Which tests are performed in the workup of giardiasis?

Updated: Oct 01, 2018
  • Author: Hisham Nazer, MBBCh, FRCP, DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
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The traditional basis of diagnosis is identification of Giardia intestinalis trophozoites or cysts in the stool of infected patients via a stool ova and parasite (O&P) examination. However, in more recent years, the advent of more objective techniques (eg, immnoassays, nucleic acid amplification techniques [NAATs]) has led to an increase in their use versus those that rely on subjective microscopic examination of fecal specimens for Giardia cysts. [55]

Stool examination may be performed on fresh specimens or after preservation with polyvinyl alcohol or 10% formalin (with appropriate staining). Ideally, 3 specimens from different days should be examined because of potential variations in fecal excretion of cysts. G intestinalis is identified in 50-70% of patients after a single stool examination and in more than 90% after 3 stool examinations.

Stool O&P testing aids in the diagnosis of giardiasis in 80-85% of patients. It remains the diagnostic method with which other tests are compared. Aspiration of duodenal contents and demonstration of trophozoites also have been used for diagnosis but this is more invasive than stool examination and, in direct comparison studies to stool microscopy, may have a lower diagnostic yield.

Stool antigen enzyme-linked immunosorbent assays also are available. [56] These tests are similar to the stool O&P test in terms of cost and have a sensitivity of 88-98% and a specificity of 87-100%. These tests are best used as a screening test in high-incidence settings such as day-care centers or for identification of subjects during an epidemic, but they should not take the place of stool microscopy.

If the results from 3 O&P tests are negative and giardiasis is still suspected, stool antigen enzyme-linked immunosorbent assay (ELISA) may be helpful. If both of these methods result in negative findings but the patient has symptoms consistent with small bowel diarrhea/malabsorption, upper endoscopy with biopsies and duodenal aspirate is a reasonable alternative.

Stool culture is not routinely used because of the difficulty of reproducibly isolating Giardia from patient fecal samples. However, stool cultures are beneficial in ruling out other pathogens as the cause of a patient's symptoms.

Routine laboratory tests (eg, CBC count, electrolyte levels) usually show normal results. Eosinophilia is an uncommon feature of infection.

Because immunoglobulin G (IgG) levels remain elevated for long periods, they are not beneficial in making the diagnosis of acute giardiasis. Serum anti-Giardia immunoglobulin M (IgM) can be beneficial in distinguishing between acute infections and past infections.

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