Interpreting ELISA Data and Establishing the Positive-Negative Threshold. Chester L. Sutula, Agdia, Inc., Mishawaka, IN. Jerri M. Gillett, Susan M. Morrissey, and Donald C. Ramsdell, Michigan State University, East Lansing. Plant Dis. 70:722. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-722.