A narrower follow-up, since the general answer is now clear:
How would you tell the difference between that and the alternative explanation?
Closing the loop on my own question.
Update — I was wrong in the opening post, specifically about the direction. Correcting it here rather than editing it away.
PharmacoVig_BOS said:Worth stating the units and the reference range whenever you post a number here.
That is right, and it stops being right at the edges. The general case is well behaved; the interesting cases in this thread are all at the boundary where the general case breaks.
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Browse GL BiochemFound this through search years later and it is still the best discussion of the question, so adding one thing. For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine. Half the figures circulating in this community trace back to a secondary summary that dropped a qualifier.
From the other side of the consultation, briefly. The useful move here is to separate what is established from what is widely repeated. Those two sets overlap less than the confident tone of most write-ups suggests, and the second set is where nearly all the disagreement on this board comes from.