Okay the official cut off point for high blood pressure is 140/90 but I wanted to know what this means and where it comes from, and similarly what low blood pressure is and means.

I am of course immediately skeptical because of those big round zeros on the end. People pick arbitrary values with zeros on the end, few people would pick say 142/93 as the cut of point unless it was based on some sort of statistical analysis, although of course it may be that 140/90 is chosen as being more memorable and that blood pressure measurement is imprecise enough that the rounded numbers are good enough.

Most pathology lab reference ranges are based on the 95th percentile range (Between the 2.5 to 97.5 percentile values) of a value based on testing a group of healthy people. These values are typically corrected for age, gender, ethnicity etc where necessary.

For blood pressure I failed to find much public data on the 2.5 and 97.5 percentile values in healthy adults but the NIH site has a chart for children noting that the 95th percentiles for systolic and diastolic are already above 140/90 at age 17. I found lots of report saying that blood pressure ranges vary with age, gender and ethnicity.

Note since we are dealing with two (potentially correlated values) simple 95th percentile ranges in healthy individuals might not be appropriate even if that was where the limit came from. But clearly the limit is not based on percentile ranges. Thus I guessed the range was based on risk, and it led me to a paper titled Systolic Blood pressure and mortality which references both where the original limit came from and looks at the Framingham study data to test if it is appropriate.

Low blood pressure (hypotension) is badly treated on many websites with a lot of places giving block diagrams suggesting average blood pressure might be low?! The NHS site gives a more plausible lower bounds 90/60 at which to be concerned. Many places fail to note that low blood pressure may be a marker for endocrine problems (diabetes, hypothyroidism) even if the condition is largely asymptomatic and naively assume lower is better till the point where you get dizzy and/or start falling over.

My take away is that whilst 140/90 may not be spot on it isn’t a dreadful cut off. It is selected based on risk, and because it may be a bit low some people will worry about their blood pressure when it isn’t especially risky. Since the selection is risk based it suggests something like 20% of people may need to do something about their risk from hypertension assuming their goal is to maximize their own life expectancy and avoid heart attacks and strokes.

My chemistry teacher correctly complained about numbers without their units,  Systolic and Diastolic values above are all in the non-SI unit Torr named after Evangelista Torriceli.