Scintillation proximity assay (SPA)--a new method of immunoassay. Direct and inhibition mode detection with human albumin and rabbit antihuman albumin.

Abstract Scintillation Proximity Assay (SPA) was used to measure concentrations of human albumin (HA) and rabbit anti-human albumin (RAHA). In one form of this assay, dilute aqueous suspensions of tritiated latex particles (LH) and scintillant latex particles (L∗) covalently coated with antigen are counted in a standard liquid scintillation counter energy gated for tritium. The count rate reflects the extent of(LH)-(L*) dimer formation and is therefore a very sensitive measure of the concentration of antibody present. Concentrations of less than 2.5 ng/ml of RAHA and, in the inhibition mode, less than 1 ng/ml of HA were easily and reproducibly detected.