Home blood pressure monitors are cheap, widely available and genuinely useful. They are also, in most homes, used in a way that quietly inflates or deflates the reading by more than the difference the reading is meant to detect. That is a strange situation: a good instrument producing a number nobody should act on.
The five minutes nobody sits still for
Blood pressure responds to what happened just before the cuff inflated. Walking in from outside, climbing stairs, finishing a phone call or arguing with a parking app all move it. The convention is to sit quietly for about five minutes first, feet flat, back supported, not talking. Most people take the reading in the middle of doing something else, which is exactly when it means least.
Where the arm sits changes the answer
An arm resting below heart level reads higher; an arm held up reads lower. Supporting the arm on a table so the cuff sits roughly level with the heart removes a difference that can be several points on its own. A cuff over a shirt sleeve, or one sized for a smaller arm, adds error in the same range.
One reading is not a measurement
A single number is a snapshot of a value that moves all day. The usual advice is two or three readings a minute apart, morning and evening, over several days, and to look at the pattern rather than the outlier. People tend to do the opposite: they take one reading when they feel odd, get a high number, and then take four more while worrying about the first one.
The white coat works in both directions
Readings taken in a clinic are often higher than the same person's readings at home, which is well described and has a name. Less discussed is the reverse — readings that look fine at home and are consistently higher elsewhere. Both are reasons to bring your home log to an appointment rather than to replace the appointment with it.
What the log is actually for
The value of measuring at home is not diagnosis. It is context: a fortnight of readings taken properly tells a doctor far more than one taken in a corridor. Write down the time and the number, not just the number, and keep the readings you did not like as well as the ones you did.
When to stop reading and ask
Readings that stay high across days, or that come with chest discomfort, breathlessness, visual changes or a headache unlike your usual ones, are for a doctor now rather than for another week of measuring. The same is true of a number that drops sharply along with dizziness on standing. A monitor is a way of noticing something. It is not a way of managing it alone.