The first immunoassay, an assay for human insulin, was described by Rosalyn Yalow and Solomon Berson back in 1959. This innovation has been a fundamental contribution to diagnostics in the areas of medicine, veterinary, agricultural, pharmaceutical and life sciences.
Proven immunoassay technology
The last 50 years has spawned a wide number of immunoassay techniques, one of these, lateral flow immunochromatography has enabled testing to move from the laboratory to the point of contact. Providing results in minutes, these lateral flow rapid tests are straightforward to use and have found wide application.
Lateral flow immunoassays can detect both small and large molecules and can be applied to any protein, hapten, nucleic acid or amplicon. Use of a visual label, such as gold, carbon, or coloured latex nanoparticles, allows for a rapid qualitative test which can be further enhanced using a reader to convert to a fully quantitative readout.
At the heart of the immunoassay is a binding reagent. For this article, we’ll use the term antibody to cover any protein, affirmer, anti-ligand or DNA-based aptamer that might be deployed. The antibody can be tuned to bind the target analyte with exceptional specificity, enabling minute concentrations to be detected in the presence of structurally related molecules.
The flexibility of the immunoassay format has led to a plethora of techniques which is impossible to cover in this article. Here we consider the two main approaches that have had wide commercial success in lateral flow devices:
- Non-Competitive (or Sandwich immunoassay format) - used for large molecular weight analytes with multiple antigenic sites. A positive test is represented by the presence of a coloured line. A negative test is represented by the absence of a line. The best-known examples of this format are the over the counter pregnancy tests. This format will not work for small molecular weight analytes.
- Competitive (or Inhibition immunoassay format) - used for small molecular weight analytes with single antigenic determinants. A positive test is represented by the absence (inhibition) of a coloured line. A negative test is associated with any hint of a line in the test region. Typical examples of this format are the more specialised tests for drugs and toxins. This format can also be applied to large molecular weight analytes: The insulin assay described in the introduction was a competitive technique.
Each format has pros and cons depending on the analyte, the antibody, sample matrix and concentration range of interest. Generally, non-competitive immunoassay has a lower limit of detection (analytical sensitivity) compared to the competitive format. This is typically picogram/mL (parts per trillion) for non-competitive compared to nanogram/mL (parts per billion) for the competitive format. In situations where significantly high analyte concentrations might be encountered, non-competitive formats can suffer from the high-dose hook effect which can yield false results. The competitive format cannot have a high-dose hook effect. With many Man years experience and over 200 contract Research and Development (R&D) projects completed, we are perfectly placed to advise and guide you on the most suitable approach for your test.
The manufacture of these lateral flow immunoassay formats
Abingdon Health routinely produce inhibition and sandwich lateral flow assays across its state-of-the-art UK facilities that boast one of Europe's largest automated rapid test manufacturing capacities. Abingdon Health is ISO 13485 certified and works to GMP.