Assumptions used in estimating a population benchmark

The assumptions used in estimating a population benchmark of referrals for cognitive behavioural therapy (CBT) of 3% per year of the adult population (defined as 15 years or older) are based on the following sources of information:

• epidemiological data on the prevalence/incidence of anxiety and depression
• current practice on the numbers of people detected in primary care
• published research on the detection of anxiety and depression in primary care and treatment preferences
• expert clinical opinion of the topic-specific advisory group, based on experience in clinical practice and literature review.

Epidemiological data

Depression and mixed anxiety and depression

The survey on psychiatric morbidity conducted by the Office for National Statistics in 2000^[1] found that the prevalence of depression among adults in Great Britain was 2.6%. The prevalence of mixed anxiety and depression was estimated to be 8.8%. This gives a combined total of 11.4% of adults of whom 36% are expected to have mild depression (with or without anxiety), 43% are expected to have moderate depression (with or without anxiety) and 21% are likely to have severe depression (with or without anxiety).

Mixed anxiety and depression was defined in the psychiatric morbidity survey report as a ‘catch-all' category that included people with significant symptoms which could not be coded into any of the other conditions included in the survey.

NICE clinical guideline CG22 on anxiety states that when someone has anxiety with depression the NICE clinical guideline CG90 on depression should be followed.

Panic disorder, generalised anxiety disorder and obsessive-compulsive disorder

The psychiatric morbidity survey found the population prevalence of generalised anxiety disorder (GAD) to be 4.4%, panic disorder (PD) to be 0.7%, and obsessive-compulsive disorder (OCD) to be 1.1%.

In some instances it is difficult to distinguish GAD and PD, and co-morbidity is very common with other anxiety disorders, depression and mood disorders^[2].

Post-traumatic stress disorder and body dysmorphic disorder

No large-scale surveys have been conducted to estimate the prevalence of post-traumatic stress disorder (PTSD) and body dysmorphic disorder (BDD) in the English population.

NICE clinical guideline CG31 on obsessive-compulsive disorder (OCD) suggests that the population prevalence of BDD in England is between 0.5% and 0.7%.

The NICE cost impact report for CG26 on PTSD suggests a population prevalence of PTSD in England of around 1.5%. However, commissioners should be aware that the prevalence is likely to be higher in some groups in the population - for example, members of the armed forces^[3], asylum seekers and refugees^[4].

The psychiatric morbidity survey did not cover PTSD or BDD but did group all people with significant symptoms who could not be coded into any of the other conditions included in the survey into ‘mixed anxiety and depression'. Therefore it has been assumed that PTSD and BDD are counted within this group.

Taking these assumptions into account, the combined results from the psychiatric morbidity survey suggest that, at any one time, 17.6% of the population have either depression, mixed anxiety and depression (which we have assumed includes those people with PTSD and BDD), panic disorder, generalised anxiety disorder or OCD.

However, it should also be recognised that anxiety disorders and depression are often undiagnosed, and many people with anxiety disorders and/or depression do not seek treatment.

The Improving Access to Psychological Therapies programme workforce and gap analysis tool supports commissioners to estimate the number of new workers required to manage the demand for psychological treatment in their area. The tool uses data from the psychiatric morbidity survey, with the assumptions that of those in the population with symptoms, 50% may present to services and of those that present to services, 50% are likely to have the symptoms detected. Applying these two assumptions to the prevalence of 17.6% estimated above suggests that 4.40% of the population are likely to present to services and have their symptoms detected per year.

Current practice

IMS Disease Analyser is a database that holds data from a sample of GP practice systems. Data were extracted to give a snapshot of 1 year's activity within general practice.

Results of analysis of these data suggest that around 4.95% of the population come into contact with GP services per year and are diagnosed as having either depression or mixed anxiety and depression.

The following estimates are based on people with panic disorder (PD), generalised anxiety disorder (GAD), obsessive-compulsive disorder (OCD) and post-traumatic stress disorder (PTSD) presenting at GP services within a year. The average rate of detection in general practice per year of:

• PD is estimated to be 0.76% of the adult population
• GAD is estimated to be 0.23% of the adult population
• OCD is estimated to be 0.07% of the adult population
• PTSD is estimated to be 0.05% of the adult population.

