The invention is directed to methods and devices for reducing interference from heterophile antibodies in an analyte immunoassay. In one embodiment, the invention is to a method comprising the steps of (a) amending a biological sample such as a whole blood sample with non-human IgM or fragments thereof by dissolving into said sample a dry reagent to yield a non-human IgM concentration of at least about 20 μg/mL or equivalent fragment concentration; and (b) performing an electrochemical immunoassay on the amended sample to determine the concentration of said analyte in said sample. Preferably, the sample is amended with IgG or fragments thereof in addition to the IgM of fragments thereof.