Dr.NutriCornell said:

PSA: get baseline labs BEFORE starting baseline bloodwork therapy.

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.

lisa_labSD said:

One addition: if the lab changes analytical platform between your draws, the comparison breaks and nobody tells you.

That reframing is the part I needed. Sending this to two other people who asked me the same thing last week.

Dr.NutriCornell said:

PSA: get baseline labs BEFORE starting baseline bloodwork therapy.

Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. I think the framing smuggles in the conclusion. Ask it the other way round and the obvious answer reverses, which usually means the question is doing the work.

One concrete data point for the thread. 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.

Moderator note: good thread. Keeping it here rather than moving it, because the question is general enough to be useful.