The prevalence and incidence, resource use and financial costs of treating people with attention deficit/hyperactivity disorder (ADHD) in the United Kingdom (1998 to 2010)

Attention deficit/hyperactivity disorder (ADHD) is common and more likely to affect boys than girls, with an estimated prevalence in the UK of 3.6% and 0.9%, respectively, in children aged 5–15 years, using DSM-IV criteria [1]. Anecdotally, there is a commonly held belief that the prevalence of ADHD has risen markedly over the previous 20 years, with a corresponding increase in the financial cost of medicines indicated for ADHD [2, 3]. ADHD is a chronic condition that is often associated with significant impairments in academic performance and social functioning [4, 5]. Over 65% of those with ADHD also have one or more comorbid disorders. These include dyslexia, developmental coordination disorder, Tourette’s syndrome, autistic spectrum disorders, conduct and oppositional defiant disorders, and substance abuse [4, 6]. ADHD is also associated with disrupted parent–child relationships and increased parent stress levels [4, 7]. Treatment costs for patients with ADHD are greater than those without [8–15].

In the UK, the National Institute for Health and Care Excellence (NICE) has recommended that diagnosis of ADHD and treatment initiation should be conducted within secondary care [16]. When medication is used the dose should also be titrated and stabilised by a specialist. Once the patient is stabilised on treatment, prescribing and monitoring can be carried out in primary care under a shared-care protocol [16]. Whilst the popular press frequently comments on increased rates of diagnosis of ADHD and questions whether ADHD is over-diagnosed and over-treated [17], data from reviews of clinical practice suggest the opposite may be true with ADHD being both under recognised and under treated [18]. There are, however, few studies characterising the epidemiology of diagnosed ADHD in the UK and the healthcare cost to the NHS of treating children both with and without ADHD.

The aim of this retrospective, observational cohort study was to characterise the incidence and prevalence of diagnosed ADHD and to determine the corresponding resource use and financial cost of care for children, adolescents, and adults with ADHD compared with a matched control group over a 12-year period to 2010.

In this retrospective study, the prevalence of diagnosed ADHD was notably lower than previously reported. We estimated that in 2009 the incidence of ADHD was 9.9 per100k population and the prevalence 81.5 cases per100k. Compared to a matched control group, those with ADHD had substantially increased resource use and related financial costs (four-fold).

A systematic review and meta-analysis characterising the worldwide prevalence of ADHD reported that the pooled prevalence was 5.3%, with significant variability [35]. In the UK in 1999 in children aged 5–15 years, the actual prevalence of ADHD—when estimated using the Development and Well-Being Assessment (DAWBA)—was 3.6% in boys and 0.9% in girls [1]. The difference between these two figures may be related to the sensitivity of the DAWBA compared with other diagnostic instruments. At 0.44% in boys and 0.05% in girls the estimates of prevalence of diagnosed ADHD in 1999 in children (6–17 years) in our study was much lower than either of these. The most likely explanation for this is that the epidemiological studies screened the population and aimed to identify both diagnosed and undiagnosed cases. In the UK only a minority of patients with ADHD currently seek or receive medical treatment for their condition [36, 37]. The reason for the under-diagnosis of ADHD in the UK [38] is likely to be multifactorial. For example, parents of children with ADHD are likely to identify a problem and consult education professionals, but the presentation to primary care is limited and less than one in three children with ADHD access specialist services [37]. In addition, there is limited recognition of children at risk of ADHD in primary care [36] and uncertainty among many GPs over whether ADHD should be classed as medical disorder [39]. Even in the USA, where ADHD has been recognised longer, it was estimated that, between 2001 and 2004, less than half of the children meeting DSM-IV criteria received treatment [40]. In contrast to this, the percentage of children in the USA aged 4–17 years with a parent-reported ADHD diagnosis increased from 7.8% to 9.5% between 2003 and 2007 [41]. As the prevalence and incidence figures for this study relate to diagnosed ADHD, it is possible that any change in incidence or prevalence rates during the study period is an ascertainment effect.

