Comparison of a logistic and a mass-action curve for radioimmunoassay data.

This paper compares a logistic curve and a mass-action curve in their ability to represent radioimmunoassay standard curves. A data base from 10 different assays is used in this comparison. Six of the 10 assays could be modeled equally well by either the logistic or the mass-action curve. In one case the mass-action curve gave a slightly better fit, though the practical distinction was negligible. In the remaining three assays the mass-action curve performed very much worse than the logistic curve. It is shown that the mass-action curve can assume only a very limited range of shapes, which explains its difficulty with these three assays. In particular, the mass-action curve cannot represent assays where the standard curve slope is less than a specified value. The paper also discusses the extension of each of these curves to more complicated equations, and their application to immunoassay data.