ZFaktor

ZFaktor, commonly written as Z-Faktor or Z'-Faktor in German-language contexts, is a statistical measure used to assess the quality and separability of high-throughput screening (HTS) assays. It compares the signal distributions of positive and negative controls to quantify their dynamic range and variability, providing a single scalar value that reflects overall assay performance across a plate or screen.

Definition and formula: The ZFaktor is typically defined as Z' = 1 - [3*(SD_positive + SD_negative)] / |Mean_positive - Mean_negative|. Here Mean and SD refer to the means and standard deviations of the signal from the positive and negative control distributions. The statistic is dimensionless and predominantly ranges from negative values up to 1, with higher values indicating better separation and lower variability between controls.

Interpretation: Values above 0.5 are commonly described as excellent for HTS, values between 0 and 0.5 indicate a marginal assay, and non-positive values suggest the assay is unsuitable for screening. ZFaktor is usually calculated for each plate, with aggregate estimates obtained by combining plate-level results.

History and usage: The statistic was introduced in the late 1990s by Zhang and colleagues as a simple, interpretable quality metric for HTS assays. It is widely used in drug discovery, enzymology, and cellular assays to determine whether an assay provides reliable discrimination between active and inactive compounds.

Variations and limitations: The ZFaktor assumes relatively normal distributions and can be sensitive to outliers. Variants such as the robust Z'-factor use medians and robust dispersion measures to mitigate non-normality. Other related metrics include plate-by-plate Z-scores and additional quality-control measures.

See also: Z'-factor; high-throughput screening; assay quality control.