BACKGROUND: Despite widespread use, the accuracy of community-based automated blood pressure machines has been questioned. We sought to determine if these machines are as accurate and reliable as those obtained by a clinician with a mercury manometer.
METHODS: We randomly selected 25 pharmacies and compared blood pressure readings obtained from their automated machines with from a mercury manometer. We used 3 volunteers with arm circumferences at the low, medium, and high ends of the acceptable range of a normal adult cuff size.
RESULTS: For the subject with the small arm size, store machines reported systolic pressure readings that were, on average, 10 mm Hg higher than those obtained by the clinician (P <.001) and diastolic pressures 9 mm Hg higher (P <.001). The mean systolic pressure readings for the subject with the medium arm size were not significantly different between the store machine and the mercury manometer, and the readings were only modestly different for diastolic pressure. For the subject with the large arm size, store machines reported diastolic pressure readings that were, on average, 8.3 mm Hg lower than those obtained using the mercury manometer (P <.001), but with no significant difference in the systolic pressure.
CONCLUSIONS: We found that automated blood pressure machines from a representative community-based sample of pharmacies did not meet the accepted standards of accuracy and reliability. Accuracy of readings is especially uncertain for patients having arm sizes larger or smaller than average.
Hypertension affects approximately 50 million adults in the United States.1 Frequent blood pressure readings are needed for diagnosing disorders and for monitoring patients’ health status. Self-monitoring may provide a more accurate record of patients’ blood pressure if they have white-coat syndrome and can be used to construct a record of the patient’s response to medications and to improve compliance.
Patients can conveniently monitor their blood pressure by using free automated machines found in pharmacies. Despite their widespread availability, the accuracy of these machines has been questioned.2-6 The Association for the Advancement of Medical Instrumentation (AAMI) published standards for the accuracy of blood pressure measuring devices that state, “for systolic and diastolic pressures, treated separately, the mean difference of the paired measurements of the test system and the comparison system shall be ±5 mm Hg or less, with a standard deviation of 8 mm Hg or less.”7 We sought to determine if the readings obtained from the automated machines available in the community meet the recommended standards and whether these machines are accurate and reliable when compared with the mercury manometer. We also sought to determine if arm size would affect the accuracy of automated machine readings.
Methods
Between June and November 1998, we randomly selected 25 of the 80 pharmacies within 7 miles of the University medical clinics. On the basis of previously reported variances we calculated that 25 stores would be required to provide approximately 80% power to detect a true blood pressure difference of 5 mm Hg between machine and clinician.2 We used 3 volunteers with arm circumferences at the low (26 cm), medium (29 cm), and high (33 cm) ends of the acceptable range of normal adult cuff size. Standard cuff sizes allow for arm circumferences between 26 and 33 cm.8 None of the subjects was known to be hypertensive, and none was taking antihypertensive medications or medications known to increase blood pressure. All readings were taken during a 2-month period.
Mercury manometer readings were obtained using the standardized technique and a standard cuff recommended by the American Heart Association.8 One member of the research team obtained all mercury manometer readings. The subjects obtained store-derived blood pressure readings by following each machine’s printed instructions. The model and manufacturer of each machine were recorded, and the pharmacy staff was asked about the machine’s most recent calibration or maintenance.
Blood pressures were measured after the subjects had a 5-minute rest. Six measurements were recorded for each subject in random order: 3 readings with the store blood pressure machine and 3 with the mercury manometer. Readings were taken from the same arm with a 1-minute rest between them. This is the method recommended by the AAMI and the British Hypertension Society to compare 2 methods of measurement.9
The subjects self-recorded the machine values, and the clinician was blinded to those readings. The 3 systolic and diastolic readings obtained using each method were averaged to determine the final estimate for using each source. We assessed differences between machine and clinician readings using the paired t test. The variance ratio of these readings was compared using the F statistic. We conducted analyses separately by arm size and for systolic and diastolic readings. All reported P values are 2-tailed with statistical significance set at an a of 0.05.