The Eagle Biosciences IGFBP-3 ELISA is a so-called Sandwich-Assay. It utilizes two specific antibodies of high affinity. First the IGFBP-3 in the sample binds to the immobilized antibody on the microtiter plate. In the following step, the complex of biotinylated anti-IGFBP-3-Antibody and Streptavidin-Peroxidase binds in turn to the immobilised IGFBP-3. Subsequently, the peroxidase catalyzes an enzymatic reaction resulting in a blue coloration. The intensity of the blue color depends on the IGFBP-3 content of the sample. The reaction is stopped by the addition of stop solution and color intensity is quantified by measuring the absorption. Sample Types: serum, plasma. Range: 0.03 - 15150 ng/mL. Sensitivity: 0.03 ng/mL. Time: 2.5 hours ? Insulin-like growth factors (IGF)-I and -II are bound to specific binding proteins (IGFBPs) in the circulation. To date, at least six binding proteins can be distinguished on the basis of their amino acid sequence. They are designated as IGFBP-1, IGFBP-2 to IGPBP-7 (1-2). The predominating IGFBP in blood is IGFBP-3, which largely determines the total IGF-I and IGF-II concentration. In contrast to the other binding proteins, IGFBP-3 has the property to associate with an acid-labile non-binding subunit (ALS) after binding of either IGF-I or IGF-II (3-5). Most of the IGFBP-3 in plasma is present as the high molecular weight ternary complex, however, small amounts of free IGFBP-3 are also found (6,7).