Ethnic differences in blood pressure monitoring and control in south east London

Study design

The dataset comprises electronic patient records from practices in Lambeth. Lambeth has the second highest proportion of ‘black’ or ‘black British’ residents in the UK, at 25.8% (neighbouring Southwark has the highest proportion at 25.9%).17

The database was originally set up to improve both the level and quality of ethnicity coding in GP data, in order to facilitate ethnic monitoring of health inequalities.18 Several strategies were used to improve both the level and quality of ethnicity coding.19

Study sample

To facilitate data extraction, the study sample was restricted to only those practices using the EMIS (Egton Medical Information Systems) LV computer system, comprising just over half (27/53) the GP practices in Lambeth. The study sample covered all patients (192 432) from these practices, and data were extracted from local practice computer systems in March 2009, using MIQUEST software.

Outcome measures

Outcomes for blood pressure monitoring and control were primarily based on QOF targets for each chronic disease. Blood pressure monitoring was coded as positive for those with diabetes, CHD, stroke, and CKD, if there was a record of blood pressure measured in the past 15 months and, for hypertension, the past 9 months. Blood pressure control was coded as positive for those with hypertension, CHD, or stroke if their last recorded blood pressure was 150/90 mmHg or less. For patients with diabetes, the target was 145/85 mmHg or less, and for those with CKD 140/85 mmHg or less. The most recent systolic, diastolic, and mean arterial pressures were also examined; the latter was calculated as: [(systolic blood pressure) + (2 × diastolic blood pressure)]/3.20

Predictors

Patient ethnicity codes were mapped on to UK census ethnic categories.17 Initially outcomes were compared for white (white British), black (black or black British) and Asian (Asian or Asian British) patients. It has been argued that more detailed ethnic definitions are justified when exploring differences in blood pressure treatment.9 Therefore, a subgroup analysis was conducted comparing patients in the more detailed ‘other white’ (excluding those coded English, Scottish, Welsh, or Northern Irish), black African and black Caribbean census categories. For brevity, the terms African and Caribbean will be used when referring to the latter ethnic groups, and white when referring to the white British group.

Relative social deprivation was assessed at patient level by mapping patient postcodes to lower super output areas and assigning the Index of Multiple Deprivation (IMD 07) score21 for each patient's immediate neighbourhood. Age, sex, and number of comorbidities (of the five specified chronic diseases) were also adjusted for.

Statistical analysis

The effect of ethnicity on whether patients met QOF blood pressure targets was assessed using logistic regression in Stata (version 10). Recent blood pressure levels were also analysed as a continuous outcome using multiple regression. For each analysis, the standard errors were adjusted to account for the effect of clustering at practice level, using the Huber/White sandwich estimating procedure.

Summary of main findings

The study found that overall blood pressure monitoring was at least as good for BEM patients with chronic conditions as for the majority white population. However, black patients were consistently less likely to have their blood pressure adequately controlled.

Therefore, despite evidence that inequality in blood pressure control according to social deprivation has steadily narrowed over the past few years, there has not been a corresponding reduction in ethnic health inequality.

Strengths and limitations of the study

This study benefited from a large sample of patient records covering a range of ethnic groups and chronic conditions. The dataset was particularly suited to this study as Lambeth is an area of contrasting social deprivation with a large BEM population. While this provided enough statistical power to compare the main ethnic groups, it proved to be underpowered for the ethnic subgroup analysis. Also, the data were cross-sectional and, given that Lambeth has a highly mobile population, it is possible that this has affected overall treatment outcomes. For example, some BEM patients may have just begun treatment, after recently moving into the practice area and, therefore, their blood pressure levels may be higher. It was, however, possible to adjust the model to take into account the number of years registered, and this did not make any appreciable difference to the results.

The study was based on data collected as part of the QOF, the limitations of which have been well documented elsewhere.22,23 One possible source of bias in data based on QOF returns is the use of exception reporting to exclude patients.14,15 However, it was possible to overcome this in the present study by accessing data for all patients. As routine administrative data were used, the recorded diagnoses were not independently verified and there might be systematic bias in recording comorbidities. Lifestyle factors are another area where QOF data are relatively limited, and it was not possible to account for differences in exercise, diet, or obesity, all of which may be relevant confounders. It is worth noting that when the effect of smoking was examined, it was found that this made little difference to the results; hence this was excluded from the analysis.

