JRSM >
Volume 101, Number 11 >
Pp. 558-563
1 Department of General Practice and Primary Care, University of Aberdeen UK
2 Department of Public Health and Epidemiology, University of Manchester UK
3 Community Health Sciences, University of Nottingham UK
4 Allergy & Respiratory Research Group, Division of Community Health Sciences, University of Edinburgh 20 West Richmond Street, Edinburgh EH8 9DX, UK
Correspondence to: Aziz Sheikh aziz.sheikh{at}ed.ac.uk
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Aim To investigate recent trends in the recorded incidence, lifetime prevalence and consulting behaviour of patients with multiple allergic disorders in England.
Methods QRESEARCH is one of the world's largest national aggregated health databases containing the records of over nine million patients (including those who have left or died). Data were extracted on all patients with a recorded diagnosis of multiple allergic disorders, and annual age–sex standardized incidence and lifetime period prevalence rates were calculated for each year from 2001 to 2005. We also analysed the consulting behaviour of these patients when compared with the rest of the QRESEARCH database population.
Results The age–sex standardized incidence of multiple allergic disorders was 4.72 per 1000 person-years in 2001 and increased by 32.9% to 6.28 per 1000 patients in 2005 (p<0.001). Lifetime age–sex standardized prevalence of a recorded diagnosis of multiple allergic disorders increased by 48.9% from 31.00 per 1000 in 2001 to 46.16 in 2005 (p<0.001). Over this period, the mean consultation rate to general practitioners for these patients increased from 4.68 to 4.90 consultations per person per year.
Conclusions Recorded incidence and lifetime prevalence of multiple allergic disorders in England have increased substantially in recent years.
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0.4kU/l HDM specific IgE)3 indicating the very high proportion of young people who are at high risk of manifesting allergic problems. The presence of one allergic disorder significantly increases the risk of developing other allergic disorders affecting different organ systems.4,5 This has been attributed to the shared predisposition and mechanism for development of these diseases, whereby an immunological response to environmental allergens is driven by type 2 T cell cytokines.6 Both patients and healthcare providers find managing multiple allergies to be particularly problematic, with patients often requiring referrals to a number of different specialists.7
The Health Survey for England (HSE 1995 to 1997) found that 11% of children (2–15 years), 10% of young adults (16–44 years) and 5% of older adults (45+years) experienced multiple allergic disorders.1 While such survey data provide useful information on variations in period and lifetime prevalence of self-reported diagnoses of allergic disorders, particularly in children and adolescents, there are relatively little reliable national data describing clinician-diagnosed disease prevalence; furthermore very few data exist on the overall population trends over time for all ages.
Exploitation of large national healthcare data-sets, with their key strengths of large numbers and representative data, offers an important opportunity to develop insights into the epidemiology of multiple allergic disorders.8 Studying primary care databases provides a window onto overall national trends – something that is not possible with large scale surveys such as the International Study of Allergies and Asthma in Childhood (ISAAC), which has studied only children,9 and the European Community Respiratory Health Survey (ECRHS), which has surveyed only adults.10 Large primary care data-sets, recording information at the point at which the majority of patients with multiple allergic disorders are likely to be managed does, however, offer an important opportunity to study changing patterns of disease. Building on previous work,11–13 we sought to describe recent trends in the primary care diagnosis of patients with multiple allergic disorders in England.
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Patients were included in an 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. Patients were considered to have an allergic disorder if they had a relevant computer-recorded diagnostic Read code (see below) in their electronic health record during the time period of interest.
Incidence was defined as the number of patients with a new case of having multiple allergic conditions diagnosed in a specific year, with the denominator being the number of patient-years of observation (calculated from the number of patients registered with practices and their length of registration). Lifetime prevalence was defined as the number of people having multiple allergic conditions ever recorded on at least one occasion in the GP records; the denominator used to calculate the lifetime prevalence rate was the number of patients registered with the study practices.
Definitions
An allergic condition was defined as patients who have the following: peanut allergy (SN582), eczema (M11.., M111., M113–4, M11z, M12z and below), allergic rhinitis (H17 and below, H18., Hyu21 and Hyu22), asthma (Read code H33 and below) or anaphylaxis (SN50 and below, SP34).
Statistical methods
Because 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 multiple allergies in England. The Mantel-Haenszel 
2 test were used to investigate trends over time, this analysis being undertaken using EpiInfo2000 (World Health Organization, Geneva, Switzerland).
