In one aspect, a homogenous assay method for determination of blood levels of legumain molecules (an asparaginyl endopeptidase) is disclosed. In one aspect, the assay utilizes specific sizes of nanoparticles that are coated with antibody or antibodies specifically towards legumain molecule or its fragment(s). In one aspect, the assay is designed in a homogenous manner with a dynamic range from 0.2 to 160 ng/mL. In another aspect, disclosed herein is a kit for assaying blood levels of legumain comprising two parts of reagents (R1 and R2), which kit is adaptable to be used on clinical chemistry analyzers. In one aspect, the cut-off values for differentiating normal from high risk of cancers such as breast cancer, colorectal cancer and stomach cancer are 18±3 ng/mL.