Hi peter, yes it is possible to do IgA ELISA on stool samples.Secretory IgA (sIgA) ELISA (Saliva, Stool) manual by IBL international and a paper by Peters et al 2004. Measurement of Immunoglobulin Concentrations in the Feces of Healthy Dogs. Clinical and diagnostic laboratory immunology p. 841–848 will be helpful. see links