Reliability of the mercury sphygmomanometer as a reference standard for the measurement of blood pressure

Background: Guidelines for blood pressure measurement do not have explicit requirements for calibration of mercury manometers and suggest they may be used as reference standards for calibration of sphygmomanometers. This is in contrast to the requirements for industrial metrology, e.g. the National Association of Testing Authorities of Australia (NATA) recommends that working liquid manometers be calibrated against a national reference standard at least every three years. National reference standards are traceable to the International System of Units (SI units). Methods: We reviewed guidelines for measurement of blood pressure and management of hypertension for recommendations regarding sphygmomanometer calibration and reference standards against which manometers should be calibrated. We also reviewed sphygmomanometer surveys in which mercury sphygmomanometers were checked against reference standards. Results: No guidelines recommend that sphygmomanometer calibration be traceable to national reference standards. Six guidelines recommend that non-mercury manometers should be calibrated against a mercury sphygmomanometer. Seven sphygmomanometer surveys found that between 1 and 33% of mercury sphygmomanometers had errors > 3 mm Hg. One survey found that 1% had errors > 10 mm Hg, and another found that 17% had mercury levels below zero at zero input pressure. Conclusions: While many hypertension guidelines recommend that mercury sphygmomanometers do not require calibration and can be used as reference devices, survey evidence suggests that this practice may lead to significant systematic errors in blood pressure measurements. Recommendations should be developed for mercury sphygmomanometers and reference instruments used to calibrate sphygmomanometers to be calibrated regularly against references traceable to national reference standards.