Abstract
Summary
Background
Observations in the UK at the end of the last century found increasing trends of asthma prevalence over time. However, it has been reported that the number of new cases of asthma presenting to general practice has declined, especially among younger children.
Aim
To study national trends in the epidemiology of asthma.
Methods
A cross-sectional observation analysis was performed using the QRESEARCH database, which is one of the world's largest national aggregated health databases containing records from 422 English practices yielding 30 million patient-years of observation. Data was extracted on 333,294 individuals with a recorded diagnosis of asthma and calculated annual age–sex standardized incidence, lifetime period prevalence and asthma-related prescribing rates for each year from 2001–2005.
Results
The incidence rate of asthma decreased in all patients (2001: 6.9 (95% confidence intervals [CI] 6.8–7.0); 2005: 5.2 (95% CI 5.1–5.3) per 1000 patient-years, p<0.001), but most particularly in children under 5 years of age (–38.4%) where a decrease in the lifetime prevalence of asthma (–34.3%) was also found. However, the lifetime prevalence rate of asthma for adults increased (15–44 years: 23.3%; 45–64 years: 27.7%; >65 years: 21.5%) with an estimated 5,658,900 (95% CI 5,639,700–5,678,200) or approximately one person in nine having being diagnosed with asthma in England. The number of asthma-related prescriptions also increased over the study period (17.1%), such that in 2005 an estimated 32,577,300 (95%CI 32,531,600–32,623,000) prescriptions were issued.
Conclusions
This large national study reveals that the rate of new diagnoses of asthma appears to have passed its peak; however, the number of adults with a lifetime asthma diagnosis continues to rise. Whether these trends are genuine or are a result of the introduction of incentives and guidelines to improve identification and recording of asthma or changing diagnostic trends is a question with important public health implications and one, therefore, that warrants detailed further enquiry.
Introduction
Asthma is one of the most important chronic conditions in the UK, affecting patients' quality of life and posing a high level of burden on health services. 1 Observations in the UK at the end of the last century found increasing trends of asthma prevalence over time. 2, 3 However, it has been reported that the number of new cases of asthma presenting to general practice has declined, especially among younger children. 4
This study of national trends in the epidemiology of asthma was commissioned by the Chief Medical Officer for England because of growing concern about the high prevalence, disease burden and healthcare costs (£800 million [€880 million] annually) associated with asthma (and other respiratory disorders) and is being used to inform policy deliberations on allergy and respiratory service provision in England. 5
Methods
Version 10 of the QRESEARCH database was used for these analyses. This database contains broadly representative anonymized aggregated health data derived from 422 primary care practices throughout England. Data were available for each year during the period 1 January 2001–31 December 2005, these comprising of between 2.8 and 3 million individual patients who collectively contributed over 30 million patient-years of observation. All individuals resident in England (including children) are registered with primary care, which is free at the point of contact. The methods used to collect primary care data for the QRESEARCH database have been previously described. 6, 7, 8, 9
Patients were characterized by gender, age (under 5 years; 5–14 years; 15–44 years; 45–65 years; >65 years), deprivation (area-based Townsend deprivation quintiles), and were included in the analysis year if they were registered for the entire year of study. Patients with incomplete data (i.e. temporary residents, newly-registered patients and those who joined, left or died during the study year) were excluded.
Asthma Read codes used in the analysis
All analyses were conducted using de-identified data and were subject to the QRESEARCH research governance process.
Definitions
Asthma was defined as patients who have Read codes H33 and below (Box 1). Asthma-related prescriptions were defined according to the new General Medical Services contract. 10 These drugs included bronchodilators (British National Formulary [BNF] chapter 3.1), inhaled corticosteroids (BNF chapter 3.2), and cromoglycates and related therapies (BNF chapter 3.3). In England, these drugs can only be prescribed by a clinician, usually based in a primary care practice.
