Enzyme-Linked Immunosorbent Assay, or ELISA, is a method used to measure reproductive hormones (eg. proteins or steroids) in blood samples. The technique uses antibodies that are linked to an enzyme. The antibodies specifically bind to the hormone in the blood sample. At the end of the reaction, a chemical is added and the enzyme causes a change in color (from clear to blue). The amount of hormone in the sample is directly proportional to the change in color.