1. The coherent SI unit is the mole per second mol/s), also called the “katal” (kat). 2. Throughout this Recommendation the “component” is an enzyme. 3. The quantity “catalytic activity” relates to an amount of active enzyme, not its concentration (see 2.25). 4. The measurement procedure employing defined indicator substance is an essential element for the definition of the measurand. 5. In many instances, instead of the conversion rate of the substrate ascribed in the short name of the enzyme analyte, e.g. “creatine kinase”, the conversion rate of an indicator substance as substrate of a combined reaction, e.g. NADH, is measured. Then the measurand should be defined as “catalytic activity of the enzyme as measured by the conversion rate of an indicator substance in a specified system according to a given measurement procedure”, e.g. “catalytic activity of creatine kinase as measured by the rate of conversion of NADH in the IFCC reference procedure in human serum”. [ISO 18153, clause 3.2]