What are Cysts(Stool)? Stool examination (see the images below) for trophozoites or cysts is the traditional method for diagnosing giardiasis. At least 3 stools taken at 2-day intervals should be examined for ova and parasites. Trophozoites may be found in fresh, watery stools but disintegrate rapidly. If the stool is not fresh or is semiformed to be formed, trophozoites will not be found.

Cysts are passed in soft and formed stools. The fresh stool can be mixed with an iodine solution of methylene blue and examined for cysts on a wet mount. If not immediately examined, the stool should be preserved in polyvinyl, alcohol, or 10% formalin, with subsequent trichrome or iron hematoxylin staining. Cyst passage is extremely variable, not related to clinical symptoms, and may lag behind the onset of symptoms by a week or more.

Cysts are smooth-walled and oval, measuring 8-12 ?m long and 7-10 ?m wide. Iodine stains the cyst's brown and accentuates their intracystic structures, especially their curved median bodies, axonemes, and nuclei. By focusing through the plane of the sample, 4 nuclei may be visualized, representing 2 daughter trophozoites.

Trophozoites are leaf-shaped, measuring 9-21 ?m long and 5-15 ?m wide. Stained organisms have a characteristic facelike image with 2 nuclei and 4 pairs of flagella. [26]

Because many antibiotics, enemas, laxatives, and barium studies mask or cause the disappearance of parasites from the stools, the microscopic examination should be postponed for 5-10 days following these interventions.
Fecal leukocytes should not be visualized in stool samples of patients with giardiasis.