An observation about fasting versus non-fasting that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.
Fasting versus non-fasting and which tests care Writing it up because I had to work it out twice and would rather nobody else did.
Fasting versus non-fasting: 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.
The opening post is the version of this I will quote in future. One addition.
Fasting versus non-fasting has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.
Analytical imprecision: any measurement has an error margin. A small difference in consecutive tests is usually measurement noise, not a real change. Knowing the imprecision helps distinguish noise from signal.
Happy to expand any of that if it is the useful part.
Fasting versus non-fasting: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.
An observation about fasting versus non-fasting that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.
The reason fasting versus non-fasting is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.
Post #5 and I disagree about the size of the effect, not about the direction.
Fasting state: some lab tests require fasting; others do not. Lipids on a fasting draw differ from lipids on a fed draw. Always note whether the test was fasting when comparing results.
That is the distinction I keep failing to hold on to. Written down now.
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.
Written from notes rather than memory, which is why the numbers are specific.
Taking post #11 at face value and following it one step further.
A definition problem is doing most of the work in this fasting versus non-fasting discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small.
Post #12 and I disagree about the size of the effect, not about the direction.
What I would check first on fasting versus non-fasting is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
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.
It is one reading of the data and not the only reasonable one.
Fasting versus non-fasting was covered in the wiki last year and the page has a review date on it, which is a better starting point than my memory of a thread.
Building on post #14 rather than restating it.
The number people quote for fasting versus non-fasting is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
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.
That is a description of practice, not a recommendation of it.
The claim about fasting versus non-fasting upthread is stronger than its source supports. I have read the source. The source says "associated with" and the post says "causes".
Coming back to post #16, because the follow-up matters more than the original answer.
My understanding of fasting versus non-fasting is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.
On post #17 — agreed on the reasoning, with one qualification.
Repeat before acting is standard practice for an isolated abnormal result on most analytes, and it is standard for a reason.
I have changed my mind on this once already, so take it as current rather than settled.
Picking up post #21: that is the part I would want checked first.
I disagree with the framing of fasting versus non-fasting above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.
The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.
Fasting state, time of day, recent exercise and hydration all move several common analytes. Standardising the conditions matters more than the interval between draws.
Two things can be true about fasting versus non-fasting at once: the mechanism is plausible and the evidence for the size of the effect is thin. Most of the argument here is people defending the first against attacks on the second.
Adding the measurement that post #25 says would settle it.
I keep a log for fasting versus non-fasting specifically because my memory of it turned out to be systematically wrong in one direction. Six weeks of notes cost nothing and settled it.
Where I part company with post #25, and it is a narrow parting.
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 have separated what I observed from what I concluded, which does not always happen.
Thank you for the correction. I would rather find out here than later.
Agreed, and I will stop repeating the version of this I had been repeating.
General information rather than advice about anyone's care. A single value outside a reference interval, on a single draw, in an otherwise unremarkable panel, is usually uninformative.