The accurate and reliable determination of biologically active endogenous compounds in plasma, urine, and other body fluids for scientific or diagnostic purposes has been a challenge for many decades. In the past, the assessment of such compounds was difficult and tedious because specific and practical analytical tools were not available. In the early days, decisive and convincing conclusions about the significance of a biologically active molecule in disease or health could only be made after purification and isolation of the analyte and identification of its chemical structure. A definite improvement in the analysis of biologically active endogenous compounds was the introduction of bioassays using an intact animal model or in vitro tissue preparations. Although the bioassays possessed sufficient SENSITIVITY, there were problems with their lack of SPECIFICITY. Other analytical procedures such as liquid chromatography, electrophoresis, or photometric procedures have also been developed for in vitro diagnosis. However, these approaches are either tedious or time-consuming and require expensive equipment and specially trained personal. A landmark in diagnostics was the introduction of immunoassays, which are inexpensive and easy to perform with high reproducibility, SENSITIVITY, and SPECIFICITY.