Social inequalities in prevalence of diagnosed and undiagnosed diabetes and impaired glucose regulation in participants in the Health Surveys for England series

Introduction

Diabetes is a condition that can result in significant morbidity and mortality,¹ exacerbated by the latent period of several years during which diabetes is asymptomatic and may therefore remain undiagnosed.² During this time, people are at risk of the complications of diabetes but being undiagnosed, they do not receive treatment to reduce the risks of cardiovascular or other diabetes-related diseases: at the time of diagnosis, up to 50% of people with diabetes already have microvascular and macrovascular complications.^{3 4} Early glycaemic control can reduce complications,⁵ so targeted testing for diabetes is advocated by Diabetes UK⁶ and the American Diabetes Association (ADA).⁷ In 2012, the National Institute for Health and Care Excellence (NICE) published guidelines recommending targeted screening to identify undiagnosed diabetes in asymptomatic populations.⁸

Recently, a further high-risk category has been recognised: people with impaired glucose metabolism but with blood glucose levels below the diabetic range, referred to in this paper as having impaired glucose regulation (IGR). Different definitions of IGR exist but an International Expert Committee on the role of glycated haemoglobin (HbA1c) in diagnosing diabetes suggested that people with HbA1c of 42–47 mmol/mol (6.0–6.5%)⁹ should be included in this group. NICE adopted this recommendation in its 2012 guidance.⁸ Studies have shown an annual progression rate from IGR to diabetes of 5–10%^{10 11} and that treatment can prevent or delay progression to both diabetes^12–15 and its complications.^{16 17} For these reasons, NICE recommends treating people with IGR, primarily with intensive lifestyle interventions but with metformin and orlistat where lifestyle changes fail or are inappropriate.⁸ These guidelines have not yet been incorporated into the Quality and Outcomes Framework (QOF) for general practitioners (GPs).

Socioeconomic inequalities in health and risk factors are well known,¹⁸ as is Hart's¹⁹ inverse care law, that those at highest need are least likely to receive healthcare. Rising obesity levels, which are socially patterned,²⁰ have been paralleled by the increasing prevalence of type 2 diabetes;^{21 22} information regarding trends in diagnosed diabetes is available from QOF and the National Diabetes Audit.^{23 24} These databases provide information about patients, so may not accurately reflect diabetes in the general population. In particular, they do not tell us about IGR or undiagnosed diabetes, nor do they provide information about social inequalities.

Nationally representative information about those with undiagnosed diabetes and IGR is required to understand, manage and reduce the potential impact of diabetes on population health. A recent report used Health Survey for England (HSE) data to count and characterise people with non-diabetic hyperglycaemia (IGR) and assess the relative utility of four risk scores.²⁵ We hypothesised that more deprived groups are likely to have higher prevalence of IGR and undiagnosed diabetes, both for absolute prevalence and as a proportion of all cases of diabetes. The objectives of this study were to: determine the prevalence, and trends in prevalence, of diagnosed and undiagnosed diabetes and IGR in nationally representative data from the HSE; examine socioeconomic and ethnic inequalities in IGR and undiagnosed and diagnosed diabetes; and examine the health status and biomarkers for cardiovascular disease risk in people with these conditions. We also examined the summary performance characteristics of various current screening strategies.

Methods

Results

Discussion

Our study provides up-to-date information on prevalence and trends in prevalence of diabetes, undiagnosed diabetes and IGR in the general population and social inequalities in these. These survey data show the average prevalence of diagnosed diabetes in adults aged 16+ in England in 2009–2013 was 6%, the same as the prevalence reported by QOF over the 2012/2013 period among those aged 17+.²³ As expected, diabetes was more prevalent with increasing age and obesity as well as among males, people of black and Asian ethnicities and those in lower socioeconomic positions. Around a quarter of people with diabetes are still undiagnosed. Consistent with previous work,^38–42 HSE participants with IGR have health characteristics between those with normoglycaemia and with diabetes, supporting the idea of a continuum of disease.^{43 44} We also showed that low socioeconomic status, known to be associated with diabetes,^{45 46} is also associated with IGR; education was the best marker. The prevalence of people with IGR is lower than expected and appears to be static. There is a graded association of cardiovascular characteristics across the continuum of impaired glucose metabolism; a diagnosis of diabetes appears to have beneficial impacts on risk factor modification. A number of different screening criteria can each identify the majority of those with impaired glucose metabolism but support the use of a validated risk assessment tool as the most sensitive screening method.

The main strengths of this study are the analysis of data from nationally representative random samples of the free-living population, rather than limiting analysis to those in the healthcare system with diagnoses or tests of blood glucose levels. Non-response weighting, recommended as the best approach to enhancing representativeness,⁴⁷ was used, making these data as generalisable as possible for England. Being a general health examination survey, the richness of data about each participant enables wider examination than a diabetes-focused study, hence the breadth of information on socioeconomic position and the range of variables that could be examined. However, all surveys are limited in the extent of the questionnaire; we had no information on family history of diabetes, one component of the Diabetes UK/Leicester risk score tool. A sizeable minority of data were missing for explanatory variables, especially income and BMI. These participants were retained in the analysis, coded as a separate ‘missing’ category. The nature of the study results in other limitations, particularly in how the categories of impaired glucose metabolism were defined. First, to be included in the diagnosed diabetes group, participants needed to self-report a diagnosis or be on diabetic medication. For this reason, a proportion of diagnosed diabetics may have been missed. The effect of this is likely to be small; furthermore, if people do not report that they have diabetes and are not treating their condition, it may be appropriate to consider them as undiagnosed diabetics. Another problem is the use of a single HbA1c test to categorise participants. In practice, symptoms or a confirmatory test would often be used to diagnose diabetes in people with an HbA1c of ≥48 mmol/mol. Applications of such strict definitions might alter the number of patients in each category but any such alteration and its impact are likely to be small at a population level. As discussed, impaired glucose metabolism is a continuum, and although categories are defined for practical reasons, all those with raised HbA1c levels are at risk and could benefit from identification and risk reduction. Although the direction of association cannot be determined in cross-sectional surveys, these are the only ethical way to monitor trends in IGR and undiagnosed diabetes; cohort studies should always release such results to their participants, thus new participants are required for these analyses.