Statistical methods
As a result of known age and sex variations, rates of disease and prescribing were standardized by sex and five-year age bands. The mid-year population estimates for England in each year of study were used as the reference population. These results were then used to estimate the numbers of people with asthma in England. Where appropriate, χ2 tests were used to test whether there were statistical associations between categorical variables. The Mantel-Haenszel χ2 test was used to investigate trends over time, this analysis being undertaken using EpiInfo2000 (Centers for Disease Control and Prevention, Atlanta, Georgia, USA). Where appropriate, 95% confidence intervals (95% CI) are reported.
Results
We estimated that 261,400 (95% CI 257,200–265,700) of 50 million people in England were newly diagnosed with asthma in 2005. We estimated that 5,658,900 (95% CI 5,639,700–5,678,200) had a GP recorded diagnosis of asthma, this translating into approximately one person in nine being diagnosed with asthma at some point in their lives. An estimated 3,257,000 (95% CI 3,242,400–3,271,600) people with asthma were prescribed an asthma-related drug. An estimated total of 32,577,300 (95% CI 32,531,600–32,623,000) asthma-related prescriptions were issued to patients in England in 2005.
Trends in incidence rate
Incidence and lifetime prevalence rates of asthma 2001–2005
Age–sex standardized rate per 1000 patients (95% CI)
Incidence rate of asthma by gender, age and socioeconomic status
Crude rates are reported by age band
Most affluent group of patients
Trends in lifetime prevalence
Lifetime prevalence rate of asthma by gender, age and socioeconomic status
Crude rates are reported by age band
Most affluent group of patients
Trends in asthma-related prescribing
Asthma-related prescribing by gender, age and socioeconomic status
Crude rates are reported by age band
Most affluent group of patients
Socioeconomic variations
There were substantial socioeconomic differences found in the rates of asthma, with the most deprived (quintile 5) having higher incidence (Table 2) and lifetime prevalence of asthma (Table 3) and higher rates of being prescribed asthma-related drugs (Table 4) than the most affluent patients (quintile 1; p<0.001).
Discussion
This study, using routinely collected electronic data from one of the world's largest national data-sets, has confirmed that asthma is extremely common, and that despite an apparent decrease in the number of people newly presenting with the disease, the number of those with a lifetime diagnosis and prescriptions of asthma-related drugs issued by primary care continues to grow. However, conversely, we have found evidence that in children, the rate of new and lifetime diagnosed prevalence of asthma and the issuing of asthma-related prescriptions (for pre-school children < 5 years) appears to be declining. Substantial socioeconomic differences were also found in the rates of asthma, with the most deprived having higher incidence and lifetime prevalence of asthma and higher rates of being prescribed an asthma-related drug.
Main strengths and limitations of this work
The main strengths of this study include our interrogation of patient-level computerized data from an extremely large nationally representative data-set, the fact that all contributing practices used the same computing systems for electronically recording clinical data, the approach used to ensure that all contributing practices were accustomed to electronically recording routine data, and the use of contemporaneous clinician recording of a diagnosis of asthma as opposed to patient self- or parental reporting of historical diagnoses or symptoms. The study design employed ensured that there was no risk of selection bias due to non-responders or recall bias.
There are a number of limitations related to the use of large routinely collected data from primary care, including the dependence on clinician-recorded diagnosis of asthma (especially as diagnostic tests [e.g. spirometry] used to confirm or refute diagnoses were not available), possible improvements in recording over the study time period and a lack of information on the dispensing of drugs from pharmacies or evidence of patient adherence to prescribed medications. The relatively short time window over which trends were studied is another limitation, and although this did have the advantage of confining analysis to a period during which there were relatively few changes in disease definition, the introduction of the new General Medical Services Contract to UK primary care in April 2004, which introduced incentives to create and maintain a registry of patients with asthma (aged over 8 years and with one asthma-related prescription), may have influenced the prevalence of asthma toward the latter end of the study period. Data regarding childhood incidence and prevalence may be underestimated, as the ascertainment of disease present in the community will be dependent on parents bringing their children for consultation. 11 The inadequacy of Read codes for asthma (as well as other allergy-related conditions) has previously been reported and this may have contributed to under-recording. 12 Prescribing data are not available by clinical indication and, therefore, it is possible that for a proportion of older patients, prescriptions for other respiratory diseases such as chronic obstructive pulmonary disease are included.
