What method validation means

Every number on a certificate of analysis comes from an analytical procedure, and every analytical procedure can be wrong in ways that are not visible in a single result. Method validation is the work done, before a procedure goes into routine use, to show with data that it is fit for its intended purpose. The international reference for what that work involves is ICH Q2(R2), Validation of Analytical Procedures, read together with its companion ICH Q14, which covers how procedures are developed.

Fit for a stated purpose

Validation is always relative to a purpose. A procedure meant to confirm identity has different requirements from one meant to measure the main component’s content, which in turn differs from one meant to find impurities at low levels. Before validation starts, the intended use is written down: what is measured, in what material, over what range, and how well the result needs to be known. The validation then tests the procedure against that statement and nothing else.

The characteristics that are tested

ICH Q2(R2) describes a set of performance characteristics. Which ones apply depends on the type of procedure, but the core list is consistent.

Specificity, also called selectivity, is the ability to measure the target substance without interference from anything else likely to be present: impurities, degradation products, other ingredients, the sample matrix. It is usually shown by analyzing samples deliberately spiked with those substances, or degraded on purpose, and confirming that the target’s result is unaffected.

Accuracy is closeness to the true value. It is typically shown by analyzing material of known content, or by adding a known amount of reference standard to a sample and measuring how much is recovered.

Precision is the closeness of repeated results to each other. It is assessed at more than one level: repeatability, meaning the same analyst on the same equipment over a short time; intermediate precision, meaning different days, analysts or instruments within the same lab; and, where methods move between sites, reproducibility between laboratories.

Range is the interval between the lowest and highest levels for which the procedure has been shown to give acceptable accuracy, precision and response. Within that range the relationship between signal and amount must be demonstrated, which for many methods means showing that it is linear. Results outside the validated range are not supported by the validation.

For impurity methods, the detection limit and the quantitation limit matter: the lowest amount that can be reliably detected, and the lowest amount that can be measured with acceptable accuracy and precision. A method cannot report an impurity below its quantitation limit as a number.

Robustness is how well the method tolerates small, deliberate changes in its conditions, such as a slightly different mobile phase pH, column temperature or flow rate. Under the current guidance robustness is normally explored during development, which is one reason ICH Q14 and Q2(R2) are read together.

Validation versus verification

A laboratory does not always validate a method from scratch. Methods published in a pharmacopoeia or a recognized standard have already been validated by their authors. A lab adopting one is expected to verify it: to show, with its own analysts, equipment and samples, that the method performs as described. ISO/IEC 17025 draws the same distinction, requiring validation of non-standard and laboratory-developed methods, and of standard methods used outside their intended scope, and verification of standard methods before they are put into use.

What a validation record contains

A validation is documented in a protocol, written and approved before the work starts, and a report afterward. The protocol states the purpose, the characteristics to be tested, the experiments and the acceptance criteria. The report presents the data, compares each result with its criterion, and records any deviation from the protocol with an explanation. Setting criteria before seeing the data is what separates validation from description: a method is not validated by showing that it produced whatever results it produced.

When validation is repeated

Validation describes a procedure as it was at a point in time. When something material changes — the route by which the material is made, the composition of the product, the method itself, the class of equipment — the affected characteristics must be examined again. ICH Q14 frames this as a lifecycle: the procedure is monitored in routine use, and the knowledge gathered during development defines which changes need new data and which do not.

For a reader of certificates, validation is mostly invisible. It shows up as a method reference beside each result, and as the absence of results reported outside a method’s range or below its quantitation limit. When a certificate names a method, the question behind it is whether that method was validated or verified for this material, and whether the lab can produce the record that says so.