Typically, an aged sample should be used to ensure that there are significant

levels of related substance in the sample. Alternatively, if different samples are

available with different levels of related substance (e.g., fresh sample and sample

atexpiry),onecandeterminetherepeatabilitybyperformingthreereplicate

preparations for each sample. ICH guidelines require a minimum of three samples

with three different levels of related substance.

Instead of using spike samples (as in accuracy determination), drug product

lots that are representative of the commercial products should be used for preci-

sion (repeatability, intermediate precision). This is to ensure that the commercial

drug product is used in at least one part of the method validation and that the

repeatability results are representative of those that can be expected in the future.