Assay performance and the Z’-factor in HTS

High-throughput screening (HTS) is a common method of drug discovery, wherein a large library (sometimes containing more than a million compounds) is screened using an in vitro or phenotypic assay. The assay is used to identify an active compound that brings about the desired change (known as a ‘hit’) from many inactive compounds, and hence assay quality is critical. Hit identification is strongly dependent on the difference in behaviour between active and inactive compounds (the ‘signal window’), and the variation within the data (as measured by the standard deviations). Intuitively, it will be easier to identify a hit when there is a large signal window and low variability.