Hundreds of thousands of people in Britain may not be receiving the blood pressure lowering medications they need, or could be taking drugs unnecessarily, as doctors switch to mercury-free aneroid devices for measuring blood pressure, according to a study reported on Friday.

Researchers assessed 279 blood pressure measuring devices in 45 general practices in London against an electronic reference pressure sensor. They found 53 percent of aneroid instruments were off by more than 3 points - i.e., millimeters of mercury, or mm Hg - compared with just 13 percent of mercury-column devices.

Aneroid devices provided for promotional purposes by pharmaceutical companies were particularly inaccurate, with 36 percent off by more than 5 mm Hg and 14 percent off by more than 10 mm Hg.

Study leader, Andrew Shennan, chair of the Government’s Committee on Blood Pressure Monitoring in Clinical Practice, told Pulse magazine: “If you overestimate by 3 mm Hg, you increase the number classified as hypertensive by 24 percent. And if you underestimate by 3 mm Hg you cause 19 percent misclassification.”

He added: “This has huge implications. People are on life-long anti-hypertension treatment when they don’t need it or we are not treating. This translates into a lot of mortality due to something simple like a blood pressure machine.”

The Blood Pressure Association estimates over 16 million people in the UK have high blood pressure, suggesting the inaccuracies could affect hundreds of thousands of patients.

Phil Johnson, Editor of Pulse, said in a statement: “Treating high blood pressure is the most routine of all GP tasks, so it seems ridiculous that GPs are now left with equipment that does not work. It is ironic that by rushing to get rid of mercury devices for reasons of environmental health, the Government has put at risk the lives of many thousands.”