Validating a method you did not develop — does this still hold? posts 61–90

Post #72 answers the question as asked. The question underneath it is different.

Checked the Validating a method claim against the primary source this morning. It survives, with a narrower scope than the version quoted here. Posting the narrower scope.

A stability-indicating method is one demonstrated to resolve the analyte from its degradation products, usually through forced degradation. Calling a method stability-indicating without that work is a claim rather than a property.

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.

Agreed, and I will stop repeating the version of this I had been repeating.

Thank you for the correction. I would rather find out here than later.

Where I part company with post #77, and it is a narrow parting.

The failure mode on Validating a method is boring rather than dramatic. It is almost always the step everyone assumes was done correctly because it is too simple to get wrong.

Answering the question post #80 raises rather than the one it answers.

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.

That is a fair summary of where the discussion has got to.

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.

Adding it in case it saves somebody the afternoon it cost me.

Post #83 is the version of this I will quote in future. One addition.

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.

That is the version I use. It may not be the version that is correct.

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.

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 not lead a decision with this, but I would not ignore it either.

Post #83 put the caveat in the right place and I want to underline it.

I read the earlier replies on Validating a method twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.

Building on post #87 rather than restating it.

Specificity is the first question: does the method separate the analyte from everything reasonably expected to be present? A method that has not been challenged with its own degradation products has not answered it.

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.

Two sources, same conclusion, and I could not rule out that one copied the other.

Picking up post #87: 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.

I would hold that lightly until someone with a larger sample weighs in.