The burden of hypertension in sub-Saharan Africa: a four-country cross sectional study

The parent study was conducted using a cohort study design between January 2011 and July 2012. Participants were followed for up to six months, with evaluations at baseline, and at six months only. Results presented in this article are a cross-sectional analysis of the baseline data.

The study was conducted in four sub-Saharan countries, including Tanzania, South Africa, Uganda and Nigeria. Participants were enrolled from 5 different population groups, three defined by occupation, and two by degree of urbanization. Tanzania and South Africa enrolled school teachers, Nigeria enrolled nurses, whereas Uganda enrolled rural and peri-urban residents. Eligible subjects were adults aged 18 years or older, with no intension of migrating outside of their community of residence within the next 6 months (in Uganda), or retiring from service in the next 6 months (for Tanzania, South Africa and Nigeria).

School teachers in Tanzania were enrolled from 18 randomly selected public schools in Dar es Salaam. In South Africa, school teachers were enrolled from government schools in Cape Town Metropolitan area, where 111 schools with 20 or more teachers were invited to participate in the study. In Nigeria, a random sample of nurses were enrolled from two urban hospitals; one located in Abuja city and another in a semi-urban setting 1.5 h outside of Abuja city. In Uganda, participants were enrolled from two geographic locations; a peri-urban community in the Wakiso District 10 miles north of Kampala city, and from a rural community in Bushenyi District 200 miles west of Kampala city. In Wakiso District, a random sample of households was selected from two parishes comprising 13 villages. In Bushenyi District, households were randomly selected from an enumerated list of all the households in each village.

Informed consent was obtained from each subject either by voluntarily posting back a signed form with a completed questionnaire (South Africa and Tanzania) or through documentation with trained interviewers (Nigeria and Uganda). The pilot studies were approved by the: a) Harvard School of Public Health Institutional Review Board, b) Institute of Human Virology Heath Research Ethics Committee in Nigeria; c) Health Research Ethics Committee of the Faculty of Health Sciences, Stellenbosch University in South Africa; d) National Institute for Medical Research in Tanzania; and in Uganda; e) Makerere University School of Public Health Higher Degrees, Research and Ethics Committee; f) Mbarara University of Science and Technology Ethics Committee, and g) the Uganda National Council of Science and Technology.

We used a standardized questionnaire to collect data on socio-economic characteristics, history of infectious and chronic disease diagnoses, mental health, injuries, and common risk factors for non-communicable diseases (NCDs) including tobacco use, alcohol use, and physical activity. Most of the questions were adapted from the World Health Organization STEPS instrument developed for use in resource-limited countries [11]. Physical and biochemical measurements were also made following a standardized protocol across the five study sites. Physical measurements included weight, height, and blood pressure. Height was measured without shoes, to the nearest centimetre. Weight was measured with the participants in light clothing and without footwear using a weighing scale to the nearest tenth of a kilogram. Blood pressure measurements were taken on the left arm with the participant in the sitting position using a cali-brated electronic blood pressure device (Welch-Allyn®). Three systolic and diastolic blood pressure measurements were taken at least five minutes apart. The average of the last two blood pressure readings was used in this analysis.

After administering the questionnaire, interviewers requested participants for an appointment to return the following morning so that a blood sample could be obtained to measure plasma glucose levels. Participants were instructed to fast overnight and no exercise or smoking in the morning, in preparation for obtaining a blood sample to conduct a fasting plasma glucose (FPG) measurements. The following morning the interviewer returned and collected the participant’s finger prick blood sample for an FPG test using a digital glucometer (On-Call® Plus, ACON Laboratories). Only participants reporting compliance with an overnight 8-h fast, no exercise or smoking that morning were eligible for the finger prick blood sample collection. Non-compliers were rescheduled to a future date where possible; otherwise this procedure was omitted in such participants. FPG levels were recorded in milli moles per liter (mmol/L).

Participants were classified in different blood pressure categories based on the World Health Organization (WHO) cut-off criteria [12, 13]. Thus a participant was classified as being hypertensive if their average systolic blood pressure (SBP) was at least 140 mm Hg, and/or their average diastolic blood pressure (DBP) was at least 90 mm Hg, or if they reported being on regular anti-hypertensive therapy. A participant was classified as being pre-hypertensive if their average SBP was between 120 and 139 mm Hg (inclusive), and/or their DBP was between 80 and 89 mm Hg (inclusive), and not on any anti-hypertensive therapy.

The prevalence of pre-hypertension and hypertension were calculated as the percentage of participants classified as being pre-hypertensive or hypertensive, respectively. To enable comparison of prevalence across the five population groups with differing age-structures, prevalence was directly age-standardized to the World Health Organization’s 2000–2025 world standard population age structure [14, 15].

Body Mass Index (BMI) was computed by dividing the weight (kg) by the height in meters squared (m²) and used to develop the categories of: underweight (less or equal to 18.5), normal weight (18.5–24.9), overweight (25.0–29.9), and obese (30 or higher). Participants were also categorized on the basis of their plasma glucose levels status using the WHO definition and diagnosis of diabetes mellitus criteria as follows: Normal (less than 6.1 mmol/L), Pre-diabetes (6.1 to 6.9 mmol/L), and diabetes or probable diabetes (greater than 6.9 mmol/L, or currently on anti-diabetes mellitus medication) [16].

