Validating a method you did not develop — does this still hold? posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
Reporting a result to more decimal places than the method's precision supports is a small dishonesty that appears everywhere. A method with a two per cent relative standard deviation does not support a figure quoted to a hundredth.
Adding the measurement that post #32 says would settle it.
Documented validation is what separates a number from an opinion expressed numerically. That is the whole reason to ask for the procedure identifier rather than the method name.
Post #30 describes the usual case. This is about the unusual one.
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.
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.
Post #34 put the caveat in the right place and I want to underline it.
Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal).
Adding a note of thanks rather than an opinion. I did not know most of that.
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Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
It is a small point and it changes the answer, which is an awkward combination.
Post #36 is right about the mechanism and I think understates the practical bit.
Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements.
An independent laboratory's method being different from the supplier's is not a discrepancy. It becomes one only when the results differ by more than both methods' demonstrated precision.
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Linearity across the working range is a routine demonstration and it constrains how far a result can be extrapolated. A method linear from 80 to 120 per cent of nominal says nothing about a sample at ten per cent.
I would rather say I do not know than round it up to an answer.
Where I part company with post #39, and it is a narrow parting.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
The disagreement above is smaller than it looks once the terms are fixed.
Robustness: the method gives consistent results when minor parameters vary. Tested by deliberately varying pH, temperature, flow rate, and mobile phase composition within reasonable ranges and demonstrating that results stay within acceptance.
Confirming post #46 from a second method, which matters more than confirming it from a second person.
Two sentences on Validating a method and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
On post #48 — agreed on the reasoning, with one qualification.
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 checked the source rather than the summary, and they differ.
Picking up post #48: that is the part I would want checked first.
Specificity: the method can distinguish the intended compound from related impurities and degradation products. Tested by comparing results on pure compounds, mixtures of compounds, and degraded samples.
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.
That reframing is the whole thing. The facts I already had.
The arithmetic in post #50 is right; the assumption feeding it is the part to check.
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.
That matches what I was told, which is not the same as knowing it.
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Answering the question post #53 raises rather than the one it answers.
The distinction between a qualified instrument and a validated method is worth keeping. Both are needed and they fail in different ways.
Not the whole picture, but the part of it I can speak to.
Building on post #53 rather than restating it.
Accuracy: the method measures what you intend to measure. For purity methods, this is tested by spike-and-recover experiments: add a known amount of impurity to a sample and measure whether you recover the added amount.
Someone should write this up properly, and it should probably not be me.
Robustness testing deliberately varies the parameters most likely to drift — organic percentage, pH, temperature, flow — and shows the result does not. It is the part of validation that predicts whether a method will transfer.
Take it as a starting point and not as a specification.
Everything in post #56 holds. The case it does not cover is the one I have.
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.
I am reporting what happened, not recommending it.
Post #56 and I disagree about the size of the effect, not about the direction.
Precision and repeatability: within-run and between-run variability of the method. Acceptance criterion is typically a relative standard deviation of ≤2% for area measurements.
That is where I would start, not where I would stop.