Comparison with existing literature

The encouraging results for blood pressure monitoring for BEM patients are comparable with those of previous studies.8,11 Cappuccio et al found that black people with hypertension were more likely to be detected, although less well managed, than white people.8 It has been argued that blood pressure monitoring rates are relatively good for BEM patients, due to a greater awareness among GPs of ethnic differences in hypertension.10 Another factor may simply be that monitoring can be more easily conducted on an opportunistic basis, while achieving adequate blood pressure control itself demands sustained therapeutic effort and patient engagement, posing a greater clinical management challenge.24

The study results also confirm those of previous studies reporting poorer blood pressure control for black people with chronic conditions.8,16,25 It is notable that those studies that have so far failed to show any ethnic difference were based on smaller samples of BEM patients, because either they relied on national survey data,11,13 or they were concentrated in a small locality.12 For example, Nazroo et al looked at patients treated for hypertension in four waves (1998–2004) of the Health Survey for England, and detected only minimal ethnic differences in blood pressure control.11 Despite being confined to one local authority area, the present study benefited from a larger sample of BEM patients, and therefore had potentially greater power to detect a difference.

It is notable that the results showing a significant ethnic difference remain well within the 95% confidence intervals of those reported in the latter study. A further factor may be that the present study, by concentrating on one large urban area, is well placed to represent the experience of BEM patients, given that the majority of the BEM population is concentrated in a few urban areas in the UK. Despite some effort to represent more concentrated BEM populations, much of the Health Survey for England sample covers BEM participants in areas that are predominantly white.26 It is possible, therefore, that ethnic differences in treatment may be much less distinct in these areas, explaining the overall lack of difference reported by studies using these data.11,13,27 Conversely, the present results are very similar to those reported in a similar study in south west London.16 Using data collected up to the beginning of 2006, the latter study looked at records for GP patients with hypertension soon after QOF was introduced. That the present findings are so similar suggests that there has been little subsequent reduction in ethnic health inequalities.

The results of the present study also suggest that African individuals may have poorer blood pressure control compared to Caribbean people. However, there is very little previous research in this area to compare this with. While the present study looks at those with chronic diseases, only two previous studies have compared blood pressure for African and Caribbean patients overall, and both failed to show any significant difference, although both studies were based on much smaller samples.6,8 There has, though, been some interesting recent research that points to important differences in cerebrovascular risk between these two ethnic groups.28

Implications for future research and clinical practice

Poor blood pressure control has important implications for cardiovascular disease risk and, it has been argued that because hypertension is largely treatable, even small ethnic differences can have major implications for health resources.29 It has been established that lowering blood pressure levels results in a proportional decrease in cardiovascular disease risk, with a reduction of 5 mmHg diastolic blood pressure corresponding to a 34% reduction in the risk of stroke and a 21% reduction in the risk of ischaemic heart disease.30 On this basis, the present findings translate into a substantial additional cardiovascular disease risk for black patients with hypertension. The study was not confined to patients with hypertension but looked also at those with other chronic diseases for which poor blood pressure control increases overall cardiovascular disease risk. Of particular concern are stroke patients, where the mortality risk is much higher in the black population.1 While the present results for stroke patients did not achieve statistical significance they are consistent with the overall pattern of poorer blood pressure control in black patients. The study also looked at different ethnic subgroups, and further study is now needed to see if the results are replicated in other African–Caribbean communities.

While this study was able to show some clear ethnic differences in achievement of blood pressure targets, the mechanism behind this can only be speculated at this stage. It is possible that medication adherence, differentiated by ethnic group, may partly explain these differences, as some US studies have demonstrated,31,32 although further research is needed to establish this. Another question is whether black patients are being prescribed lower volumes of drugs to treat hypertension. More fundamentally, poorer blood pressure control in black patients with hypertension and other chronic diseases related to cardiovascular disease, may reflect higher population norms for mean blood pressure values in the black population.7 In consequence, this could mean that greater blood pressure reductions are required for this group to achieve fixed blood pressure targets. Another factor may be whether or not GPs adhere to ethnic-specific treatment guidelines for blood pressure management,33 and further work is needed to explore the different classes of medication prescribed and how these might contribute to ethnic differences in blood pressure outcomes. The authors are, therefore, currently working on a follow-on study looking specifically at the relationship between blood pressure control and the class of blood pressure drugs prescribed.