Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark for referrals to a service for the diagnosis and management of attention deficit hyperactivity disorder (ADHD) in adults and young people in transition of 25 per 100,000 population per year are based on the following sources of information:

• epidemiological data on the prevalence of ADHD
• expert clinical opinion of the topic-specific advisory group, based on experience in clinical practice and literature review.

Epidemiological data

Prevalence estimates of ADHD are rare in the published literature, especially in relation to Diagnostic and statistical manual, 4^th edition (DSM-IV) and ICD-10 (International classification of diseases, 10^th revision) criteria. Therefore to calculate the prevalence of ADHD in adults and young people where evidence is limited we have made the following assumptions:

• The prevalence of ADHD in the population aged 13-15 years based on the 1999 British Child and Mental Health Survey is 3.62% in males and 0.85% in females^[1].
• The prevalence of ADHD in the population aged 18 years and older is based on the assumption that 65%^[2] of the population aged 13-15 years with ADHD will continue to have symptoms of ADHD in adulthood that meet the DSM-IV criteria for either a full or partial diagnosis of ADHD. This equates to a prevalence of ADHD in the population aged 18 years and older of 2.35% in males and 0.55% in females.
• The prevalence of ADHD in the population aged 16-17 years is assumed to be the midpoint between the prevalence in the population aged 13-15 years and the prevalence of ADHD in the population aged 18 years and older. This equates to a prevalence of ADHD in the population aged 16-17 years of 2.99% in males and 0.70% in females.

We have assumed that 30% of the population aged 16-17 years with ADHD will require referral to adult ADHD services. This is basedon a survey of community paediatricians and the proportion of children with ADHD they expected to require follow-up in adulthood^[3]. The weighted average of the responses from the survey is 20% (lower range) and 40% (upper range), the mid point of which is 30%.

Of the population aged 16-17 years with ADHD who require referral to an adult ADHD service, we assumed that 50% would be referred in any one year. The topic-specific advisory group concurred with this assumption. Therefore the number of referrals to an adult ADHD service would be around 7.5 per 100,000 population per year. This is based on the following calculations:

• the combined prevalence of ADHD in males and females is around 50 per 100,000
• around 30% of the above (15 per 100,000 population) will require referral at some point to an adult ADHD service
• in any one year 50% of the above (7.5 per 100,000 population) will require referral to an adult ADHD service.

Expert clinical opinion

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

• Between 1% and 2% of the population with ADHD aged 18 years and older may be referred to an adult ADHD service each year; however, this estimate is subject to a high degree of uncertainty and local variation, and should be audited when services have been developed.
• The estimate that 30% of the population aged 16-17 years with ADHD may require a referral to an adult ADHD service is lower than the percentage of referrals expected from CAMHS and paediatric services because it refers to the population aged 16-17 years with ADHD, rather than young people aged 16-17 years who are receiving care from CAMHS and paediatric services.

Conclusions

Based on the epidemiological data and other information outlined above, it is concluded that population benchmark rate for referrals to a service for the diagnosis and management of ADHD in adults is 25 per 100,000 population per year. This is based on the following assumptions:

• the average number of referrals in the population aged 16-17 years (based on an equal proportion of people being referred in each year) is 0.007%, or 7.5 per 100,000 population per year
• around 1.5% of the population aged 18 years and older with ADHD may be referred per year to an adult ADHD service. This is equates to around 17 per 100,000 population per year.

Therefore the population benchmark for referrals to a service for the diagnosis and management of ADHD in adults and young people in transition, based on the combined estimates above, is 25 per 100,000 population per year.

Use the service for the diagnosis and management of attention deficit hyperactivity disorder in adults and young people in transition 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. Ford T, Goodman R, Meltzer H (2003) The British Child and Adolescent Mental Health Survey 1999: the prevalence of DSM-IV disorders. Journal of the American Academy of Child and Adolescent Psychiatry 42: 1203-11.

2. Faraone SV, Biederman J, Mick E (2006) The age-dependent decline of attention deficit hyperactivity disorder: a meta-analysis of follow-up studies. Psychological Medicine 36: 159-65.

3. Marcer H, Finlay F, Baverstock A (2008) ADHD and transition to adult services - the experience of community paediatricians. Child: Care, Health and Development 34: 564-6.