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For a long time I've noticed that if I take my blood pressure a second time, the readings vary especially for the systolic reading. If I don't like the reading, I just take it again (recording both).

I wonder if these expected variances are known.

As an example, this morning I made the following three readings, one after the other, without readjusting the cuff on my left arm on a Medline brand of home blood pressure monitor:

SYS 119 DIA 79 Pulse 83
SYS 105 DIA 76 Pulse 79
SYS 109 DIA 75 Pulse 79

I would have expected the readings to differ no more than plus or minus 2, but perhaps what I am getting are normal variances.


This is from the comments:

  • I think the key point here is that BP is used as a trend, and a rough idea. If your BP is always <120 and >100, the exact range doesn’t matter because as Carey said, BP itself changes rapidly, and the measurement methods (even manually) have limited accuracy. If you‘d want to be as accurate as possible, it’s probably best to measure twice, once left and once right. This not only gives you an estimation of the error bar, but if your BP constantly varies between l&r (i.e. right is always +20mmHg), it is also a sign that something is wrong (Thanks to @Narusan)
  • Those numbers are all within the range of normal variation. Measurement error of BP machines is going to be specific to the machine so your best source would be the manufacturer. – Carey Gregory Dec 30 '18 at 16:55
  • Your BP changes minute to minute and can easily vary more than +/- 2 mm from one measurement to the next, so you can't determine accuracy of the machine by looking at variation. You need to take the machine with you to someone who can measure your BP manually and compare the results. – Carey Gregory Dec 30 '18 at 17:17
  • In my experience , your readings are surprisingly uniform. – blacksmith37 Dec 30 '18 at 20:52
  • 1
    @CareyGregory The variance is +/-10. // I think the key point here is that BP is used as a trend, and a rough idea. If your BP is always <120 and >100, the exact range doesn’t matter because as Carey said, BP itself changes rapidly, and the measurement methods (even manually) have limited accuracy. If you‘d want to be as accurate as possible, it’s probably best to measure twice, once left and once right. This not only gives you an estimation of the error bar, but if your BP constantly varies between l&r (i.e. right is always +20mmHg), it is also a sign that something is wrong. – Narusan Dec 30 '18 at 23:26
  • The variance is +/-10 was a response to Carey who said that the variance was super accurate with +/-2. I didn't claim that the expected variance of BP measurements are +/-10. – Narusan Jan 2 '19 at 9:08

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