Question

How could a sandwich ELISA differentiate between a patient with many antibodies versus few antibodies?

Answer 1

A more precise ELISA to detect antibodies concentration in a sample will be Indirect ELISA where antigens coated on wells bind to specific antibodies, which in turn bind to enzyme-linked antibodies.  

In sandwich ELISA, an antigen from the sample has two antibody binding regions- one for primary antibody coated on well and other for secondary antibody bound to the enzyme. The amount of colour developed in the reaction is used to quantify the amount of antigen. This can be used to quantify the amount of antigens and thus indirectly the amount of antibodies also in the patient. That is if a patient has more antigens, there will be more antibodies and vice versa.