scintillation proximity assay Scintillation proximity assay (SPA) eliminates the separation step of unbound radioisotopes and is performed using low energy radioisotopes (3H and 125I) as labels, which have short range electron emission. SPA involves the use of microscopic beads which are first coated with a receptor protein of interest and contain a scintillant. When stimulated by the binding of radio-labeled ligand molecules, they emit light, which is detected by the photomultiplier tube. SPA is suitable for high throughput since the protocol has no wash or separation steps. SPA scintillation counting