Reference intervals: how they are constructed and why one in twenty flags — does this still hold?

Asking directly, because I could not find a straight answer: Reference intervals: how they are constructed and why one in twenty flags — does this still hold?

A methods question rather than a substantive one, about Reference intervals.

Everyone quotes the same figure and I cannot find anyone who says how it was arrived at. That is not an accusation; it usually means the derivation is somewhere obvious and I have missed it.

Where I part company with the opening post, and it is a narrow parting.

Comparing results across laboratories is harder than it looks because reference intervals and methods differ. The same sample can produce different numbers and different flags.

Where the Reference intervals reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.

Agreed, and I will stop repeating the version of this I had been repeating.

Confirming post #2 from a second method, which matters more than confirming it from a second person.

Reference intervals has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.

Lipid panel interpretation: total, LDL, HDL, triglycerides all on one panel. Reading them together is more informative than reading one value in isolation. An elevated triglyceride with low HDL is different from triglyceride elevation alone.

Reference intervals are constructed to contain the central ninety-five per cent of a reference population, so one healthy person in twenty falls outside one by definition.

I am describing what is, rather than arguing for what should be.

On post #5 — agreed on the reasoning, with one qualification.

Offering a way to settle Reference intervals rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.

Time of day: some biomarkers vary across the day. Cortisol in the morning differs from cortisol in the evening. Comparing results from different times of day is comparing things that are not the same.

I would want a second opinion before relying on that.

Answering the question post #7 raises rather than the one it answers.

The most useful thing this subcategory does is help people tell an interesting result from an uninteresting one before booking an appointment about it.

Reference intervals: constructed to contain the central 95% of a reference population, which means one in twenty healthy people falls outside one by definition. Add biological variation and analytical imprecision and the base rate of a meaningless flag is substantial.

I have no interest in any supplier named above.

Two claims get bundled together under Reference intervals and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.

Almost every disagreement in threads like this one dissolves once you say which of the two you are making.

On Reference intervals, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.

If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.