What is the role of stool exam in the diagnosis of giardiasis?

Updated: Oct 01, 2018
  • Author: Hisham Nazer, MBBCh, FRCP, DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
  • Print

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 formed, trophozoites will not be found.

Cysts are passed in soft and formed stools. Fresh stool can be mixed with an iodine solution or methylene blue and examined for cysts on a wet mount. If not immediately examined, 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 μm long and 7-10 μm wide. Iodine stains the cysts 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 μm long and 5-15 μm 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, 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.

Giardia intestinalis trophozoites on stool examina Giardia intestinalis trophozoites on stool examination from a patient with diarrhea.
Giardia cyst. Giardia cyst.
Giardia trophozoite Giardia trophozoite

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!