Reference intervals: how they are constructed and why one in twenty flags — setting out what I have, and where I think it stops being reliable.
Reference intervals: what I expected, what I found, and the gap between them.
I am posting the gap rather than a theory about it. My theories about gaps like this one have not held up well.
Adding a note of thanks rather than an opinion. I did not know most of that.
Glycaemic markers integrate over different periods. A point glucose and a longer-term marker disagreeing is not a contradiction; it is two different questions.
It took me longer than it should have to see that.
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.
Filing this under things that are true until someone shows me otherwise.
Filing a mild objection to the consensus on reference intervals. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit.
Taking post #7 at face value and following it one step further.
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.
Adding the measurement that post #7 says would settle it.
A trend needs at least three points to be a trend. Two points are a line and a line through noise is still a line.
This follows post #8 rather than contradicting it.
Liver enzymes: elevation does not specify cause. ALT and AST can rise from many things. Bilirubin helps narrow down the cause. Multiple markers together are more informative than one alone.
Adding a source would improve this post and I do not have one to hand.
Worth separating two things that post #10 runs together.
What I would tell a new member reading about reference intervals for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
An update on my earlier reference intervals post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
I think the reference intervals question is answerable and has not been answered, which is a more optimistic position than most of this thread.
On post #15 — agreed on the reasoning, with one qualification.
Biological variation: the person-to-person variation in a biomarker for a healthy person is larger than most people realize. Comparing your result to the reference interval is one thing; comparing your result today to your result from months ago is more sensitive to change.
Somebody will have a better source than mine, and I hope they post it.
Post #15 is right about the mechanism and I think understates the practical bit.
A trend needs at least three points to be a trend. Two points are a line and a line through noise is still a line.
It is worth stating the boring hypothesis before the interesting one.
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.
A modest claim, modestly supported.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
It cost nothing to check and would have cost something not to.
Agreed, and I will stop repeating the version of this I had been repeating.
Bringing the actual numbers with intervals to an appointment is far more useful than describing them, and clinicians here consistently say so.
This is the sort of thing that ought to be settled and apparently is not.
The honest answer on reference intervals is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
Everything in post #24 holds. The case it does not cover is the one I have.
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.
Narrowing post #27, because the general version has more than one answer.
On reference intervals I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
I had written a reply contradicting post #27 and deleted it. Here is what survived.
The most useful thing this subcategory does is help people tell an interesting result from an uninteresting one before booking an appointment about it.
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.