The HSE gives unique information about undiagnosed populations and social inequalities in health, not available in routine data; until recently, prevalence estimates of undiagnosed diabetes and IGR were based on models.⁶ Unlike the recent Public Health England report,²⁵ we found marked socioeconomic inequalities in IGR. They examined only deprivation quintiles but failed to adjust for confounding by age; we also examined three individual-level markers of socioeconomic position and found similar health inequalities, with education and income remaining significant in a multivariate regression model with IGR or undiagnosed diabetes as the outcome. We found the average prevalence of undiagnosed diabetes in 2009–2013 was 1.6%. Approximately 22% of people with diabetes remain undiagnosed, a higher proportion than the Association of Public Health Observatories (APHO) Diabetes Prevalence Model estimates.⁶ This proportion has fluctuated over the 5 years, but the prevalence of both undiagnosed diabetes and IGR have not changed appreciably over this time, possibly mirroring the stabilising prevalence of obesity in adults in England.²⁰ It is too soon to assess the impact of 2012 NICE guidance and the introduction of targeted screening for earlier diagnosis.

The average IGR prevalence was 11%, with no discernible trend over our 5 years of data. This contrasts starkly with another recent study using HSE data that suggested an increase in IGR prevalence from 11.6% in 2003 to 35.3% in 2011.⁴⁸ This is explained by the differences between definitions of IGR. We used an HbA1c range of 42–47 mmol/mol, as recommended by an international expert panel, NICE and Diabetes UK,^{6 8} whereas Mainous et al used a range of 39–47 mmol/mol (5.7–6.4%) as recommended by ADA,⁷ finding similar 2011 prevalence in the UK as in the USA using the same definition.⁴⁹ Our own findings, using the same data set for this paper, are that using a definition of 39–47 mmol/mol, IGR was 31.2% (30.4% to 32.1%) 2009–2013, and tended to decrease since 2010 (34%) to 28% in 2013. Overall, the proportion with HbA1c 39 mmol/mol or above was 38.4%, increasing from 2009 (37.7%) to 2011 (41.2%) before decreasing again to 35.9% in 2013.

The differences between the results for the different thresholds suggest that there is an increasing prevalence of people with an HbA1c of 39–41 mmol/mol but not with an HBA1c of 42–47 mmol/mol. There are two potential explanations for this. First, there may be a lag period and the impact of the increasing prevalence of those with lower range IGR is yet to be felt. The second explanation is that people with lower range IGR do not necessarily develop increasingly deranged glucose metabolism, meaning a change in prevalence of this group would not necessarily be reflected in groups with higher HbA1c levels. If this is the case, it would provide significant support for the use of an HbA1c of 42 mmol/mol as the lower threshold for IGR as per current UK recommendations.

We showed a continued increase in the prevalence of diabetes over the 5 years, consistent with information from other databases.^{23 24} This is unsurprising in the context of rising levels of obesity but provides concerning insight into the potential impact of this serious and costly condition on population health in the future. A large proportion of people with diabetes are still undiagnosed, so do not benefit from the potential advantages that a diagnosis carries. Modelling studies have suggested a benefit in using screening to detect early diabetes.^{50 51} In contrast, recent data from the ADDITION trial⁵² does not show a reduction in mortality with screening for diabetes. In terms of biomarkers, our data support the assumption that a diagnosis can be beneficial,⁵³ particularly for lipid and blood pressure modification: people who are diagnosed will benefit from QOF and guideline-driven risk factor modification, resulting in a stalling or reduction of cardiovascular risk factor progression despite advancing disease. However, self-reported health was worst in this group.

There is a clear case for identifying IGR as cost-effective interventions exist to prevent or delay diabetes^{6 8 54 55} for people in this category. Our lower prevalence of IGR implies that interventions to manage this population may be more feasible and less costly than was anticipated based on an earlier study.³⁸ IGR and undiagnosed diabetes are more prevalent in more deprived groups, even after adjusting for demographic factors and obesity. Undiagnosed diabetes as a proportion of all cases is also higher in these groups. It is recognised that individual ‘agentic’ interventions may widen inequalities, as disadvantaged groups are less likely to participate.⁵⁶ Thus, only if GPs manage to target deprived groups successfully for detection and support with lifestyle modification, this programme could reduce social inequalities in health. Similarly, focusing screening and advice by occupational health departments on employees in lower supervisory and technical and semiroutine occupations could also reduce health inequalities and employee morbidity. Future health examination surveys are needed to monitor continuing trends in changes in national prevalence and social inequalities of IGR and undiagnosed diabetes, as clinical commissioning groups pilot and roll out detection and intervention programmes.