The figures reported here are similar to those reported in a government-sponsored audit of ADHD services in Scotland [42]. In 2012, the overall prevalence had increased slightly to 0.7% with a similar variation across regions of Scotland and no change in the male-to-female ratio [42]. A UK study using the General Practice Research Database (GPRD; forerunner of CPRD) estimated that the prevalence of treated ADHD for patients aged 15–21 years was 0.88 per 1,000 in 1999, increasing to 5.09 per 1,000 in 2006 [43]. A slightly higher prevalence, though in a different age range, was reported by another study: 2.6 and 5.5 per 1,000 for 1999 and 2006, respectively, in patients aged 6–17 years [43].

We found that diagnosed cases of ADHD were more common in males than in females. Epidemiological studies have also reported a greater prevalence in males, with a male-to- female ratio of 2–3:1 [35]. In adults, however, the male-to-female ratio for ADHD has been reported to be approximately equal [44]. The higher ratios reported here and in other studies of diagnostic prevalence or treatment suggest that, in the UK, girls with ADHD are even less likely to be recognised and diagnosed than boys. It is possible that this is at least partly due to the fact that that females present with different symptoms and, most importantly, that they are less likely to have coexisting oppositional or disruptive behaviours [45]. However, a firm consensus on this matter has not been reached [16].

In our study, the diagnosed prevalence of ADHD in children age 6 – 17 years old increased from 192.4 to 506.4 per100k between 1998 and 2007. An increasing incidence rate was also observed between 1998 (39.3 per100k) and 2007 (79.0 per100k). An increase in the prevalence of ADHD has been reported in the USA between 1997 and 2007 [41, 46]. Since 2007, the incidence and prevalence rates have decreased, suggesting that recognition rates may have peaked for the time being. This is broadly in line with the findings of the most recent NHS Scotland audit [42] and coincides with the publication of the NICE guidelines, although we do not expect this to have resulted in a decrease in the recognition of ADHD [16].

A systematic review with meta-analysis has suggested that the prevalence of ADHD declines with age (although the strict application of DSM-IV criteria designed for use in children may have led to an underestimation of prevalence in the adults) [47]. However, many people do continue to have significant ADHD-related impairments as adults [16]. A meta-analysis reported that the rate of persistence of a full DSM-IV diagnosis of ADHD was 15% at the age of 25 years, but when those patients fulfilling the DSM-IV definition of ADHD in partial remission were included, the rate of persistence increased to approximately 65% [48]. It has been estimated that this level of persistence equates to an estimated prevalence of 0.6–1.2% of adults by the age of 25 [16]. Our estimate of less than 0.02% prevalence in adults in 2009 (approximately 7,800 adults with ADHD in the UK [49]) is therefore much lower than expected [47, 50], suggesting that the under-recognition of ADHD in adults exceeds that for children and adolescents. One possible explanation for this low prevalence rate could be that clinicians in the UK have only been diagnosing children over the last 20 years or so. As a consequence most adults were not diagnosed as children and, as services for adults are still not generally available, they are not yet getting diagnosed in large numbers as adults. Also, many adolescents are not transitioned to adult services. A study using data from GPRD identified that, for people aged 15–21 years between 1999–2006, prevalence of prescribing of ADHD medication decreased with increasing age but increased with increasing calendar year [43]. During the study period, we found a large increase in the prevalence of ADHD in adults (1.4 to 16.1 per 100,000 between 1998 and 2009), suggesting that either ADHD is now being increasingly recognised in adults or that children with a diagnosis of ADHD have grown and are still recognised to have the condition as adults.

The magnitude of the difference in annual mean costs was surprising. Prescription costs in year 1 were higher for cases compared to controls (£308 and £37, respectively), largely due to the cost of ADHD medicines. NICE guidance indicates that drug treatment should be first line when ADHD is severe and can be considered for moderate ADHD and impairment in school-aged children and young adults when non-pharmacological approaches are unsuccessful [16]. In adults, drug treatment is recommended by NICE as first line unless the patient prefers psychological treatment. Drug treatment is not recommended for pre-school children. Within the context of significant under-recognition it is likely that those individuals receiving a diagnosis would be at the more severe end of the ADHD continuum. As a consequence medication treatment would often be considered the first-line treatment for all except the very young.

