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Previous studies have reported results on 24-hour ambulatory blood pressure (ABP) in Europe and Japan, but no data exists from South America. In this study, we conducted a population survey to identify reference values and to compare ambulatory blood pressure with clinic, home, and self-measured values. A random sample of 2650 adults was selected among 190 000 people covered by our prepaid healthcare institution. Clinic (physician and nurse) and home (nurse) blood pressure measurements were performed 3 times each, with semiautomatic electronic equipment. Self-measurements were performed by the subjects manually activating the ambulatory device. We analyzed 1573 individuals who were not receiving antihypertensive therapy from 1921 participants. Self-measurement was available in a subgroup of 577 participants younger than the whole sample. Normal ambulatory blood pressure limits were estimated as those that best correlated with 140/90 mm Hg at clinic. Estimated values were 125/80 mm Hg for 24-hour ambulatory (range: 122 to 128 and 77 to 83 mm Hg) and 129/84 mm Hg for daytime ambulatory (range: 127 to 132 and 81 to 86) blood pressure, depending on gender and age. Ambulatory and clinic blood pressures increased with age. The age-dependent increase in ABP was similar in women and men. Average blood pressure at clinic was 124/79 mm Hg by physician and 123/78 mm Hg by nurse. Nurse measurement at home was 125/78 mm Hg, daytime ambulatory was 121/77 mm Hg, and 24-hour ambulatory was 118/74 mm Hg. The values of the subgroup with self-measurement were physician 119/77 mm Hg; nurse at clinic 118/77 mm Hg; nurse at home 121/78 mm Hg; self-measured 115/72 mm Hg; daytime ambulatory 119/77 mm Hg; and 24-hour ambulatory 115/73 mm Hg. This study shows that a 24-hour ABP average value of 125/80 mm Hg and a daytime ABP average value of 129/84 mm Hg are suitable upper limits for normality. Higher limits would yield an artificially higher prevalence of white coat hypertension. Most subjects showed higher blood pressure levels when measurements were performed by healthcare personnel at a clinic or at home than when self-measured at home.