Why the trip is worth making

Home monitors drift, and their owners cannot tell by looking. In a 2017 study published in the American Journal of Hypertension, researchers tested 85 upper arm and wrist monitors that people had brought from home against readings taken by two trained observers using a stethoscope. About 69 percent of the devices were off by 5 mm Hg or more on the systolic or diastolic number. About 29 percent were off by 10 mm Hg or more, and 7 percent by 15 mm Hg or more. People with larger arms tended to get higher readings from their own devices, a hint that cuff fit was part of the problem.

Manufacturers describe their monitors as accurate within about plus or minus 3 mm Hg, but that figure comes from laboratory testing of a new device under controlled conditions. It says nothing about the monitor on your kitchen table after two years of use, a tumble off the counter, and a cuff that has been folded into a drawer. The 2025 American Heart Association and American College of Cardiology blood pressure guideline says devices should be recalibrated according to the manufacturer's guidance. One manufacturer's manual, for example, recommends testing accuracy every 2 years or after any mechanical impact.

Technique matters as much as the machine. A 2017 review in the Journal of Hypertension pooled 328 studies and catalogued 29 sources of measurement error. Talking during a reading raised systolic pressure by 4 to 19 mm Hg across studies. An arm held below heart level added anywhere from about 4 to 23 mm Hg. Crossing the legs at the knee added 2.5 to nearly 15 mm Hg, and a cuff that was too small added 2 to 11 mm Hg. Any of these habits can make a perfectly good monitor look wrong, or make a faulty one look fine. The only way to separate the two problems is for someone who knows the technique to watch you take a reading.

The kit to bring

Pack these items the night before so nothing gets left behind.

1. 1 The monitor, with the batteries in it or the power adapter packed alongside. Bring it in the condition you use it at home. If the low battery symbol has been flashing, replace the batteries before the visit so a weak battery does not become one more variable.
2. 2 The cuff you actually wear, attached to the unit. The international standard used for validation testing requires a separate evaluation for each cuff type, and the cuff is where fit problems live. If you own two cuffs, or switched cuffs recently, bring both.
3. 3 A photo of the label with the model number. The label is usually on the back of the unit or under the battery cover, and many cuffs print the model and the arm circumference range as well. A series name often covers several models, so the exact model number is what lets you or the health care professional look the device up on the validatebp.org listing. A separate article in this series walks through that check.
4. 4 The manual, or photos of its accuracy and cuff pages. The manual states the arm size range, the stated accuracy, and the recommended interval for accuracy checks. A separate article in this series explains where to find a manual when the paper copy is long gone.
5. 5 Your readings. Bring the paper log, or bring the monitor itself if its memory is where the readings live. The American Medical Association's protocol for health care professionals calls for 7 days of readings, taken twice in the morning and twice in the evening, with 3 days and at least 12 readings as the minimum. If someone else in your household also uses the monitor, make clear which stored readings are yours.
6. 6 A list of your medicines and when you take them. Home monitoring guidelines describe morning readings taken before blood pressure medicines, while office readings often happen hours after a dose. Your health care professional needs both pieces of information to make sense of any gap between the two sets of numbers. Include vitamins and nonprescription products on the list, since the 2025 guideline notes that some common pain relievers and decongestants can raise blood pressure.
7. 7 A few notes about your routine. Write down which arm you use, whether you sit at a table or on the couch, roughly how long you rest first, and whether you usually have coffee or a cigarette within the previous half hour. These details tell the health care professional what to watch for when you demonstrate a reading.
8. 8 A tape measure, if you have one. The American Medical Association lists a tape measure alongside the monitor, cuff, and manual as the tools for confirming cuff size. Many offices keep one, but bringing your own avoids a hunt.

What the check looks like in the exam room

Say when you book the appointment that you would like your monitor checked, so a few extra minutes get built into the visit. A nurse or medical assistant may do the comparison rather than the doctor. That is fine; Focus on that someone trained in standardized measurement is watching.

The technique check comes first. You take a reading with your own monitor exactly as you would at home, while the health care professional watches. They are looking for the elements the 2017 American College of Cardiology and American Heart Association guideline lists for a valid reading: seated with your back supported, feet flat on the floor, legs uncrossed, at least 5 minutes of quiet rest beforehand, an empty bladder, the cuff on bare skin with its lower edge just above the bend of the elbow, and the arm resting on a surface so that the middle of the cuff sits at the level of your heart. No talking during the reading. If any of that differs from your home habit, say so. The purpose is to find out what you actually do at home.

The device comparison comes next. A common approach is to take readings on the same arm, alternating between your monitor and the office device, with about a minute of rest between each, and to repeat the pair two or three times. Guidelines call for repeat readings to be spaced 1 to 2 minutes apart, and alternating the devices helps cancel out the natural drift in blood pressure over the course of a sitting.

Ask which device the office is using and whether it is validated. The 2025 guideline recommends only automated cuff devices validated under a rigorous standardized protocol, and the same independent listing that covers home monitors has a category for office devices. Office equipment is not immune to drift either. The 2017 review of measurement error found that anywhere from about 1 to 70 percent of the dial style manual gauges checked in various studies were off by more than 3 mm Hg, and 4.5 to 26 percent of automated devices were. A comparison is only as good as the reference.

