Methods and devices for reducing interference from leukocytes in an analyte immunoassay are provided. In one embodiment, a method is provided comprising the steps of amending a biological sample such as a whole blood sample with one or more leukocidal reagents that reduce or eliminate the metabolic activity of leukocytes, and performing an immunoassay on the amended sample to determine the concentration of analyte in the sample. Preferably, the sample is amended with one or more enzymes and optionally one or more enzyme substrates and cofactors.