Insulin autoantibody affinity measurement using a single concentration of unlabelled insulin competitor discriminates risk in relatives of patients with type 1 diabetes

Development of high-risk combinations of multiple islet autoantibodies and type 1 diabetes is associated with high-affinity insulin autoantibodies (IAA), but IAA affinity measurements require large serum volumes. We therefore investigated whether a simplified method of IAA affinity measurement using a low concentration of unlabelled insulin (ULI) competitor discriminated between moderatehigh- and low-affinity IAA and identified individuals at highest risk of disease. Samples were assayed by radiobinding microassay using high (4.0 x 10(-5) mol/l) and low (7 x 10(-9) mol/l) ULI concentrations for competitive displacement in three cohorts of IAA-positive individuals; (1) 68 patients with newly-diagnosed type 1 diabetes; (2) 40 healthy schoolchildren; and (3) 114 relatives of patients with type 1 diabetes followed prospectively for disease development (median follow-up 13 years). IAA results obtained with low ULI were expressed as a percentage of those obtained with high ULI and this was used to classify samples as low or moderatehigh affinity (050% and >50%, respectively). Sixty-eight patient samples were positive with high and 67 (99%) with low ULI. Forty schoolchildren were IAA-positive with high and 22 (55%) with low ULI (P < 0.001). Of the relatives, 113 were positive with high and 83 (73%) with low ULI (P < 0.001). In relatives, moderatehigh affinity IAA were associated with multiple islet antibodies (P < 0.001) and greater diabetes risk than low affinity IAA (P < 0.001). A single low concentration of ULI competitor can act as a surrogate for complex IAA affinity measurements and identifies those IAA-positive relatives at highest risk of disease progression.