Defining and measuring biological activity: applying the principles of metrology

A definition of biological activity is proposed that is superficially analogous to the equation relating the thermodynamic activity of a solute to its concentration via an activity coefficient. The biological activity of a molecular entity is defined as A=cf, where A is the activity, c the amount-of-substance concentration, and f is a parameter designated as “inherent activity.” Units and dimensions are determined by the type of activity, catalytic (katal) or binding (mol⁻¹ L). The measurand is described by a chemical equation that identifies the entity for which an activity is being monitored. This definition of biological activity has the advantage of separating the chemical characterization of the entity in terms of structure and amount from the assessment of biological activity. Ideally, a homogeneous entity is used for the measurement of f. In instances where impure materials are used or the chemical equation defining the activity is unknown, the evaluated parameter should be designated as f′ to denote its empirical nature. Any measurement of f or f′ should be qualified with an appropriate estimate of measurement uncertainty.