How do primary and secondary reference standards differ, and how is each qualified?

Why

Most QC results are relative. An HPLC assay compares the sample's peak with a standard's peak, so an error in the standard's assigned purity moves every result by the same amount, and no amount of method precision will reveal it. ICH Q7 sets out the hierarchy.

Primary standards from an officially recognized source are normally used without testing if stored as the supplier recommends, with their source, storage and use recorded. Where no official standard exists, an in-house primary standard must be established with testing that fully establishes identity and purity. Secondary standards are prepared, identified, tested, approved and stored under control, compared with the primary before first use, and periodically requalified under a written protocol.

US regulation also requires complete records of testing and standardization of reference standards. Biologics add a twist, because potency is often defined by the standard itself, so replacing a bioassay reference standard needs a bridging study to avoid a step change in reported potency. For example, if a new working standard is assigned 99.2 percent when it is really 98.0 percent, every assay result reads about 1.2 percent low.

The limit of this system is that it relies on careful storage and handling. A hygroscopic standard left open absorbs water and quietly changes its effective purity.