who are referred but not subsequently diagnosed with epilepsy produces the figure of 0.03%.

Therefore the population benchmark is estimated to be 0.03%.

Use the service for the accurate diagnosis of the epilepsies 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. Stokes T, Shaw EJ, Juarez-Garcia A et al. (2004) Clinical guidelines and evidence review for the epilepsies: diagnosis and management in adults and children in primary and secondary care. London: Royal College of General Practitioners.