For the development of sandwich ELISAs to measure natural and recombinant human Fibroblast Growth Factor 23 (FGF-23). The Reagent Diluent recommended may be suitable for most cell culture supernate, serum, and plasma samples. The Reagent Diluent selected for use can alter the performance of an immunoassay. Reagent Diluent optimization for samples with complex matrices such as serum and plasma, may improve their performance in this assay. This kit contains sufficient materials to run ELISAs on at least five 96 well plates, provided the following conditions are met: - The reagents are prepared as described in this package insert. - The assay is run as described in the General ELISA Protocol. - The recommended microplates, buffers, diluents, substrates, and solutions are used. This package insert must be read in its entirety before using this product.