Why we measure your blood pressure twice

The first number is the one we trust least

If you have had your blood pressure taken here you will have noticed that nobody writes anything down after the first go. We inflate the cuff, read it, wait, and do it again. Most of the time the second number is lower than the first, and the third is lower still, and it is the average of the last two that goes in your record.

People sometimes read that as us fishing for a better answer. It is closer to the opposite. The first reading is reliably inflated by the act of being measured — walking in, sitting down, having a stranger wrap something around your arm — and treating it as the real one is how people end up on medication they do not need.

What the cuff is actually measuring

The number is a pressure, and pressure changes constantly. It rises when you stand, when you talk, when you are cold, and when you are concentrating on the fact that your blood pressure is being taken. What we want is not the pressure right now but the pressure you spend most of your life at, and no single measurement can give us that.

Two or three readings a minute apart, averaged, gets much closer. A week of readings taken at home gets closer again, which is why we so often send people away with a monitor rather than deciding anything in the room.

One reading tells you about a moment. An average tells you about a person.

The four things that move it before we start

There are four that matter enough to be worth naming, because all four are avoidable and all four push in the same direction.

Sitting still for five minutes first is the largest. A reading taken the moment you sit down runs higher than one taken after five quiet minutes, sometimes by a lot. Feet flat on the floor rather than crossed at the knee is the second. An arm supported at heart height rather than hanging or held up is the third. And talking during the reading — including answering our questions — is the fourth. We will usually go quiet at exactly the moment you want to tell us something, and it is not rudeness.

Caffeine and a full bladder both matter as well, though they are harder to plan around in a clinic you drove to in traffic.

Cuff size is the one we get wrong most

A cuff that is too small for the arm squeezes harder than it should and reads high. The effect is not subtle and it is common, because the standard cuff on most machines does not fit a lot of people. If your reading is taken with a cuff that visibly strains to close, ask for the large one. Any clinic worth the name has one within arm’s reach.

What a high reading actually triggers

A single high reading in a clinic is not a diagnosis of anything. What it triggers is a second appointment, or a monitor to take home, or a twenty-four hour recording — because the question we are answering is whether the number is high most of the time, and one afternoon cannot answer it.

Perhaps one in four people whose clinic readings are high turn out to have entirely normal pressure at home. The reverse also happens, and matters more: normal in clinic, high everywhere else. Both are only visible if you measure more than once.

What to do with this before your next appointment

Sit for five minutes. Do not cross your legs. Let us wrap the cuff over a bare arm rather than a shirtsleeve, and let us be quiet while the machine runs. If you already measure at home, bring the log — on paper, in the portal, or as photographs of the screen. It is worth more to us than anything we can take in ten minutes at a desk.