Methods for detecting in a sample the presence of an antibody to a conjugate of an antigen associated with a first carrier by a first association are disclosed. The method comprises contacting a conjugate of the antigen associated with a second carrier by a second association with said sample under conditions that allow binding of the antibody to the antigen; and introducing an agent to detect the presence of the antibody bound to said antigen. The first association and the second association are covalent associations and the first association is different from the second association. Also provided are kits, multiwell plates and conjugates that are useful in the method and further uses of the method. Also provided is a method of releasing a batch of a vaccine comprising a conjugate of an antigen associated with a first carrier by a first association and antibodies useful in this method.