A stool sample is collected and analyzed for the presence of the
parasite G. 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
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.
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
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:
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.
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.
September 9, 2011
E. Gregory Thompson, MD - Internal Medicine
& W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
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