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Age–sex standardization of lifetime prevalence of patients with multiple allergic disorders and changes over time
The lifetime prevalence of multiple allergic problems for 0–14-year-olds was 6.0% (95% CI 5.9–6.1), 5.4% in 15–44-year-olds (95% CI 5.3–5.5) and 3.3% (95% CI 3.2–3.4) for those aged over 45 years. Figure 1 shows the lifetime prevalence by age and sex per 1000 registered patients in QRESEARCH. Multiple allergic disorders were more common among women in people aged under 20 years or over 75 years. In 2005, the lifetime prevalence rate of multiple allergies was highest in boys aged 10–14 years.
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Main strengths and limitations of this work
The main strengths of this study include our interrogation of an extremely large nationally representative data-set, the fact that all contributing practices used the same computing systems for electronically recording clinical data and the approach used to ensure that all contributing practices were accustomed to electronically recording routine data. The study design employed ensured that there was no risk of selection bias due to non-responders or recall bias. Another strength of this study was the use of contemporaneous clinician recording of a diagnosis of allergic disorders as opposed to patient self-reporting of historical diagnoses or symptoms.9,10
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 allergic disorders and the possible improvement in recording of allergic disorders over this time period. The relatively short time window over which trends were studied is another limitation, but this does also have the advantage of confining analysis to a period during which there were no changes in disease definition or classification. Data regarding childhood prevalence may be underestimated, as the ascertainment of disease present in the community will be dependent on parents bringing their children for consultation.14 There may have been an underestimate inthe rates of multiple allergic disorders reported. Firstly, egg and egg-protein allergy (SN580, SN581)was not included in the final analysis. Secondly, it has been previously reported that Read codes for allergy, in particular for food allergies, are missing, and therefore some practices may have had difficulties in recording some allergic disease.15
Comparison of findings with other published work
We found a lower lifetime prevalence of multiple allergic disorders when compared to the prevalence rates found using the HSE (QRESEARCH database: 0–14 years: 6.0%; 15–45 years: 5.4%; 45+ years 3.3% when compared to HSE: 2–14 years: 9%; 15–44 years: 8%; 45+ years 4%), this most probably reflecting differences between diagnosed disease prevalence (electronically recorded in general practice) and prevalence based on patient recall of doctor diagnosis, as is the case with the HSE.1
Meaning of the study results: possible mechanisms and implications for clinicians and policy-makers
There may be several possible reasons for this increase in multiple allergic disorders this including an increase in sensitization, which would then in turn predispose the development of any of a number of allergic disorders. Supporting evidence for such a possibility comes from an important study by Law et al. which found significant increases in atopic sensitization in the UK over a 25-year window.16 Increased predisposition to atopy,17 possibly reflecting changing exposure to known and unknown risk factors,18 may also be important. Increases in the rate of these conditions could however result from increased clinician awareness of allergic problems, which may then have led to improved identification and recording of allergic disorders. Similarly, increased patient awareness, or parental awareness of the potential of accessing effective treatments may have resulted in increased case presentation in primary care.
The House of Lords Allergy Inquiry published in 2007 has identified several issues highlighted by this work and other previous research that require further attention.19 In particular, given the large and possibly increasing numbers of people with multiple allergic problems, there is a need to invest in improved co-ordinated diagnosis, management and support for patients in order to reduce avoidable morbidity and inconvenience that many patients currently experience in moving between organ-based specialist care providers.
Conclusions and future research
This large national study reveals that the recorded incidence and lifetime prevalence of patients with multiple allergic disorders increased in England during the first half of this decade, which may reflect a genuine increase in the incidence of multiple allergic disorders, improved awareness, diagnosis and recording in primary care, or, perhaps most plausibly, a combination of genuine increases and improved identification and recording. Future work needs to try and distinguish between these different possible explanations. A key related important unanswered question concerns the quality of care and symptom control of these patients.20 Given the relatively higher prevalence of multiple allergic problems in children when compared with adults, the overall numbers of people in England with multiple allergic problems is, for the present at least, likely to continue to increase.
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DECLARATIONS
Competing interests JHC is Director of QRESEARCH (a not-for-profit organization owned by the University of Nottingham and EMIS, commercial supplier of computer systems for 60% of GP practices in the UK). CS, JN and AS have no competing interestsFunding NHS Health and Social Care Information Centre
Ethical approval Ethics approval was not required as this analysis was conducted using only anonymized data
Guarantor JHC
Contributorship AS, JHC and JN were involved in designing the studyand CS contributed to literature searches and led the drafting of the paper with all co-authors commenting on drafts of the manuscript
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