Thousands of people use home blood-pressure (BP) monitoring machines for optimal control, but how accurate are the readings? Not very, say cardiologists, and advise users of home units to get their devices calibrated against a mercury sphygmomanometer at a clinic at least once a year.A Canadian study of 85 home devices published in the American Journal of Hypertension last year found most home-monitors were inaccurate, with readings off the mark within 5 mm Hg of blood pressure about 70% of the time, and by at least 10 mm Hg 30% of the time. One in four adults in India have hypertension, which is defined as chronic blood pressure above 140/90 mm Hg. Readings less than 120/80 mm Hg are healthy. The risk of death from ischemic heart disease (heart attack) and stroke doubles with every 20 mm Hg systolic or 10 mm Hg diastolic increase among 40 to 89 year olds, according to the American Heart Association.One in eight young people between the ages of 18 and 25 years in India have hypertension, which is the second biggest risk factor for heart disease after unhealthy diets high in salt, fat and sugar. Cardiovascular disease, which includes stroke, account for 28.1% of the total deaths in India, according to The Lancet Global Health.Best of threeOf the three types of sphygmomanometers used to measure blood pressure, the mercury device is the best while the aneroid machine (unit with a moving needle) is the least accurate, say experts.With most countries dispensing with mercury sphygmomanometers because of environmental reasons, digital devices are becoming the norm. Among them, cuff-style, upper-arm monitors are recommended as wrist and finger monitors give less reliable readings.“The digital sphygmomanometers is generally accurate for systolic blood pressure, which is the higher reading, but it under-reads the diastolic or lower number, which tends to slightly lower than the actual reading,” said Dr Srinath Reddy, president, Public Health Foundation of India, and former head of cardiology, All India Institute of Medical Sciences.A small study from West Bengal found the aneroid device had better accuracy than the digital device as compared to mercury sphygmomanometer, but it had several limitations. “The fact that digital monitors like Omron differ in readings from the mercury readings is known, especially in the diastolic (lower) reading, but unless we compare both in cath lab with haemodynamically-measured intra-aortic pressure, it will be difficult to comment on comparative accuracy. Even the mercury method is subject to the observer’s hearing ability and whether they are recording phase 5 (as they are supposed to) or phase 4 (as some observers inaccurately do) and the speed of cuff deflation,” explains Dr Reddy.“It’s best to measure blood pressure on your left arm but cardiologists may measure on both if you have diagnosed cardiovascular disease. A slight variation between readings on the right and left arms is normal, but a difference of more than 10 mmHg could indicate circulatory problems,” said Dr Ravi Kasliwal, chairman of clinical and preventive cardiology, Medanta.Do it rightMost people don’t take readings correctly. You shouldn’t smoke, drink alcohol or caffeine, have a full bladder, or exercise 30 minutes before measuring blood pressure. Sit still, preferably at the dining table, with your back straight, feet flat on the floor and legs not crossed. “The readings taken lying down are always higher than sitting, and sitting measurements are higher than standing,” said Dr Reddy.Support your arm on a table, with the upper arm at level with the heart. The bottom of the cuff should be placed on the bare arm (do not take readings over clothes), two fingers above the bend of the elbow.“For accuracy, take multiple readings, at least three, one minute apart and record the average. The readings should be taken at the same time each day, ideally beginning two weeks after a change in treatment and during the week leading up to an appointment with a doctor,” said Dr Kasliwal, who encourages patients to take their home devices on appointments so clinicians can check whether they are using it correctly.The calibration should be done at least once a year, though people with acute hypertension or uncontrolled/frequently fluctuating blood pressure should get it done every six months. “The common sense thing is to periodically calibrate the digital instrument against a good mercury instrument used by a clinician who observes the correct BP measurement protocol to make sure the home instrument is accurate,” said Dr Reddy.