Impact of the number of blood pressure measurements on blood pressure classification in US adults: NHANES 1999-2008.

Clinical guidelines recommend averaging ≥ 2 blood pressure (BP) measurements on each visit. Only one BP is measured on many clinical visits, especially if the value is <120/<80 mm Hg, ie, normal. The impact of this practice on accurate assignment of BP category is incompletely defined. Data were analyzed from 22,641 adults 18 years and older who had 3 BP readings in the National Health and Nutrition Examination Surveys 1999-2008. BP category defined by initial measurement was compared with the category determined by mean of the first and second, first through third, and second and third readings. Among 8553 nonhypertensive patients with initial BP <120/<80 mm Hg, 2.9%, 3.3%, and 6.7%, respectively, were reclassified as prehypertensive, ie, BP 120-139/80-89 mm Hg, and two patients as stage 1 hypertension (140-159/90-99 mm Hg). In 733 treated hypertensive patients with initial BP <120/<80 mm Hg, 5.1%-8.9% were reclassified as prehypertensive and only one patient as hypertensive. Among nonhypertensive and hypertensive patients with initial BP in the prehypertensive range, 8.0%-23.6% were reclassified as normal. Among stage 1 and 2 hypertensive patients based on initial BP, 18.2%-33.5% were reclassified to lower BP categories. By multivariable logistic regression, older age and higher systolic and diastolic BP were associated with reclassification to a lower BP category. In nonhypertensive and hypertensive patients with normal initial BP values, one BP measurement appears adequate as <10% are re-classified as prehypertensive and <0.5% as hypertensive. In contrast, patients with an initial BP above normal are often reclassified to a lower category, which supports recommendations for additional measurements.