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Ova and Parasites (O & P)

Testing for stool pathogens has been addressed extensively in the literature for the last several years, because of the low yield and high cost of stool cultures and ova & parasite (O&P) exams. Therefore routine O&P exams should be converted to a Giardia antigen screen. The sensitivity and specificity of this test are 98 and 99%, respectively. Although 3 specimens are recommended for microscopic exams, one specimen is sufficient for Giardia antigen testing due to the test’s excellent sensitivity. Fewer specimens per patient result in significant cost savings to the patient.

More effective strategies may also be useful in testing inpatients for stool pathogens. It is now widely recognized that the vast majority of diarrhea that develops after 3 to 4 days of hospitalization is not due to community acquired bacterial pathogens or parasites. A significant number of those patients test positive for Clostridium difficile toxin. Stool specimens submitted for O&P and routine culture from inpatients hospitalized > 3 days should be initially tested for C. difficile toxin. The specimen is then held for 7 days and additional testing is performed if the requesting physician informs Microbiology that special circumstances exist.

Microscopic examinations are still available if circumstances warrant a more comprehensive evaluation, and should be ordered as a “microscopic O&P”. Indications for microscopic O&P evaluation include:

  • History of travel outside the U.S.
  • Immunocompromised patients
  • Negative Giardia antigen & stool culture with persistent symptoms.

Stool specimens are grossly examined for adult worms. Microscopic examination of saline and iodine wet preps is performed for protozoan cysts, trophozoites, and helminth eggs.

Trichrome stained smears are used to detect trophozoites. The varying, cyclic production of diagnostic stages of parasites and the inherent heterogeneity of fecal content necessitate the need for multiple specimens. Therefore, a minimum of three stools should be collected during a week's time and submitted for examination. In hospitalized patients, single stools on three consecutive days can be submitted to avoid lengthening the hospital stay. All parasites seen will be identified.

Ova and parasites

Several substances will cause fecal specimens to be unsatisfactory for microscopic examination. Water and urine contaminations destroy fragile trophozoites. Barium, bismuth, anti-diarrheal compounds, and antibiotics may decrease the number of organisms present or interfere with their visualization. Dirt or lake water may sometimes contaminate outpatient specimens and introduce free-living soil and water parasites.

Delay in delivery of outpatient samples is also harmful. If delay is unavoidable, the patient should be given a vial of polyvinyl alcohol (PVA) fixative for each specimen to be collected. One part of feces should be mixed well with three parts of PVA fixative, capped tightly, and delivered to the laboratory, along with a portion of unpreserved feces in a separate, sealed container.

Reference values are:

  • Negative Giardia antigen
  • Negative C. difficile toxin
  • No ova or parasites seen.

Specimen requirement is a random stool specimen in a screw capped container. If the specimen cannot be immediately delivered to the laboratory it should be preserved with polyvinyl alcohol and formalin.

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