I am setting up a sandwich ELISA assay with human serum. My standard is purified His-tagged protein. I am supposed to check the level of my protein of interest in human serum. I found that my blank has higher absorbance value than my human serum. I am suspecting thaat there may be interfering antobodies in the serum so I diluted with blocking buffer by two folds. I still get the same result. Does this mean that the human serum does not contain my protein of interest or the antibodies cannot detect the protein? How can I strengthen my experimental design? Thanks.