Determination of antibacterial antibodies in serum by immunoradiometric assays.

When used to determine immunopurified antibacterial globulins in saline-phosphate buffer, the detection sensitivities of direct competitive and indirect immunoradiometric assays were respectively 25 and 2 ng. Normal human and rabbit sera interfered in both types of assay, markedly decreasing sensitivity and precision. Various pre-treatments of sera and modifications of reaction conditions substantially decreased interference in the competitive assay without affecting the activity of added antibody, but they had only marginal effects in the indirect assay. When serum samples taken from rabbits after vaccination with the Live Vaccine strain of Francisella tularensis were tested with the two assays and agglutination tests, newly synthesised circulating antibody was usually detected earlier by the competitive assay than by the other methods.