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A 22-year-old man is evaluated for diarrhea and weight loss. The patient has a 3-week history of foul-smelling, large-volume, watery stools associated with abdominal bloating. There is no visible blood or mucus in the stools. He reports a 4.5-kg (10-lb) weight loss, which he attributes to a poor appetite. One month before symptoms developed, he took a 2-week hiking trip along the Appalachian Trail, where he slept in primitive camp sites without running water. His girlfriend, who accompanied him on the trip, is well.

On physical examination, the patient appears thin but is in no acute distress. Vital signs are normal. Abdominal examination discloses high-pitched, increased bowel sounds and diffuse tenderness to palpation, without peritoneal signs.

Results of a stool examination are negative for occult blood and fecal leukocytes.

Which of the following stool studies is most likely to be diagnostic?

A: Stool assay for Giardia antigen
B: Stool culture for bacteria
C: Stool examination for ova and parasites
D: Stool sample for modified acid-fast staining for Cryptosporidium

MKSAP Answer and Critique

The correct answer is A: Stool assay for Giardia antigen.

The study most likely to establish a diagnosis is a stool assay for Giardia antigen. This patient’s clinical presentation of a prolonged gastrointestinal illness characterized by watery diarrhea and weight loss and his history of camping in the preceding month are strongly suggestive of infection with Giardia lamblia. This parasitic infection is typically transmitted by ingestion of Giardia cysts found in natural bodies of water. Giardiasis can be prevented by boiling or filtering water or by iodine treatment. Most infections are asymptomatic. Symptomatic infection often involves the small bowel and is characterized by large-volume liquid stools and bloating or belching. Although fever is rare, weight loss due to anorexia and malabsorption is an almost universal finding. A monoclonal antibody assay that detects Giardia antigen directly on a stool sample is recommended for diagnosis.

Stool cultures for bacteria have a low yield in diagnosing a diarrheal illness lasting more than 7 to 10 days and, in addition, would not be useful in this patient who has a protozoal infection.

Although giardiasis may be diagnosed by microscopic examination for ova and parasites identifying Giardia trophozoites or cysts, the intermittent shedding of these organisms makes this a less sensitive test than the Giardia antigen assay. In situations in which antigen testing is not readily available, examination of three stool specimens for ova and parasites may be diagnostic.

Modified acid-fast staining of a stool sample is needed to visualize Cryptosporidium, Isospora, and Cyclospora organisms but is not indicated for the diagnosis of Giardia.

Key Point

• The most sensitive test for diagnosing giardiasis is a stool assay for Giardia antigen.

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