This assay is a competitive ELISA which utilizies a 96-well microtiter plate which was pre-coated with a human RBP4. A purified polyclonal which recognizing native human RBP4 reacts with a series of predetermined recombinant human RBP4 standard proteins or the test samples under competition in the human RBP4-coated plate. Their relative reactivity is plotted with that of the standard proteins. This ELISA is specific for the measurement of natural and recombinant human RBP4. It does not cross-react with mouse RBP4, rat RBP4, human adiponectin, rat adiponectin, human resistin, human vaspin, human clusterin, human leptin, human IL-23, human IL-33, human GPX3, human Nampt, human ANG1, human ANG2, human ANGPTL3, human ANGPTL4, human ANGPTL6, human FABP4, human RELM-beta, rat RELM-alpha, mouse Nampt, human PAI-1. The assay range is 0.001 - 5 µg RBP4/mL and a detection limit of 1 ng/mL (based on adding two standard deviations to the mean value of the (50) zero standards).