In addition to bacterial isolation, serological tests can be performed. CDC utilizes a test called the Brucella microagglutination test (BMAT), a modified version of the serum (tube) agglutination test (SAT), that can detect antibodies to Brucella species – abortus, melitensis or suis. There is no serological test available to detect antibodies to B. canis.

For a diagnosis to be made using serology, two serum samples are required. The first serum sample should be taken when a person is acutely ill (≤7 days after symptom onset); the second serum sample should be drawn 2-4 weeks later to check for a rise in antibodies (a fourfold or greater rise in antibodies would mean an individual is positive for brucellosis).

If submission of paired sera is not possible, a probable diagnosis can be made with a single serum sample.

Serology is not currently available to monitor persons for RB51 vaccine exposure or for Brucella canis exposure.

Serological Monitoring After Laboratory Exposure

Serology for laboratory workers exposed to Brucella are drawn at 0, 6, 12, 18 and 24 weeks post exposure.

The immune response to Brucella is characterized by an initial production of IgM antibodies followed afterward by the production of IgG antibodies. The major antigens that are useful for diagnosis of brucellosis are the smooth (S) lipopolysaccharide (LPS) of the outer membrane and internal proteins.

The serum (tube) agglutination test (SAT) detects antibodies to the S-LPS. Antibodies reacting against S-LPS can also be detected by other tests, such as ELISA (enzyme-linked immunosorbent assay) and the Coombs test. It is important to note that the Coombs test remains positive longer than other agglutination tests.

CDC recommends that Brucella serology testing only be performed using tests approved by the Food and Drug Administration (FDA), or validated under the Clinical Laboratory Improvement Amendments (CLIA) and shown to reliably detect the presence of Brucella antibodies. Results from these tests should be considered supportive evidence for recent infection only and interpreted in the context of a clinically compatible illness and exposure history.