Why two devices rarely agree exactly

Suppose your monitor reads 138/86 mm Hg and the office device, a minute later, reads 131/84 mm Hg. It is tempting to conclude that your monitor runs 7 points high. One pair of readings cannot support that conclusion.

Blood pressure moves from minute to minute, and it tends to fall over the course of a visit as you settle. A 2018 study in JAMA Internal Medicine looked at more than 38,000 adults with high blood pressure whose pressure was measured twice at the same primary care visit. The median systolic reading dropped 8 mm Hg between the first and second measurement, and 36 percent of the repeated readings ended up below 140/90 mm Hg. The researchers attributed nearly two thirds of the drop to pressure settling back toward its usual level rather than to anything the staff did.

Add the setting. Many people read higher in a medical office than at home, and some read lower. The 2017 review found the white coat effect ranged from about 13 mm Hg below to 27 mm Hg above out of office values across studies. That gap comes from the setting rather than from either machine, and it will not go away with a new monitor.

This is why the comparison relies on a pattern rather than a pair. If your monitor reads higher than the office device on every one of three alternating pairs, and by roughly the same amount each time, that consistency points at the device or the cuff. If it reads 7 high on the first pair, 2 low on the second, and 4 high on the third, that scatter looks like ordinary variation, and the average difference is small.

How big a gap matters

No guideline from the American Heart Association, the American Medical Association, or the American College of Cardiology sets a number of millimeters that separates an acceptable home monitor from one that should be replaced. What exists instead is a set of reference points, and a judgment call that belongs to your health care professional.

The manufacturer's stated tolerance of plus or minus 3 mm Hg is the tightest reference point, and it is unrealistic for a comparison in an exam room, because blood pressure itself will wander more than that between readings. The 2017 American Journal of Hypertension study used differences of 5, 10, and 15 mm Hg as its thresholds and found that 29 percent of owners' devices missed by 10 mm Hg or more. A consistent gap of that size, present across several pairs and in the same direction each time, is a reasonable point to ask whether the cuff fits, whether the technique was sound, and whether the device should be serviced or replaced.

Cuff fit is the first suspect. In a 2023 randomized crossover trial published in JAMA Internal Medicine, a regular adult cuff on a person who needed an extra large cuff read systolic pressure about 19.5 mm Hg too high, and a regular cuff on someone who needed a small cuff read about 3.6 mm Hg too low. A monitor that seems to run high may simply be wearing the wrong cuff. A separate article in this series covers measuring your arm.

What to do about a gap is the health care professional's decision. They may decide the difference is small enough to ignore, note it and mentally adjust when reading your log, suggest a differently sized cuff, or suggest a new device. Do not change anything about your medicines on the basis of the comparison. If the visit ends with a clear number in mind, such as "your monitor reads about 5 points high," write it on the log or tape it inside the monitor case so it is not forgotten by the next visit.

Questions to ask while you are there

1. 1 Did anything in my technique look off, and what one change would matter most?
2. 2 Does this cuff fit my arm? If not, what size range should I look for?
3. 3 How close were the two devices, and is that close enough for the way you use my home readings?
4. 4 Is the office device validated, and when was it last checked?
5. 5 Which arm should I use at home? The 2019 American Heart Association scientific statement on blood pressure measurement reports that about 11 percent of people with high blood pressure have a difference of 10 mm Hg or more between arms, and guidelines say to use the arm with the higher reading.
6. 6 When should I take readings, and how should I time them around my medicines? The 2017 guideline suggests home readings during the week before a clinic visit and beginning 2 weeks after any change in treatment, so ask whether that schedule fits your situation.
7. 7 What home readings should prompt a call to the office, and what should prompt emergency care? The American Heart Association's guidance is that a reading of 180/120 mm Hg or higher should be rechecked after waiting at least 1 minute. If it stays that high without symptoms, contact your care team as soon as possible. If it comes with chest pain, shortness of breath, back pain, numbness or weakness, a change in vision, or difficulty speaking, call 911 without waiting to see whether it comes down.
8. 8 How often should I bring the monitor back? The general answer is yearly or as the manufacturer directs, but your health care professional may want to see it sooner after a change in treatment.

When to ask for an earlier recheck

Some situations should move the check up rather than waiting for the annual visit.

1. 1 The monitor was dropped. One manufacturer's manual calls for an accuracy check after any mechanical impact, and a fall is the most common one.
2. 2 Your readings changed suddenly without a change in your health, your medicines, or your routine.
3. 3 You replaced the cuff or switched to a different size.
4. 4 Error messages keep appearing even after you have checked the cuff connection, the batteries, and your position. A separate article in this series covers troubleshooting.
5. 5 You bought a new monitor. The American Heart Association's advice to bring a new device to the next appointment applies to every new device, not only the first one.
6. 6 Your home readings and office readings have told different stories for months, not just at one visit. That pattern may reflect white coat or masked blood pressure rather than a device problem, and it is exactly the kind of information your health care professional needs.

Photograph the label today, tuck the log and the medicine list into the monitor's case, and the next appointment needs no preparation beyond picking up the case on your way out the door.