Nearly every physiological threshold in routine use is a population
statistic: a range derived from some cohort of people, at some point
in time, under some set of conditions. When the person in front of you
resembles that cohort, the range works. When they do not, the range
quietly stops being a measurement and starts being an assumption.
The failure is not that clinicians are careless. It is that the
instrument itself encodes a comparison the clinician did not choose
and often cannot see. A patient whose resting physiology sits at one
end of a distribution can deteriorate substantially and still read
“normal.” Another can be flagged, worked up, and labeled
for being exactly who they have always been.
Both are errors of the same kind: a person compared against a
distribution they were never part of.