Giardiasis Diagnosis

There is no current gold standard diagnostic test for detecting giardiasis.¹

The traditional diagnosis for giardiasis consists of performing an ova and parasite (O+P) exam of one to three stool specimens on non-consecutive days² Several days of specimen collection are needed to improve sensitivity. Stool specimens may be preserved by fomalin or zinc sulfate concentration methods.³ Approximately 85%-90% of cases are detected when three separate stool samples are examined. However, in clinical practice, typically only a single stool exam is performed. The passage of cysts is often sporadic so many cases (>50%) of giardiasis will be missed with a single O+P examination, resulting in underdiagnosis. Stool microscopy is relatively inexpensive, but it does require a skilled technician and may be a time consuming process.⁴

The introduction of Giardia stool antigen tests has improved giardiasis diagnostic capabilities. Immunodiagnostic assays are available for the detection of Giardia and are both more sensitive and more specific (>90%) than the traditional ova and parasite (O+P) examinations.^2,4 A single antigen test is able to detect 50% more infections than O+P examinations. Antigen tests also require less time than an O+P exam and can be combined with antigen testing for diagnosing Cryptosporidium, making the antigen test more cost-effective. Giardia specific antigen may be detected in stool specimens even during an absence of cyst passage or visible signs of trophozoites. Giardia specific antigen in stool specimens are detected by several different methods: EIA, indirect and direct immunofluorescent assays using monoclonal antibodies and direct fluorescent assays. Antigen tests must be specifically requested by the healthcare provider. They are available from several different commercial sources, including Meridian, Alexon-Trend, Tech-lab, and Biosite.

Some patients repeatedly return a negative stool exam despite a high suspicion for being infected with giardiasis. Alternative assays include duodenal aspirate biopsy and collection of duodenal fluid with the string test. The least invasive method of the three is the string test, otherwise known as the Enterotest^®. This test requires the patient to swallow a gelatin capsule containing a string. The proximal end of the string is taped to the patient's cheek and the distal end in the capsule moves to the duodenum after the capsule dissolves in the stomach.⁴ Several (4-6) hours later, the string is removed and microscopically examined for trophozoites. A duodenal aspirate biopsy, or needle aspiration, is more invasive and requires an endoscopy. These alternative diagnostic methods should only be used when stool examination is repeatedly negative and there is a high clinical suspicion of infection.⁵