Comparison of liquid phase radiobinding assay and solid phase ELISA for the measurement of insulin antibodies and insulin autoantibodies in serum.

Sera containing insulin antibodies from 20 insulin-treated diabetic patients, sera containing insulin autoantibodies from 20 insulin-naive non-diabetic patients, and from 10 normal controls, were tested at 1:20 dilution in three different radioimmunoassays (RIA) and an enzyme linked immunosorbent assay (ELISA), using a highly purified human insulin ligand. The RIA using insulin radiolabelled at multiple sites detected insulin antibodies in 17/20 and insulin autoantibodies in 13/20 sera. The same RIA using A-14-monoiodinated insulin was sensitive to antibodies and autoantibodies in all the sera. The same RIA using sera after insulin extraction detected only 13/20 diabetic sera and 9/20 autoimmune sera as positive, owing to a substantial rise in non-specific binding of the control sera. ELISA was sensitive to insulin antibodies and autoantibodies in every case. When binding curves for ELISA and the most sensitive RIA were compared using serial dilutions of four insulin antibody containing sera and four insulin autoantibody containing sera, antibody titres varied from 1.1 to 3.8 times higher in ELISA, and autoantibody titres from 10.6 to 28.6 times higher in ELISA. These studies indicate that ELISA is more sensitive than RIA to insulin antibodies, and in particular to insulin autoantibodies.