This International Standard specifies requirements for the content of a reference measurement procedure for in vitro diagnostic medical devices and medical laboratories. NOTE 1 It is intended that an experienced laboratory worker who follows a measurement procedure written in accordance with this International Standard can be expected to produce measurement results with a measurement uncertainty not exceeding the stipulated interval. This International Standard applies to reference measurement procedures providing values of differential or rational quantities. Annex A provides information on nominal properties and ordinal quantities. This International Standard is valid for any person, body or institution involved in one of the various branches of laboratory medicine whose intention is to write a document to serve as a reference measurement procedure. Full descriptions of measurement methods are usually published in scientific literature, in which methods are described in sufficient detail that they can be used as the basis of a documented measurement procedure. NOTE 2 In this International Standard, “international measurement standard” designates a material standard. The term “international standard” is used by WHO for reference materials.