Fluorescence polarization assay for the diagnosis of brucellosis: a review

Fluorescence polarization assay (FPA) is based on the rotational differences between a small soluble antigen molecule in solution (labelled with a fluorochrome) and the antigen molecule complexed with its antibody. A small molecule will rotate randomly at a rapid rate, resulting in rapid depolarization of light, while a larger complex molecule will rotate slower and depolarize light at a reduced rate. The rate change in depolarization can be measured. The FPA is a homogeneous assay which does not require removal of unreacted reagents and can, therefore, be performed very quickly and, given portable equipment, in the laboratory and in the field. The latter obviates the need for shipping samples and eliminates waiting for results, as well as reducing test costs. The FPA technology has been developed and validated for the serological diagnosis of brucellosis in cattle, swine, sheep, goats, bison, and cervids. Sufficient cross reactivity of the common epitopes of Brucella abortus, B. melitensis and B. suis O-polysaccharide (OPS) allowed for the use of a single antigen for all species of smooth Brucella and animals. The OPS prepared from B. abortus S1119.3 was conjugated with fluorscein isothiocyanate (FITC). The FPA was initially developed for testing serum; however, the technology has been extended to testing whole blood and milk from individual animals or bulk tank samples pooled from 2000 or fewer animals. The accuracy of the FPA equalled or exceeded those obtained using other serological tests such as the buffered antigen plate agglutination test (BPAT), the milk ring test (MRT), the complement fixation test (CFT), the indirect enzyme immunoassay (IELISA), and the competitive enzyme immunoassay (CELISA).