The competitive antigen spot test (CAST). A rapid and simple means of quantitatively screening antisera

A method is described which permits the simultaneous testing of several antisera and an estimation of their antigen binding capacities. The sera to be tested are preabsorbed with different quantities of antigen and subsequently assayed for unbound antibodies by a second incubation with antigen immobilized on nitrocellulose. When the absorbing antigen exceeds a definite concentration, which is specific for a particular antiserum, a clear cut-off is observed when immunostaining is used in the following step. This competitive antigen spot test (CAST) is both quick and easy to perform and was found useful when comparing different antisera.