A farmer presenting with fever off-and on for the past 4 years was diagnosed to be suffering from chronic brucellosis. All of the following serological tests would be helpful in the diagnosis at this state except:

Standard agglutination is mainly useful for identification of IgM antibodies. But in chronic brucellosis (and other chronic infections), the IgM levels will be very low. IgG will be predominant. Hence it is not useful in diagnosing chronic brucellosis. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition