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Giardia lamblia (also known as Giardia duodenalis, Giardia intestinalis) are flagellated protozoa that can be found contaminating natural streams, lakes, and surface water municipal reservoirs. The human host ingests them in water, food, and by the fecal-oral route.
Giardia infect primarily the small intestine causing malodorous watery diarrhea and flatulence after attaching by their ventral sucker. Malabsorption and lactose intolerance may also occur.
Giardiasis is the most common intestinal parasitic infection in the United States and is a common cause of diarrhea in children (especially in day care centers), travelers, and in waterborne epidermics. Although Giardia may be detected using the microscopy-based stool parasitic exam (“ova and parasite exam”), up to 7 specimens may be necessary for optimal sensitivity. Instead, antigen detection using an enzyme-linked immunosorbent assay (ELISA) is a more sensitive method for detection and is therefore a preferred method for detection.
See Parasitic Investigation of Stool Specimens Algorithm and Laboratory Testing for Infectious Causes of Diarrhea in Special Instructions for other diagnostic tests that may be of value in evaluating patients with diarrhea.
Sensitive screening for the detection of Giardia antigens present in stool specimens.
A positive enzyme-linked immunosorbent assay (ELISA) indicates the presence in a stool specimen of Giardia lamblia antigens.
As per the manufacturer, the assay has a sensitivity of 96%, specificity of 97%, and a positive predictive value of 95%.
Interpretation of results should be correlated with patient symptoms and clinical picture.
Small numbers of Giardia residing only in the duodenum may not yield a positive enzyme-linked immunosorbent assay (ELISA).
Giardia antigen detection should be used as an aid in diagnosis of giardiasis. A single diagnostic assay should not be used as the only criteria to form a clinical conclusion.
Testing of at least 2 consecutive stool specimens by ELISA is recommended before considering the results negative.
Stool containing large amounts of leukocytes or red blood cells may give falsely positive results.
1. Janoff EN, Craft JC, Pickering LK, et al: Diagnosis of Giardia lamblia infections by detection of parasite-specific antigens. J Clin Microbiol 1989;27:431-435
2. Hanson KL, Cartwright CP. Use of an enzyme immunoassay does not eliminate the need to analyze multiple stool specimens for sensitive detection of Giardia lamblia. J Clin Microbiol 2001;39(2):474-477