Trophozoites may be found in fresh, watery stools but disintegrate rapidly. If the stool is not fresh or is semiformed to formed, trophozoites will not be found. Cysts are passed in soft and formed stools. If not immediately examined, stool should be preserved in polyvinyl, alcohol, or a formalin preparation. Cyst passage is extremely variable, not related to clinical symptoms, and may lag behind the onset of symptoms by a week or more.
Trichrome stain is useful for identification of the cysts and trophozoites.
Iodine will stain the cysts brown and accentuate their intracystic structures, especially their curved median bodies, axonemes, and nuclei. Stained organisms have a characteristic facelike image with 2 nuclei and transversely placed median bodies. Since many antibiotics, enemas, laxatives, and barium studies mask or cause the disappearance of parasites from the stools, microscopic examination should be postponed for 5-10 days following these interventions. Fecal leukocytes should not be visualized in stool samples of giardiasis patients. Stool antigen detection Several commercially available tests to detect Giardia antigen in the stool exist.