Noncompetitive ELISA for human serum insulin-like growth factor-I

Measurement of serum insulin-like growth factor I (IGF-I) is important in assessing the growth hormone/IGF axis. Competitive immunoassay methods for IGF-I are complicated by substantial interference from IGF-binding proteins (IGFBPs) due to the relatively low ratio of specific antibody to IGFBPs in the sample, even after standard acid-ethanol sample extraction. We report the development of a noncompetitive ELISA for human IGF-I that avoids this problem by utilizing excess amounts of capture and detection antibodies. Serum samples were prepared by using an abbreviated acid-ethanol extraction method. Neutralized supernatant was added to microwells coated with IGF-I capture antibody; horseradish peroxidase-labeled detection antibody was then added, incubated for 2 h, and then developed. Compared with the 'gold standard' method of acid-column chromatography, the simplified acid-ethanol extraction yields a mean ± SD recovery of 103% ± 5.5% despite the presence of residual IGFBPs in the extracted sample. Comparisons with a centrifugal filtration sample extraction method are also shown. The ELISA is specific for IGF-I with an absolute sensitivity of 0.03 μg/L and inter- and intraassay CVs of 3.9-8.8% and 2.66.7%, respectively. The availability of a rapid IGF-I ELISA combined with a simple and reliable sample preparation procedure should facilitate clinical and research studies of this important growth factor.