Comparison of findings with other published work
Comparison of UK epidemiological data for asthma
MSGP – Morbidity Statistics from General Practice; ISAAC – The International Study of Asthma and Allergies in Childhood; HSE – Health Survey for England
Meaning of the study results: possible mechanisms and implications for clinicians and policymakers
Our data could be interpreted as indicating that asthma may now be decreasing in children. However, these results need to be interpreted in the context of other studies, which have reported increases over the same time period (using QRESEARCH data) in the prevalence of eczema, allergic rhinitis and multiple allergies in children, 7, 8, 9 increases in atopic sensitization in repeated UK birth cohorts over a 25-year window, 16 and increases in the prevalence of asthma globally. 17 It is, therefore, likely that rather than a reduction of more persistent wheezing associated with allergic sensitization (i.e. the ‘classic’ asthma phenotype), 18 what we may be witnessing is the impact of revised national asthma guidelines, 19 with the recognition that infants and pre-school children who wheeze may not be asthmatics, but are wheezing secondary to small airway or respiratory viral infections thus resulting in a diagnostic shift from ‘asthma’ to labels such as ‘wheeze’ or ‘acute respiratory infection’. 20 For adults, the reduction in incidence rates could also reflect a growing concern that, after years of under-diagnosis, asthma may now be over-diagnosed in primary care. 10 It is also possible that the observed changes in disease incidence and prevalence could have been affected by the introduction of the new incentivized General Medical Services contract, whereby payments are made to practices to keep accurate registers of patients with diseases such as asthma. 21 However, despite these changes in recorded incidence, increases in the lifetime prevalence of asthma and asthma-related prescribing appears to be continuing, such that by 2005 an estimated 5.7 million people in England were diagnosed with asthma, with over half of these (3 million) requiring recent medical treatment. It is possible that this is the result of changes in environmental factors over time which has favoured the expression of allergic disease in those who are genetically susceptible. 22 However, it is also possible that increases in the rate of these conditions could have resulted from increased clinician awareness of allergic problems, which may then have led to improved identification and recording of asthma. Similarly, increased patient awareness of the potential of accessing effective treatments may have resulted in increased case presentation and prescribing in primary care. Although data were not available on smoking, it is likely that higher rates of smoking among the more socioeconomically-deprived groups of patients 23 will have contributed to the substantial socioeconomic differences found in the rates of asthma and asthma-related prescribing.
Whatever the cause, the increasing number of people with asthma in England poses a substantial disease burden on primary care services. 24, 25
Conclusions and future research
This large national study reveals that the rate of new diagnoses of asthma appears to have passed its peak. However, the number of adults with a lifetime asthma diagnosis continues to rise. Whether these trends are genuine or are a result of the introduction of incentives and guidelines to improve identification and recording of asthma or changing diagnostic trends is a question with important public health implications and one, therefore, that warrants detailed further enquiry.
Footnotes
DECLARATIONS
Footnotes
Acknowledgements
We would like to record our thanks to the contributingEMIS practices and patients and for EMIS for providing technical expertise in creating and maintaining QRESEARCH. We thank QRESEARCH staff (Julia Hippisley-Cox, Govind Jumbu, Alex Porter, Justin Fenty, Mike Heaps and Richard Holland) for their contribution to data extraction, analysis and presentation. These findings have been reported in Primary care epidemiology of allergic disorders: analysis using QRESEARCH database 2001–2006, which is published by the NHS Health and Social Care Information Centre