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Stool Analysis for Giardiasis

A stool sample is collected and analyzed for the presence of the parasiteG. lamblia or its cysts. 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 directly 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 show that you may have giardiasis. A stool analysis also may be able to find other parasites as well as Giardia lamblia and may be helpful when the diagnosis is unclear.


If a giardia infection is present, the parasite or its cysts can be seen when the stool is looked at under a microscope. Evaluating 3 stool samples detects up to 90% of Giardia lamblia infections.1

If giardiasis is suspected, an antigen test 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 collected by endoscopy. In rare cases, a string test may also be done to look for the parasite.

What To Think About

People may have symptoms before the parasite shows up in their stool. So a single stool sample taken when symptoms first appear often does not contain any parasites. For an accurate diagnosis, most doctors recommend analyzing at least 3 samples, collected on alternate days.

To reduce costs, you may want 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), no more tests are 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, 3 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. More parasites seem to be passed in loose stools.

Substances that may interfere with test results include:

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

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

Complete the medical test information form (PDF)(What is a PDF document?) to help you prepare for this test.

Related Information


  1. Hill DR, Nash TE (2010). Giardia lamblia. In GL Mandell et al., eds., Mandell, Douglas, and Bennett's Principles and Practice of Infectious Disease, 7th ed., vol. 2, pp. 3527-3534. Philadelphia: Churchill Livingstone Elsevier.

ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerW. David Colby IV, MSc, MD, FRCPC - Infectious Disease

Current as ofNovember 14, 2014

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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