Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark for carotid endarterectomy of 12 per 100,000 per year are based on the following sources of information:

• epidemiological data on the incidence of transient ischaemic attack (TIA) and non-disabling stroke
• hospital activity data to establish current rates of carotid endarterectomy procedures commissioned in England.

Epidemiological data

To calculate the benchmark for carotid endarterectomy, we used the following estimates:

• The annual incidence of TIA is around 0.05% of the population^[1].
• Based on the 2005 National Audit Office report Reducing brain damage: faster access to better stroke care the annual incidence of stroke is 0.22% of the population, of which 85% are ischaemic strokes. This equates to a population incidence of ischaemic stroke of around 0.19%.
• Of the total number of ischaemic strokes, 25% are estimated to be non-disabling strokes^[2]. This means that the incidence of non-disabling stroke is around 0.05%, of the population.
• In 80% of all people who have a TIA or a non‑disabling stroke, the attack is in the carotid territory^[2].·
• The proportion of people who have a TIA or a non-disabling stroke who present with carotid stenosis of 50-99% (North American Symptomatic Carotid Endarterectomy Trial [NASCET] criteria) or 70-99% (European Carotid Surgery Trialists' Collaborative Group [ECST] criteria) is thought to be around 15%^[3].

See figure 1 for a diagrammatic illustration of the estimates outlined above.

Activity data - ‘Hospital episode statistics' data

The ‘Hospital episode statistics' (HES) database contains details of all admissions to NHS hospitals in England. It includes private patients treated in NHS hospitals, patients who were resident outside England and care delivered by treatment centres (including those in the independent sector) funded by the NHS.

Data were extracted from HES to determine the current rate of carotid endarterectomy procedures (OPCS4 code L29) carried out in England in 2006/07, where the primary diagnosis was either stroke or TIA. The mean directly age-standardised rate for England was around 8 per 100,000 population with significant variation across the country. See figure 2.

Conclusions

Based on the epidemiological data and other information outlined above, it is concluded that the benchmark rate for carotid endarterectomy is 12 per 100,000 population. This is based on the following assumptions:

• The incidence of non-disabling stroke is 0.05% and the incidence of TIA is around 0.05%. The combined incidence of non-disabling stroke and TIA is therefore 0.1%, or 100 per 100,000, population.
• In 80% of the total number of people who have a TIA or a non-disabling ischaemic stroke, the attack arises in the carotid territory.
• 15% of the above have stenosis between 50 and 99% (NASCET) and 70 and 99% (ECST).

Therefore the population benchmark for carotid endarterectomy is estimated to be 12 per 100,000 population. This is around a 50% increase in the current rate of carotid endarterectomy procedures currently performed in the NHS in England.

Use the TIA 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. Department of Health (2007) Impact assessment, national stroke strategy. London: Department of Health.

2. Ferris G, Roderick P, Smithies A et al. (1998) An epidemiological needs assessment of carotid endarterectomy in an English health region. Is the need being met? British Medical Journal 317: 447-51.

3. Lovett JK, Coull A, Rothwell PM (2004) Early risk of recurrent stroke by subtype of ischemic stroke in population-based incidence studies. Neurology 62: 569-73.