Giardia Antigen, Feces
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Giardia lamblia (also known as G duodenalis, G intestinalis) are flagellated protozoa that can be found in contaminated natural streams, lakes, and surface water municipal reservoirs. Humans become infected when ingesting the environmentally resistant cysts in water, food, and by the fecal-oral route.
G lamblia primarily infects 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 that is reported to the CDC and is a common cause of diarrhea in children (especially in day care centers), travelers, and campers. It is also responsible for waterborne epidemics. Although G lamblia may be detected using the microscopy-based stool parasitic exam (OAP / Parasitic Examination), up to 7 specimens may be necessary for optimal sensitivity. Instead, this test detects antigen using an enzyme-linked immunosorbent assay (ELISA), which 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 lamblia 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.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Small numbers of organisms residing only in the duodenum may not yield a positive test result.
Giardia lamblia 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 enzyme-linked immunosorbent assay (ELISA) is recommended before considering the results negative.
Stool containing large amounts of leukocytes or red blood cells may give falsely positive results.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
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