Properties and interpretation of the Woolf and Scatchard plots in analysing data from steroid receptor assays

The Woolf plot has been shown to have superior statistical properties compared with the Scatchard plot in analysing data from steroid receptor assays. This paper examines the properties of the Woolf plot, with a view to aiding its interpretation and improving familiarity with it so that its use may become more widespread. The Woolf plot contains the same information as the Scatchard plot, and deviations from linearity of the plot due to positive cooperativity, second binding sites, or non-specific binding are readily identifiable, and are described. It is emphasised, however, that the range of incubating concentrations should be chosen to give an even spread of data points on the plot. There is shown to be a relatively stable error distribution along the Woolf plot, and the implications of this for analysis of the data points are discussed. This error distribution is associated with consistently better fitting of a straight line to the data points when they are presented on the Woolf, as opposed to the Scatchard scale. This has further implications in the definition of receptor positivity, and in determining the reliability of a receptor assay result. Interpretation of the Woolf plot is easy, with the receptor site concentration being obtained from the slope of the plot, and the dissociation constant from the intersection of the extrapolated line with the abscissa. Familiarity with the plot should lead to its wider use.