I am working on the development of a lateral flow essay which is to test if human serum contains IgGs that bind to Au nanoparticle conjugated antigens. My positive control which is a monobody with a human IgG-Fc domain works fine on the tests. However I can't manage to make it work with human serum samples (The human serums are ELISA verified to be positive for anti-antigen IgGs). When the positive control is run with anti-antigen IgG negative serum instead of PBS the signal intensity also greatly decreases. Tried various antibodies (monoclonal/polyclonal) and treatments. The problem may be caused by the excess amount of IgGs in the human serum but the anti-IgG antibodies on the membrane aren't saturated because running the positive control after the serum and proves a signal. Any suggestions will be greatly appreciated.