Therefore, it is estimated that around 6.06% of the adult population (defined as 15 years or older) present to GP services with one or more of the above conditions - that is, depression including mixed anxiety and depression, PD, GAD, OCD and PTSD per year. Some of the people in these groups will have symptoms of recent onset whereas others will have recurrent symptoms that have caused them to present to GP services again in the 12-month period for which data were extracted.

The data used in the analysis of diagnosed depression and anxiety rely on the quality of the information recorded within patients' medical records. Therefore poor recognition and recording of these conditions may lead to an underestimation of total numbers of people who present to general practice each year.

These estimates therefore provide an example of current detection rates and recording practices in primary care, not the rates that may be expected if detection of these conditions in primary care was improved.

Published research

Treatment preferences need to be taken into account, as people who do not receive their preferred treatment often fail to begin or complete treatment^[5].

Research by Marks and co-workers (2003)^[6] suggested that a maximum of 64% of patients with mild or moderate depression and anxiety would be suitable for and willing to participate in computerised CBT (CCBT).

Expert clinical opinion

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

• The take up of CBT among people identified with depression and anxiety conditions is unknown. However, people with depression and anxiety who present to services and have their symptoms recognised tend to be people seeking help for their condition. This means that the take up of services among these groups is likely to be high compared with those people with these conditions in the population who do not actively seek help.
• On average, based on clinical experience, it is expected that around 50% to 70% of people who present to services with either depression or anxiety would take up CBT. However, there is likely to be some variation in the take up of CBT between groups of people with different conditions.

Conclusions

Based on the epidemiological data and other information outlined above, it is concluded that the benchmark for referral for CBT is 3%. This is based on the following assumptions:

• the detection rate of depression including mixed anxiety and depression, PD, GAD, PTSD and OCD (based on a year's snapshot of activity in general practice) is around 6.06% of the adult population
• the estimated levels of detection of depression, including mixed anxiety and depression, PD, GAD, PTSD and OCD in general practice, based on the prevalence of conditions in the adult population is estimated to be around 4.40% of the adult population per year
• the mid point of the above two estimates (4.40 and 6.06) is 5.23%
• the take up of CBT among people with depression including mixed anxiety and depression, PD, GAD, PTSD, OCD and BDD who present to services is expected to be around 60% (mid point of the estimates provided by the topic-specific advisory group).

Therefore the population benchmark for CBT for the management of common mental health problems is estimated to be 3%.

The NICE clinical guideline CG90 on depression states that for mild depression a number of brief psychological interventions are effective. Many patients with milder depression respond to interventions such as exercise or guided self-help, although many improve while being monitored without additional help. More structured therapies, such as problem-solving, brief CBT or counselling can be helpful. The topic-specific advisory group suggested that commissioners may wish to focus their effort on commissioning CBT (high intensity) for people with moderate to severe depression as this is the area where there are greatest deficits in service provision, and where the greatest potential exists for commissioners to make a significant contribution to service improvement.

Use the CBT 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. Office for National Statistics (2002) Psychiatric morbidity among adults living in private households, 2000. London: The Stationery Office.
2. McIntosh A, Cohen A, Turnbull N et al. (2004) Clinical guidelines and evidence review for panic disorder and generalised anxiety disorder. Sheffield: University of Sheffield/London: National Collaborating Centre for Primary Care.
3. Jones M, Rona RJ, Hooper R et al. (2006) The burden of psychological symptoms in UK Armed Forces. Occupational Medicine 56: 322-8.
4. Hollifield M, Warner TD, Lian N et al. (2002) Measuring trauma and health status in refugees: a critical review. Journal of the American Medical Association 288: 611-21.
5. Hazlett-Stevens H, Craske MG, Roy-Byrne PP et al. (2002) Predictors of willingness to consider medication and psychosocial treatment for panic disorder in primary care patients. General Hospital Psychiatry 24: 316-321.
6. Marks IM, Mataix-Cols D, Kenwright M et al. (2003) Pragmatic evaluation of computer-aided self-help for anxiety and depression. British Journal of Psychiatry 183: 57-65.