Question
Standard agglutination test for brucellosis.
Answer
The Standard Agglutination Test (SAT) is a tube agglutination test used for the serodiagnosis of brucellosis (Brucella species — B. abortus, B. melitensis, B. suis), detecting agglutinating antibody in patient serum against standardized, killed Brucella antigen.
Principle/procedure: patient serum is serially diluted in a series of tubes and mixed with a standardized suspension of killed, whole Brucella organisms; the tubes are incubated (typically 37°C for a set period, sometimes with an extended incubation to allow full agglutination to develop), and the highest dilution showing visible agglutination is recorded as the antibody titre.
Interpretation: a titre of ≥1:160, in the appropriate clinical context (fever, exposure history to livestock/unpasteurized dairy products), is generally considered significant for active or recent infection; a rising titre between paired acute and convalescent sera provides stronger evidence of current infection than a single titre.
Limitations:
- Prozone phenomenon — in some patients, an excess of antibody at low serum dilutions can paradoxically block visible agglutination, giving a false-negative result at low dilutions while higher dilutions test positive — testing across a range of dilutions is therefore essential.
- Blocking (incomplete/non-agglutinating) antibodies, particularly IgA and some IgG, can develop in chronic brucellosis and interfere with the SAT, causing false-negative or falsely low results — the Coombs’ (antiglobulin) test is used as an adjunct to detect these blocking antibodies in such cases.
- Cross-reactions can occur with other Gram-negative organisms sharing similar surface antigens (e.g., Yersinia enterocolitica O:9, Francisella tularensis, Vibrio cholerae — all share some antigenic similarity with Brucella smooth lipopolysaccharide), potentially causing false-positive results.
Clinical use: remains a widely used, practical serological method for brucellosis diagnosis, given the difficulty and biosafety concerns of culturing the organism directly.