Numerous studies investigating the healthcare costs associated with ADHD have been carried out in the USA, but their applicability to the UK NHS is questionable due to different patterns of service provision. Using information available for the UK, some estimates have been made of the cost of certain aspects of healthcare for ADHD at the population level. For health, social care, and educational services, it has been estimated that the NHS spends approximately £23 million on initial specialist assessment of ADHD in England and Wales and £14 million on follow-up care over one year [51]. In addition, the NHS spent approximately £8.5 million, £1.3 million, and £25.7 million on prescriptions for atomoxetine, dexamfetamine, and methylphenidate, respectively, in 2010 [3]. It is likely that almost all of this would have been spent in the treatment of ADHD, although dexamfetamine and methylphenidate also have an unlicensed indication for narcolepsy [21]. Furthermore, the mean annual cost of health and social care and educational resources relating to ADHD per adolescent in the UK has been estimated as £5,493 (median £2,327), where 24% of this cost relates to health [52]. In addition, ADHD commonly occurs with other conditions such as learning disorders, conduct and oppositional disorders, Tourette’s syndrome, bipolar disorder, anxiety and depression [16], and these conditions are likely to contribute to the higher healthcare costs observed for ADHD patients.

This study had inherent limitations. For cases where there was no prescription for an ADHD medication the requirement of two or more diagnoses was used in order to avoid selecting patients where the GP had recorded a provisional diagnosis of ADHD prior to referral for assessment by a specialist. However, this may have led to the exclusion of possible ADHD patients from the cost calculation and an underestimate of the incidence and prevalence rates. The care pathway for ADHD differs in comparison to many chronic conditions and will vary by site. Once the condition has been stabilised, GPs often prescribe drugs for ADHD under shared-care protocols. Prescriptions written in secondary care are not recorded in CPRD and could not be costed. Any underestimation of resource use and the related financial cost will disproportionately impact on the ADHD group and therefore the differences reported may underestimate the true difference. Those patients who are more difficult to stabilise may be less well recorded in CPRD as more of their healthcare may be provided in secondary care. In addition, our study index date may vary between patients from the date of first presentation to the GP, the date of referral back to the GP from secondary care, or the date of the first GP prescription for an ADHD medication. CPRD includes GP practices from all four UK regions and is therefore generalisable to the whole of the UK. On the other hand, the linked HES data is exclusively English, which could suggest that the healthcare cost estimates are generalisable to England only. However, the patients registered in the linked practices have been shown to be representative of the whole CPRD population [53].

Regarding the estimation of prevalence, a patient had to have received a diagnosis of ADHD or a prescription for a medication for ADHD both prior to and after 1st July each year. Although a washout period of 12 months was applied, the time between the mid-year point and the last collection date for the database becomes shorter for the more recent years and this may have contributed to the reduction in prevalence rates since 2007. However, this method was selected since clinical records in CPRD cannot be used to determine when a patient stops experiencing ADHD. The calculation of incidence is sensitive to the method used to calculate the denominator. Although, for this study, patients need not necessarily have had contact with their general practice to be included in the patient-years estimate. An underestimation in the incidence and prevalence rates could also have occurred if diagnoses were not accurately recorded in CPRD. The validity of medical diagnoses in CPRD have been confirmed in several studies [54, 55]. GPs in the UK act as gatekeepers, and referrals to and outpatient letters from secondary care should be recorded in CPRD. However, ADHD diagnoses may be less well recorded than other conditions diagnosed and treated exclusively in primary care.

In summary, the prevalence of diagnosed ADHD in the UK was notably lower than in reports that used screening. Costs in those with ADHD were more than four times higher than in those without ADHD.

This research was funded by Shire Development LLC.