Response curves for radioimmunoassay.

In quantitative estimates from radioimmunoassay, one of four types of response curves is usually used: a freehand curve, a spline function, an equation based upon mass-action considerations, or a logistic equation. This paper comments briefly on the subjectivity and labor of the first and on the overparametrization of the second. It is chiefly concerned to compare the single binding-site equation with a simple or modified logistic. Whatever the theoretical merits of the binding-site approach (these are not under discussion), estimation of parameters is difficult. The paper shows that under many but not all circumstances a four- or five-parameter logistic will fit data at least as well over a wide range of doses. This is particularly so when both the binding-site concentration and the equilibrium constant are small.