Radioimmunoassay for Insulin-Like Growth Factor (IGF). II: Interference by Pure IGF-Binding Proteins

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Abstract
A new radioimmunoassay for insulin-like growth factor-II (IGF-II) is described. Compared to recombinant DNA-derived IGF-II standard, the cross-reactivity of natural or recombinant IGF-I was 90% inhibition at 400 ng/ml IGFBP-1. In contrast, the IGF binding protein IGFBP-3 (BP-53) caused approximately 30% inhibition of tracer binding at 20 ng/ml IGFBP-3, with no further inhibition up to 400 ng/ml IGFBP-3. The influence of added IGF binding proteins on IGF-II displacement curves varied depending on both the type and concentration of binding protein added. It is concluded that interference in IGF radioimmunoassays by IGF binding proteins depends both on the types of binding proteins present, and on IGF concentration, in the test samples.

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