Table 1. Types of WHO reference preparations for epidemic infectious diseases*.

Types of reference material	Characteristic
International standards	Extensively characterized following international collaborative studies enable activity of biological preparations to be expressed in the same way globally, most commonly in international units
International reference reagents	Less extensively characterized than international standards. Not assigned international units. Reference reagents are interim and not intended to be replaced when they expire or are depleted.
International reference panels	Group of reference materials established to collectively aid evaluation of assays or diagnostic tests. Comply with requirements for WHO reference standards/reagents.
Working or secondary standard	Reference standards established by regional or national authorities, or by other laboratories, that are calibrated against, and traceable to, the primary WHO materials and are intended for use in routine tests.

*WHO, World Health Organization.