Plenty of people find that their blood pressure is higher at the doctor’s office than it ever is at home. A few find the opposite. Neither is unusual, and both are part of the reason health bodies on both sides of the Atlantic now lean on readings taken outside the clinic.
This page explains the two patterns, what NICE (which writes guidance for the NHS) and the American Heart Association (AHA) say about checking for them, and how to keep a home record that is actually useful to your doctor. It does not tell you whether you have either one. Only your doctor can do that.
White coat hypertension
Blood Pressure UK describes it simply: the stress of having your blood pressure measured in a clinic can make it go up for a while, so the reading is higher than it should be. This is known as the white coat effect, or white coat syndrome.
NICE gives it a precise definition for clinicians. It calls it the white-coat effect when there is a gap of more than 20/10 mmHg between clinic readings and the average of readings taken at home or with a 24-hour monitor, at the time of diagnosis.
It is common. An AHA News report on recommendations from the US Preventive Services Task Force says that roughly 15 to 30 percent of people have white coat hypertension, and that it can lead to high blood pressure being overtreated.
That does not make it something to ignore. In the same report, Dr. Paul Whelton, who chaired the group writing the AHA’s joint high blood pressure guidelines, notes that people with white coat hypertension are at increased risk of developing sustained high blood pressure. It is a reason to keep checking, not a reason to stop.
Masked hypertension
Masked hypertension is the reverse. The AHA News report describes it as blood pressure that is normal at a medical office but elevated at work or at home. NICE defines it as clinic readings that are normal, below 140/90, while the average of home or 24-hour readings is higher.
This is the pattern that is easier to miss, because a normal clinic reading looks reassuring. Dr. Whelton puts it plainly: missing masked hypertension is a serious issue, because those people are already at increased risk of cardiovascular disease.
The only way to find either pattern is to measure blood pressure away from the clinic.
Why readings outside the clinic help
There are two ways to do it.
Ambulatory blood pressure monitoring (ABPM) means wearing a cuff and a small monitor for 24 hours while you go about your day. The NHS says you go to a GP surgery, health center or hospital to have it fitted and return it the next day, and that wearing it will not affect your daily activities. NICE asks for at least two readings an hour during your usual waking hours, and uses the average of at least 14 of them.
Home blood pressure monitoring (HBPM) means taking your own readings with a validated monitor, on a set schedule.
NICE’s guidance is that if your clinic reading is between 140/90 and 180/120, you should be offered ABPM to confirm a diagnosis of high blood pressure. If ABPM is unsuitable or you cannot tolerate it, you should be offered home monitoring instead. For people already diagnosed who have a white-coat effect or masked hypertension, NICE says to consider ABPM or home monitoring alongside clinic readings.
In the US, the AHA says home monitoring can be used to confirm a diagnosis of high blood pressure, and recommends it for everyone who has high blood pressure. The AHA News report adds that the Task Force found 24-hour ambulatory monitoring the best way to confirm a diagnosis, and that home monitoring can also confirm it, though less research supports it. It also quotes the practical problem with ABPM: not every practice offers it, it is a nuisance to wear for a day, and it is relatively expensive. Home readings are much easier to get.
The NHS describes the same options from the patient’s side. If a check suggests high blood pressure, you may be asked to use a monitor at home over the next few days, come back for another check, or wear a portable monitor for 24 hours.
Home thresholds are lower
Because readings outside the clinic run lower, NICE tells clinicians that home and ambulatory readings are 5 mmHg lower than the equivalent clinic readings. That is why the NHS says blood pressure is usually considered high at 140/90 or higher in the clinic, but 135/85 or higher at home. Our guide to home blood pressure readings covers the numbers in more detail, including the readings that need urgent attention.
How to keep a home log your doctor can use
If your doctor has asked you to monitor, follow their schedule first. If they have not given one, this is the schedule NICE sets out for home monitoring to confirm a diagnosis:
- Each time you measure, take two readings, at least one minute apart, sitting down.
- Measure twice a day, ideally once in the morning and once in the evening.
- Keep going for at least four days, ideally seven.
NICE then has the clinician discard the first day and average the rest. You do not need to do the arithmetic yourself; bring every reading and let your doctor or nurse work with them.
For each reading, write down:
- the date and time
- both numbers from each of the two readings, plus the pulse if your monitor shows it
- which arm you used
- any medication you took and when, since the AHA suggests people with high blood pressure keep a record of their readings and medications
Take each reading the way the AHA describes, so your log reflects your real blood pressure rather than your last cup of coffee: no smoking, caffeine or exercise for 30 minutes before, an empty bladder, five minutes of quiet rest without talking, your arm supported at heart level and the cuff on bare skin, not over clothing.
A few more things help:
- Use a validated monitor in the right cuff size. A log is only as good as the device it came from. Our list of validated home blood pressure monitors shows the ones on the US and UK lists.
- Take the monitor with you. The AHA suggests bringing it to your appointment so your healthcare professional can check you are using it correctly and that it agrees with their equipment. If it stores readings in its memory, bring that too.
- Do not chase single readings. Blood Pressure UK says not to worry about one-off high readings, since it is normal for numbers to vary, but to contact your GP or practice nurse if you notice a consistent rise.
- Ask whether home monitoring suits you. Blood Pressure UK notes that it may not be suitable for some people, for example those with conditions that affect the pulse such as atrial fibrillation. Your doctor can advise.
If you are setting this up for an older parent, a monitor with one button and a clear screen makes a seven-day log far more likely to get done. Our guide to the easiest blood pressure monitors to use covers the simplest validated ones.
What home readings do not do
The AHA is firm on this: home monitoring does not replace regular doctor visits, and you should not stop taking blood pressure medication without checking with your healthcare professional, whatever your home readings show. A lower number at home is information for your doctor, not permission to change your treatment.
Your blood pressure in the clinic is one snapshot, taken on an unusual day. A week of careful home readings is a much fuller picture, and it is the picture NICE and the AHA both want your doctor to see.