The hook effect or the prozone effect is a false negative result with certain immunoassays due to very high concentrations of a particular analyte. The hook effect mostly affects one-step immunometric assays.
In an agglutination test, a person's serum (which contains antibodies) is added to a test tube, which contains a particular antigen. If the antibodies agglutinate with the antigen to form immune complexes, then the test is interpreted as positive. However, if too many antibodies are present that can bind to the antigen, then the antigenic sites are coated by antibodies, and few or no antibodies bind more than one antigenic particle. Since the antibodies do not bridge between antigens, no agglutination occurs. Because no agglutination occurs, the test is interpreted as negative. In this case, the result is a false negative. The zone of relatively high antibody concentrations within which no reaction occurs is called the prozone or the prezone.
Examples include high levels of syphilis antibodies in HIV patients or high levels of cryptococcal antigen leading to false negative tests in undiluted samples. This phenomenon is also seen in serological tests for Brucellosis. The serological test is mainly seen in the precipitation reaction. The antibody that fails to react is known as the blocking antibody which does not allow the precipitating antibody to bind to the antigens for proper precipitation reaction to take place; but when diluting the serum, the blocking antibody are also diluted and its concentration decreased for proper precipitation reaction to occur
- Jurado RL, Campbell J, Martin PD (November 1993). "Prozone phenomenon in secondary syphilis. Has its time arrived?". Arch. Intern. Med. 153 (21): 2496–8. doi:10.1001/archinte.153.21.2496. PMID 7832818.
- Stamm AM, Polt SS (September 1980). "False-negative cryptococcal antigen test". JAMA 244 (12): 1359. doi:10.1001/jama.244.12.1359. PMID 6997519.
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