ISO 18153:2003 specifies how to assure the metrological traceability of values assigned to calibrators and control materials intended to establish or verify trueness of measurement of the catalytic concentration of enzymes.The calibrators and control materials are those provided by the manufacturers as part of, or to be used together with, in vitro diagnostic medical devices.The following subjects are outside the scope of ISO 18153:2003: requirements for the design or selection of a reference measurement procedure; quantities involving mass of enzyme or immunoreactivity of enzymes; control materials that do not have an assigned value and are used only for assessing the precision of a measurement procedure, either its repeatability or reproducibility (precision control materials); control materials intended for intralaboratory quality control purposes and supplied with intervals of suggested acceptable values, each interval obtained by interlaboratory consensus with respect to one specified measurement procedure, and with limiting values that are not metrologically traceable; metrological traceability of routine results to the product calibrator and their relations to any medical discrimination limit; properties involving nominal and ordinal scales.