To identify factors associated with hypertension, the modified Poisson regression model with robust variance was used to estimate both the crude and adjusted prevalence ratios (PR) [17, 18], with their corresponding 95 % confidence intervals (95 % CI). The modified Poisson regression model, a model that uses a robust error variance, was preferred to avoid under estimation of the standard errors for the estimated risk ratios that is usually the case with logistic regression modelling when the prevalence of the outcome is greater than 10 % [17, 18]. Potential confounding and interaction variables that were assessed for inclusion in the model were: age, sex, level of education, body mass index, population group, FPG category, family history of hypertension (first degree relatives only), whether the participant met the WHO physical activity recommendations [19], and tobacco use category. A step-by-step backward elimination of these variables from the model was used to identify those to be retained in the model. The criterion used to retain variables in the model were if a variable was significantly associated with hypertension using a 5 % level of statistical significance (α = 0.05), or if addition of the variable in the model led to a change of at least 10 % in any of the significant PR estimates of any variable already in the model. Cases with missing observations were excluding from the model on a case by case basis. All statistical analyses were performed using STATA version 13 (StataCorp, College Station, Texas, USA).

A total of 1414 subjects participated in the baseline cohort pilot study. Of the 1414 participants, at least two blood pressure measurements were available from 1269 (89.7 %) participants and were used in this analysis. These included 163 nurses in Nigeria, 477 school teachers in South Africa, 167 school teachers in Tanzania, and from Uganda 297 peri-urban and 165 rural residents.

Of the 1269 participants, 820 (64.6 %) were females, 671 (52.9 %) were aged 30 to 49 years, and 879 (69.3 %) had received up to secondary school education or at least 8 years of schooling. The majority of participants (772 or 60.8 %) were over-weight with a body mass index (BMI) of 25 kilograms per squared meter (kg/m²) or higher, including the 407 (32.1 %) that were obese (BMI ≥ 30 kg/m²). Of the participants, 121 (9.5 %) reported current use of some form of tobacco and only 272 (21.4 %) reported physical activity levels that meet the WHO recommendation of at least 75 min of vigorous-intensity or 150 min of moderate-intensity physical activity per week [19]. The rest of characteristics of the participants are presented in Table 1.

Of the 1269 participants, 468 were classified as being hypertensive giving an overall crude prevalence of 36.9 %. The overall age-adjusted prevalence was 25.9 %. Hypertension was highest amongst nurses in Nigeria with age-adjusted prevalence of 25.8 %, followed by school teachers in South Africa with age-adjusted prevalence of 23.2 %, then by school teachers in Tanzania with age-adjusted prevalence of 23.1 %, whereas the peri-urban and rural residents in Uganda had an age-adjusted prevalence of 20.5 % and 8.7 %, respectively (Table 2).

Only 234 of the 468 participants with hypertension (50.0 %), were aware of their raised blood pressure condition. The level of awareness varied significantly by population group (p < 0.001), being highest among nurses in Nigeria at 77.9 %, and lowest among rural residents in Uganda at 14.3 %. Figure 1 shows the percentage of participants with hypertension who were aware of their raised blood pressure status, by population group.

A total of 378 participants were classified as being pre-hypertensive, giving a crude prevalence of 29.8 %, and an overall age-adjusted prevalence of 21.0 %. Pre-hypertension was highest among school teachers in South Africa with an age-adjusted prevalence of 26.0 %, followed by peri-urban and rural residents in Uganda with age-adjusted prevalence of 21.6 % and 19.5 % respectively, followed by nurses in Nigeria and school teachers in Tanzania with age-adjusted prevalence of 16.3 % and 7.5 %, respectively (Table 2).

Factors found to be associated with hypertension included population group, age, body mass index, fasting plasma glucose (FPG), level of education, and tobacco use. Compared to the rural residents in Uganda, the prevalence of hypertension was highest among nurses in Nigeria with an adjusted prevalence ratio (PR) of 6.49 [95 % CI = 2.26 – 18.7]. The prevalence of hypertension was significantly higher among participants aged 50 years or older compared to those aged 18 – 29 years with an adjusted PR of 2.20 [95 % CI = 1.49 – 3.25]. We found a significantly lower adjusted prevalence among participants who attained secondary school level of education compared to those reporting no formal education, with a PR of 0.54 [95 % CI = 0.31 – 0.93]. We also the prevalence of hypertension to be significantly higher among participants with BMI between 25 to 29.9 kg/m² compared to those with BMI less than 25, with an adjusted PR = 1.61 [95 % CI = 1.18 – 2.20], and even higher among the obese (BMI ≥ 30 kg/m²) participants compared to those with BMI less than 25 with an adjusted PR = 2.11 [95 % CI = 1.56 – 2.84]. The prevalence of hypertension was also more likely to be higher among participants with fasting plasma glucose (FPG) of 7.0 mmol/L or higher, compared to those with FPG less than 6.1 mmol/L, with an adjusted PR of 1.38 [95 % CI = 1.10 – 1 171]. Finally, the prevalence of hypertension was significantly higher among participants reporting to use unfiltered tobacco, compared to those reporting to had never used tobacco, with an adjusted PR of 1.7 [1.04 – 2.91] (Table 3).