Several national field studies have produced population-wide inferences regarding the prevalence of hypertension for older cohorts in the United States.23,24
However, few studies have described procedures for assuring and controlling quality of in-home blood pressure data collected by a large, centrally managed, national field staff using portable and affordable blood pressure monitors. The present report describes the methods used by Add Health, Wave IV (2008) to estimate the validity and reliability of blood pressure (digit preference; bias; intra-class correlation), and bias-adjusted prevalence of hypertension based on blood pressure data collected using the aforementioned methods.
The resulting estimates suggest that in Add Health, terminal digit preference of blood pressure is infrequent, bias is low, short-term reliability is good to excellent, and comparable to that found in well-known, exam center-based studies of cardiovascular disease.10,25,26
The estimates also suggest that, although the measured prevalence of hypertension (i.e. BP ≥ 140/90 mm Hg) is four- to-five-fold higher in Add Health, Wave IV (2008) than in NHANES (2007–2008) among participants aged 24–32, self-reported history of hypertension is similar in the two populations. Hence, the proportion of young adults in NHANES reporting a history of hypertension is twice that with measured hypertension (9% versus 4%), while the reverse holds in Add Health (11% versus 19%, see ). The Add Health findings are consistent with the expectation that blood pressure measurement will capture subclinical hypertension (i.e. hypertension unknown to otherwise healthy young people), and in doing so, will identify more measured than self-reported cases.
The striking between-survey difference exists despite examination of young adults in the same age range (24–32 years) during an overlapping time frame and despite Add Health’s efforts to examine and account for numerous methodologic concerns. These concerns included the following: digit preference; validity; reliability; survey weights; differential selection; use of anti-hypertensive medications (important given a recent report documenting improvements in hypertension treatment and control27
); consumption of food, caffeinated beverages, or smoking prior to blood pressure measurement; time of blood pressure measurement; measurement context; and interview content. Important between-survey differences in sampling frame and study design determine the distinct populations to which Add Health and NHANES findings are most appropriately inferred. These differences may help explain the observed discordance in hypertension prevalence between studies, without labeling either as incorrect. In Add Health, the appropriate inference is to persons who were adolescents in grades 7–12 in the U.S. during the 1994–95 school year, including those who subsequently joined the military or were institutionalized or incarcerated. In NHANES, the appropriate inference is to the U.S. civilian, non-institutionalized population during 2007–2008, one similar to that described by the U.S. Census Bureau’s American Community Survey (2008),28
yet less likely than Add Health to include non-Hispanic white, native-born, better-educated, and health-insured persons (eTable 6
). Furthermore, Add Health includes participants who were waiting to have blood pressure measurements taken by a field interviewer in the participant’s homes, whereas NHANES participants had to go to a mobile examination center for blood pressure measurement by physician examiners.
Although the differences in sampling frame and study design increase the possibility of divergent biases, directly standardizing rates of BP ≥ 140/90 mm Hg in Add Health Wave IV to the U.S. population aged 24–32 years based on race/ethnicity, foreign birth or education in the American Community Survey 2008 or NHANES 2007–2008 produced estimates (range: 18.6%, 19.6 %) that differed little from the overall unadjusted prevalence (18.6%–19.6% versus 19.1%; eTable 6
). Subgroup-specific rates and mean blood pressure were both higher in Add Health than in NHANES, even among those typically at lower risk of hypertension, e.g. the health-insured, better-educated and non-Hispanic whites. Propensity-scored estimates adjusting simultaneously for an array of salient study participant characteristics also diverged little from the unadjusted estimates. Furthermore, the adjusted odds of measured hypertension in Add Health was more than six times that in NHANES.
Robustness aside, estimated rates of hypertension approaching nearly one in five U.S. young adults raise questions about their biological plausibility. However, there is global precedent for such observations; for instance, among Latin American and Caribbean men aged 20–29, the rate of hypertension (defined as systolic/diastolic BP ≥ 140/90 or taking anti-hypertensive medications) is 28%.29
Though such high rates have not been described previously in the U.S., prior findings in the global context suggest that they are neither biologically implausible nor without epidemiologic precedent.
The large and unexplained differences between Add Health and NHANES merit further investigation. U.S. coronary heart disease mortality and policy models rely heavily on NHANES systolic BP distributions and, in some cases, on optimistic assumptions regarding relatively small decreases of 0–1 mm Hg per year in population mean systolic BP.30,31
Although consideration may well focus on between-study differences in measurement methods32,33
and observer bias, neither appears to be problematic insofar as NHANES blood pressure measurement is concerned.11,34
Indeed, sphygmomanometric and automated oscillometric blood pressure measures are much more similar among adult than pediatric populations.32,33,35
Study strengths and limitations
In a nationally representative field study, we investigated two important problems that can threaten the integrity of blood pressure measurements: lack of validity and reliability. This investigation is subject to several limitations. Under ideal circumstances, validity of blood pressure measurements would have been monitored on an ongoing basis throughout Wave IV of the Add Health study. Given the decentralized nature and geographic breadth of data collection, doing so was not practical. It also would have been useful to examine additional factors capable of affecting reliability of blood pressure, such as fasting status of participants or demographic mismatching of participants and field interviewers. However, the contribution of these factors to the reliability of blood pressure measurement was expected to be relatively small19
compared with the high cost of studying large enough sample sizes to provide adequate power for the examination of subgroup differences in reliability.
Finally, the proportion of Add-Health, Wave-IV participants defined as having hypertension on the basis of elevated blood pressure alone may have been affected by blood pressure measurement variation.36
The potential importance of this possibility is underscored by the similarity of the blood pressure monitor used in this study to a different monitor made by the same manufacturer, and its association with a standard deviation slightly higher than that of several other monitors examined in the literature.37,38
However, Add Health’s blood pressure monitor was manufactured to meet U.S. and European standards. The British Hypertension Society validated the monitor, assigned it an A/A grading, and gave it their highest recommendation “for clinical and home use.”39,40
Its precision, validity and reliability are documented here. In addition, the putative effect of measurement variability was attenuated in this context by obtaining and averaging multiple blood pressure readings, and by using an automatic oscillometric device, which has been associated with a decrease in terminal digit preference.41
Together, these findings suggest that measurement variation is unlikely to account for the observed magnitude of difference between populations. A small percentage of participants may have been taking anti-hypertensive medications for other indications. Medication history and inventory are nonetheless routinely used in health surveys like NHANES42
to help define hypertension.
Carefully standardized, in-home measurement using an inexpensive oscillometric monitor can produce valid and reliable estimates of blood pressure. Coupling Joint National Committee 7 classification14
of Add Health’s valid, reliable and publicly available blood pressure data with the study’s self-reported history of hypertension and inventory of anti-hypertensive medications allows researchers to study the epidemiology of hypertension in a nationally representative sample of young adults. The prevalence of hypertension among Add-Health Wave-IV participants indicates an unexpectedly high risk of cardiovascular disease among U.S. young adults and deserves further scrutiny.