Posting this under the heading it deserves: Second pass at: LOD and LOQ: definitions people get wrong Everything below is what sits behind that.
Something about LOD and LOQ does not reconcile and I would like a second pair of eyes before I decide which half is wrong.
Two sources, both reputable, giving figures that cannot both be right unless they are measuring different quantities. My suspicion is that they are, and I cannot see how.
Worth separating two things that the opening post runs together.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
Where I would look next, rather than where I would stop.
The opening post is right about the mechanism and I think understates the practical bit.
The arithmetic on LOD and LOQ is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
Where the LOD and LOQ discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.
Picking up post #4: that is the part I would want checked first.
System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.
The conclusion is tentative; the arithmetic underneath it is not.
On post #2 — agreed on the reasoning, with one qualification.
My position on LOD and LOQ is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
Precision has two levels worth distinguishing: repeatability within a run and intermediate precision across days, analysts and instruments. The second is where most methods lose the numbers people quote.
Helpful, and short, which on this subject is harder than long.
Limits of detection and quantitation: LOD is the lowest concentration that produces a signal above background. LOQ is the lowest concentration at which the method meets precision and accuracy acceptance criteria. Both are determined empirically.
Adding what did not work for me on LOD and LOQ, since the failures never get written up and they are half the useful information.
The honest answer on LOD and LOQ 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.
Post #13 put the caveat in the right place and I want to underline it.
Counterpoint on LOD and LOQ, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.
Building on post #13 rather than restating it.
The most useful reply I ever got about LOD and LOQ was a request to state my units. It sounds like pedantry and it has saved me twice.
On LOD and LOQ 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.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
Correct me on the arithmetic if it is wrong; I would rather know.
A method that reports the same number for every lot is worth a second look. Real processes vary, and a total absence of variation is a statement about the measurement rather than the process.
That is all I can say without guessing.
Two claims get bundled together under LOD and LOQ 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.
Narrowing post #22, because the general version has more than one answer.
I changed my mind about LOD and LOQ after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
Everything in post #20 holds. The case it does not cover is the one I have.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
I would rather say I do not know than round it up to an answer.
A note on scope: what I am saying about LOD and LOQ applies to the case in the first post and I would not extend it further without checking.
Worth separating two things that post #24 runs together.
Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.
I am confident about the direction and much less about the magnitude.
Bookmarking this. I will come back when I have something worth adding.
I would rather this thread reach "we do not know" about LOD and LOQ than reach a confident answer that nobody can support when asked.
Picking up post #26: that is the part I would want checked first.
LOD and LOQ would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.