All of the following serological tests would be helpful in diagnosis of chronic brucellosis except

As isolation is difficult, serological tests are of paramount impoance in the diagnosis of brucellosis.several tests have been developed such as standard agglutination test, complement fixation, and ELISA. SAT detects antibodies against antigens of smooth LPS. In acute infection, IgM antibodies appear early followed by IgG and IgA antibodies. The SAT does not differentiate between immunoglobulin isotypes. As the disease progresses IgM levels decline and avidity and subclass distribution of IgG and IgA change. Thus SAT becomes negative in chronic brucellosis. The complement fixation test is more useful in chronic cases as it detects the IgG antibody also. ELISA is sensitive and specific and detects IgM and IgG antibodies separately. It is therefore useful for differentiation between acute and chronic infection. Ref: Textbook of microbiology; Ananthanarayan and paniker's; 10th edition; Pg: 349