This chapter explains how high performance immunometric assays can be designed for low molecular weight analytes. It highlights the fundamental limitations of competitive immunoassays and why it is better to measure the bound antigen rather than the unbound antigen. The anti-complex system is the most similar to conventional immunometric assays, with a detection antibody raised against the combined antigen–capture antibody complex. The selective antibody system is a more complex approach. The principle is that a primary antibody against the small molecular weight analyte and a secondary antibody against this primary antibody are selected such that the secondary antibody can still bind to the primary antibody when it has bound to the small molecule analyte but not when, in the absence of analyte, the primary antibody has bound to a specific blocking substance. However, when an analyte is present and has bound to the primary antibody, binding between primary and secondary antibodies takes place. The universal selective antibody system involves generic anti-immunoglobulin antibody as the selective antibody, speeding up the development of assays for new analytes. Examples of products that use these systems are provided with performance data.

Professor Colin Self has a wide medical and scientific background and has been closely involved with the biotechnology industry for over thirty years. He holds the following qualifications: BSc (Chemistry), PhD (Biochemistry), DSc (awarded for work in Analytical Science), MB, BChir (Cambridge University Medical School) and is a Chartered Chemist, Fellow of the Royal Society of Chemistry and Fellow of the Royal College of Pathologists, Member of the Association of Clinical Biochemists and Fellow of the Royal Society of Medicine. Following his first degree in Chemistry he obtained a PhD in Metabolic Biochemistry and then went to the USA, and subsequently, to a Max Plank Institute in Tübingen to undertake research into immunology. During this period his major interest was in immunogenicity and immunodiagnostics. He returned to the UK to a lectureship during which he wrote an undergraduate text-book on immunology. Following this he studied Medicine at Cambridge University followed by hospital appointments in Gastroenterology, General Surgery, Rheumatology, Respiratory Medicine and Oncology. He was appointed Registrar, and subsequently Senior Registrar, in Chemical Pathology at The Hammersmith Hospital Royal Postgraduate Medical School and then head of the large Department of Clinical Biochemistry of Newcastle University. During his time as a medical student he invented and patented the technology of Enzyme Amplification, allowing the great amplification of signal (and thus speed and sensitivity) from immunodiagnostic and genetic tests. From this technology one of the first profitable Cambridge biotechnology companies, IQ Bio Ltd - of which he was Research Director, grew rapidly while he was a junior doctor. In parallel to this diagnostic work he has focused on ways of making therapeutic antibodies more specific for their intended cancer targets. Through this his group were the first to describe the activation of antibodies by light – a technology that can dramatically potentially increase the functional specificity of antibodies by making them, in essence, regionally specific within the body and thus leaving the rest of the body unharmed.

Immunoassay, non-competitive, immunometric, sandwich assay, reagent-excess, anti-complex, selective antibody system, universal selective antibody system, specificity, sensitivity, precision, antibodies, digoxin, digoxin-like immunoreactive factors, binding sites, masking, lateral flow.