Historically, the most common instrument used to measure blood pressure has been the mercury sphygmomanometer, but increasing concerns over the toxicity of mercury is causing these to be phased out across the NHS. Other types of instrument are taking their place with many of these being impressive in their initial performance.

However, as with any measurement instrument, they need regular re-calibration against traceable measurement standards to ensure that they are still fit for purpose.

Owing to a lack of awareness of traceability issues most sphygmomanometers are never calibrated against reliable pressure standards. The effect of this is illustrated by a survey that found 49 out of 279 instruments, in use in 45 general practices, produced errors in excess of the allowable tolerance.

NPL has helped Guy's and St Thomas' NHS Foundation Trust set up a facility that will allow sphygmomanometers to be calibrated against a traceable pressure standard, eliminating discrepancies and reducing incorrect diagnosis based on blood pressure measurement.

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