Discordance of mercury sphygmomanometer and ambulatory blood pressure measurements for the detection of untreated hypertension in a population study

To evaluate the extent to which ambulatory blood pressure (ABP) measurements could redefine the prevalence of untreated hypertension in the general population, we studied a randomly selected subgroup of the National Heart Foundation of Australia's Risk Factor Prevalence Study 1989. Subjects taking blood pressure (BP) lowering medications were excluded, leaving 66 patients with 24h ABP recordings for analysis. Mean awake ABPs were generally lower than survey BPs and diminished with reduced activity (away from work and during sleep). The correlation of survey BP and daytime ABP (10.00-20.00 h) classification of untreated hypertension was moderate (Cohen's correlation coefficient 0.49-0.56). Untreated hypertension was identified in 20 subjects by the mercury sphygmomanometer technique (BP ≥150/90 mmHg). Six (30%) of those subjects had a mean ABP < 135/85 mmHg suggesting 'normotension' on ambulatory criteria. Conversely, 19 subjects had a mean ABP of ≥135/85 mmHg, five (26%) of whom had 'normal' survey BPs. We conclude that ambulatory and mercury sphygmomanometer methods of BP measurement in this population study defined a similar prevalence of untreated hypertension but were discordant for a substantial percentage of individuals. The prognostic significance of ABP levels and any discordance with survey or office mercury sphygmomanometer BP readings will remain uncertain until prospective studies using both forms of measurement are completed.