From the Epidemiologist to the Physician: A Complex Issue for a Labile Risk.

Abstract

In a recent study of elderly, rural-dwelling Tanzanians, the incorrect classification of more than a quarter of patients as hypertensive was attributed to the white-coat effect. Although the white-coat effect might contribute to the overestimation of the prevalence of hypertension (HTN), systolic blood pressure (SBP) variability is known to enhance cardiovascular (CV) risk. In addition, white-coat HTN is now regarded as an intermediate phenotype between normotension and HTN. On the other hand, relatively high nocturnal blood pressure (BP) resulted in nighttime HTN in more than two thirds of the cohort of elderly, rural-dwelling Tanzanians. SubSaharan Africans were already reported to have higher nighttime BPs than Caucasians with a blunted nocturnal decline, so that ambulatory BP monitoring (ABPM) might be especially useful in this ethnic group. As recently underlined by the Global Burden of Disease study, HTN results in more deaths than any other risk factors, including diabetes and cigarette smoking. High prevalence, inadequate awareness, suboptimal treatment, and low rates of guideline-recommended target BP control are key factors that lead to severe CV complications that impose a heavy socioeconomic burden, especially in developing countries. A systematic review and meta-analysis pooling data from 33 surveys published between 2000 and 2013, involving more than 110,414 participants with a mean age of 40 years, shows that in sub-Saharan Africa the predicted prevalence of HTN at mean participant ages of 30, 40, 50, and 60 years were 16%, 26%, 35%, and 44%, respectively, with a pooled prevalence of 30% (95% confidence interval [CI], 27–34). Most importantly, of those with HTN, only between 7% and 56% (pooled prevalence: 27%; 95% CI, 23–31) were aware of their hypertensive status before the surveys. Overall, 18% (95% CI, 14–22) of individuals with HTN were receiving treatment across the studies, and only 7% (95% CI, 5–8) had controlled BP. The HTN burden is likely to grow in the next few decades as the population ages and the prevalence of obesity and diabetes increases. These data clearly highlight the need for implementation of timely and appropriate strategies for diagnosis, control, and prevention. However, the direct transfer of data obtained in an epidemiological study in a real estimate of the costs necessary to the country for the treatment of CV risk hides some uncertainties. Although data from observational studies indicate that this risk is continuous, to have a precise estimate in terms of prevalence in population studies may not be straightforward. In the individual patient, the clinician performs the diagnosis of HTN on the basis of repeated BP values above 140/ 90 mm Hg at several visits. Although these criteria may be met in epidemiological surveys performed in high-income countries (in the third report of the National Health and Nutrition Examination Survey, nearly 80% of participants had up to six BP measurements on two occasions), estimates of HTN prevalence in lowand middle-income countries (LMICs) are often based on data collected at a single visit, causing a potential systematic error. This discrepancy is mainly the result of the difficulty to bear the costs of more visits in an epidemiological study. A similar problem also arises in the case of diabetes. The strategy used in the clinic to make measurements of fasting blood glucose on separate days is not easily feasible in epidemiological studies so that attempts are made to obtain reliable estimates using different strategies based on glycated hemoglobin or the response to the oral glucose tolerance test. Comparisons between estimations of diabetes prevalence assessed with these different strategies are often given in the same survey, whereas only a few studies have assessed the impact of one or two visits on the estimation of HTN burden in LMICs. Taking BP multiple times can attenuate the influence of within-person variability because of physiological variation or measurement error. Another source of nonrandom variation is the phenomenon of “white-coat hypertension,” which may lead to an overestimation of HTN prevalence. The National Institute for Health and Care Excellence (NICE) recommends ABPM for all patients suspected of having HTN because of a previously elevated office BP measurement aiming to exclude patients with whitecoat HTN. According to the study by Ivy and colleagues, the white-coat effect was responsible for an increase in recorded BP in more than two thirds of a cohort of elderly, rural-dwelling Tanzanians. Those data are in agreement with a recent study performed in a different world area. At least three points have to be considered. First, could the adoption of a strategy based on ABPM lead us Address for correspondence: Pietro Amedeo Modesti, European Society of Hypertension Working Group on Hypertension and Cardiovascular Risk in Low Resource Settings (Chair), Department of Medicina Sperimentale e Clinica, University of Florence, Largo Brambilla 3, Florence, Italy. E-mail: pamodesti@unifi.it