The free Insulin-like growth factor-I (IGF-I) in plasma from normal adults was directly measured with a newly developed highly sensitive immunoradiometric assay (IRMA) for IGF-I. The capture antibody did not crossreact with IGF-I associated binding proteins which exist in plasma, and the assay was designed not to shift the equilibrium of the IGF-I and binding proteins. Total IGF-I concentration was measured using this assay with preliminary acid—ethanol extraction. Approximately 1 percent of total IGF-I existed in the free form. Gel filtration of plasma was also used to separate the free IGF-I from its bound form. The free/total ratio of IGF-I as determined by gel filtration was similar to that determined directly by IRMA with and without acid—ethanol extraction.
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