Stool analysis for giardiasis (also known as Ova and parasite test)

Exam Overview

A sample of stool is collected and analyzed for the presence of cysts or the parasite G. lamblia. The stool sample may be collected at home, in the doctor's office, or in the hospital.

If the stool is collected at home, it may help to have a bowel movement into a plastic bag taped to the toilet seat or into a plastic "hat" provided by your doctor. It is very important to wash your hands after collecting the sample to avoid spreading a possible infection to others.

Why It Is Done

A stool analysis is done if the medical history and symptoms indicate that you may have giardiasis. Stool analysis has the advantage of looking for other parasites as well as Giardia lamblia and may be helpful when the diagnosis is unclear.

Results

If a giardia infection is present, the parasite or cysts can be seen when the stool is examined under a microscope. Evaluating three stool samples may detect up to 90% of Giardia lamblia infections.¹

If giardiasis is suspected, antigen tests may be done on the stool or a sample of the fluid from the small intestine (duodenal contents). Fluid from the small intestine may be sampled by endoscopy. In rare cases, a string test may also be done to look for the parasite.

What To Think About

People may develop symptoms before the parasite appears in their stool, so analysis of a single stool sample taken when symptoms first appear may be negative. For an accurate diagnosis, most doctors recommend analyzing three samples, collected on alternate days.

To reduce costs, it may be appropriate to wait for the results of the first analysis before doing a second or third test. If Giardia lamblia is found in the first test (positive result), additional tests are not needed.

People with chronic giardiasis tend to pass large numbers of the parasite in their stools at about 2-week intervals. If chronic giardiasis is suspected, three stool samples may be collected 4 to 7 days apart to maximize the chances of finding the parasite. Stools may also be collected when the person is passing loose, rather than formed, stools since more parasites seem to be passed in loose stools.

Substances that may interfere with test results include:

• Antacids and antidiarrheals.
• Antibiotics.
• Antiparasite drugs.
• Enemas or laxatives.
• Barium (a contrast material used for X-rays).

If possible, these products should be avoided for 2 weeks before collecting stool samples.

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Author:	Colleen Cronin	Last Updated October 19, 2005
Medical Review:	Adam Husney, MD - Family Medicine W. David Colby, MSc, MD, FRCPC - Infectious Disease
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