Presence of fecal leukocytes 20 or more and RBC per high power field of stool microscopy suggests invasive diarrhea (possibly shigellosis).  Stool microscopy also reveals parasitic and helminthic infections.  Dark field microscopy is a rapid diagnostic test for Vibrio.  Stool cultures are available for detecting bacteria including Vibrio cholera, Salmonella, Shigella, and others.  Enzyme linked immunosorbent assay (ELISA) is done to detect rotavirus.  Antigen tests are done for different serotypes of E. coli.