The Kd is used as a measure of the affinity of an interaction. Tight interactions, also called high affinity interactions, have a low Kd value. The Kd is often used to compare affinities because it intuitively gives an idea of the concentration needed to reach saturation of the interaction: it describes the concentration of ligand at which half of the target molecules are bound to ligand molecules. In this regard, the Kd is similar to the EC50 but there are important differences between the two. See EC50 for more details. For information on how the Kd is derived from a TRIC measurement, see Kd Fit Model.