An immunoassay (IA) is a biochemical test that measures the presence or concentration of a macromolecule or a small molecule in a solution through the use of an antibody (usually) or an antigen (sometimes).

Analytes in biological liquids such as serum or urine are frequently measured using immunoassays for medical and research purposes.

Yalow accepted the Nobel Prize for her work in immunoassays in 1977, becoming the second American woman to have won the award.

[2] Immunoassays became considerably simpler to perform and more popular when techniques for chemically linked enzymes to antibodies were demonstrated in the late 1960s.

[3] In 1983, Professor Anthony Campbell[4] at Cardiff University replaced radioactive iodine used in immunoassay with an acridinium ester that makes its own light: chemiluminescence.

This type of immunoassay is now used in around 100 million clinical tests every year worldwide, enabling clinicians to measure a wide range of proteins, pathogens and other molecules in blood samples.

[5] By 2012, the commercial immunoassay industry earned US$17,000,000,000 and was thought to have prospects of slow annual growth in the 2 to 3 percent range.

Enzymes used in ELISAs include horseradish peroxidase (HRP), alkaline phosphatase (AP) or glucose oxidase.

RIAs were some of the earliest immunoassays developed, but have fallen out of favor largely due to the difficulty and potential dangers presented by working with radioactivity.

[16] Another demonstrated labeless immunoassay involves measuring the change in resistance on an electrode as antigens bind to it.

Immunoassays can measure levels of CK-MB to assess heart disease, insulin to assess hypoglycemia, prostate-specific antigen to detect prostate cancer, and some are also used for the detection and/or quantitative measurement of some pharmaceutical compounds (see Enzyme multiplied immunoassay technique for more details).

[24] Immunoassays are used in sports anti-doping laboratories to test athletes' blood samples for prohibited recombinant human growth hormone (rhGH, rGH, hGH, GH).

[25] The photoacoustic immunoassay measures low-frequency acoustic signals generated by metal nanoparticle tags.

Illuminated by a modulated light at a plasmon resonance wavelength, the nanoparticles generate strong acoustic signal, which can be measured using a microphone.