[1][2] The serum sample or solution of antibody to be tested is diluted and mixed with a viral suspension.
Depending on the virus, the plaque forming units are measured by microscopic observation, fluorescent antibodies or specific dyes that react with infected cells.
Currently it is considered to be the "gold standard" for detecting and measuring antibodies that can neutralise the viruses that cause many diseases.
[4][5] It has a higher sensitivity than other tests like hemagglutination and many commercial Enzyme immunoassay without compromising specificity.
However, the test is relatively cumbersome and time intensive (taking a few days) relative to EIA kits that give quick results (usually several minutes to a few hours).