Second pass at: LOD and LOQ: definitions people get wrong posts 31–52
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
That is the distinction I keep failing to hold on to. Written down now.
Reading this LOD and LOQ thread as someone who came in with a fixed view: the third and seventh replies moved me and the confident ones did not.
The arithmetic in post #31 is right; the assumption feeding it is the part to check.
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.
Post #31 put the caveat in the right place and I want to underline it.
LOD and LOQ 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.
Filing a mild objection to the consensus on LOD and LOQ. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
A qualification I should have led with rather than closed on.
Quietly grateful for the plain phrasing. Not every thread gets that.
Taking post #38 at face value and following it one step further.
Practical experience of LOD and LOQ, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.
The arithmetic in post #40 is right; the assumption feeding it is the part to check.
Offering a way to settle LOD and LOQ 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.
Answering the question post #43 raises rather than the one it answers.
Second-hand on LOD and LOQ, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
The bit of LOD and LOQ that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
Everything in post #46 holds. The case it does not cover is the one I have.
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.
Coming back to post #46, because the follow-up matters more than the original answer.
My understanding of LOD and LOQ 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.
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.
Happy to expand any of that if it is the useful part.
This topic was referenced in
- When to suspect the method rather than the sampleAnalytics › Method validation · 115 replies
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