The reference range for stool ova and parasite test is negative (no parasites seen).
The fecal ova and parasite test includes evaluation for cysts and trophozoites of intestinal protozoa and larvae, eggs, and adults of intestinal helminths.
A negative result does not rule out parasitic infection. Some parasites (particularly Giardia lamblia and Strongyloides stercoralis) are shed intermittently, [1, 2] and the probability of parasite detection increases from approximately 60% with a single specimen to more than 95% when 3 specimens are examined. 
When intestinal helminths, amoeba, flagellates, or ciliates are found, the report should indicate genus, species, and stage.
Results are qualitative for most parasites due to the intermittent nature of their presence in fecal samples, and significance regardless of quantity. However, Blastocystis hominis may be reported in a semi-quantitative manner (rare, few, moderate, many). Whether this organism should be considered a pathogen is currently under debate, and an indication exists that B hominis burden and clinical symptoms may be related. 
Pathogenic Entamoeba histolytica cannot be differentiated on the basis of microscopic morphology from nonpathogenic E dispar, unless E histolytica with ingested RBCs is observed. Otherwise, a report of E histolytica/E dispar will be issued. However, immunoassays exist that can differentiate these 2 species on fresh (nonpreserved) fecal samples.
The presence of nonpathogenic amoeba such as Entamoeba coli, E hartmanni, E polecki, Endolimax nana, and Iodamoeba buetschlii suggests exposure to fecally contaminated food or water sources. 
The ova and parasite examination does not routinely detect coccidia (Cryptosporidium or Cyclospora) or microsporidia. See Considerations for appropriate test methodologies to detect these parasites.
Collection and Panels
Stool samples should be collected into a clean dry container with a tightly fitting lid, taking care not to contaminate with urine or water to avoid lysis of trophozoites or introduction of free-living environmental organisms. 
Specimens should not be collected within 7 days of administration of barium, mineral oil, bismuth, antibiotics, antimalarial agents, and nonabsorbable antidiarrheal preparations because these substances can all interfere with the detection of parasites. [1, 4]
Ideally, a series of 3 samples should be submitted, with collection every other day because of the intermittent nature of parasite shedding. Samples are not routinely accepted on patients who develop diarrhea after 3 or more days of hospitalization because parasites are rarely a cause of nosocomial diarrhea.  The most likely etiologic agent implicated in such cases is Clostridium difficile. 
Freshly collected stool samples should be transported to the lab immediately. Liquid samples should be examined within 30 minutes, and all other samples within 1 hour of collection.  Otherwise, specimens should be transferred to preservative to maintain parasite morphology. A number of commercial preservatives are available. One common collection kit contains a vial of 5% or 10% formalin and a vial of polyvinyl alcohol (PVA). One part stool should be mixed well with 3 parts preservative. If collection kits are given to outpatients, they should include safety information about the chemicals being used and proper handling instructions.
Parasites causing intestinal infections in humans include protozoa and helminths. Examples of intestinal protozoa include flagellates (G lamblia), amoeba (E histolytica), sporozoans (Cryptosporidium spp), and ciliates (Balantidium coli). Helminths include nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes).
The ova and parasite examination includes the following components:
Macroscopic examination: This includes gross examination to determine fecal consistency, abnormalities (blood, excessive mucus), and the presence of larval or adult worms or proglottids. If the specimen is being transferred to preservative before delivery to lab, the collector can note consistency, as this will be difficult to ascertain once mixed with preservative solution.
Microscopic examination of fresh specimen: This includes examination for protozoal trophozoites and cysts and helminth larvae and eggs. When performed on unfixed specimens, this allows for determination of trophozoite motility.
Microscopic examination of concentrated specimen: Concentration by sedimentation or flotation aids detection of small numbers of organisms by removing background debris and concentrating parasites. Although trophozoites may be destroyed by the procedure, it increases sensitivity for cysts and helminth eggs and larvae.
Microscopic examination of permanent stained smears: Wheatley’s trichrome or iron hematoxylin staining aid in identification of protozoan cysts and trophozoites, as well as providing a permanent and reviewable record of findings.
The etiology of most infectious diarrhea cases is viral or bacterial, rather than parasitic. The Infectious Disease Society of America guidelines suggest evaluating patients with persistent diarrhea (> 7 days duration) for parasites, especially if immunocompromised.  Although G lamblia and E histolytica are the most common intestinal parasites worldwide, pathogen prevalence varies regionally.  Patient travel history, exposures, and immune status should help direct test selection.
The routine ova and parasite examination has very poor sensitivity for 2 of the most common parasites found in the United States, G lamblia (66-79% sensitivity) and Cryptosporidium spp (< 5%). [7, 8, 9] Another method, such as an immunoassay for parasite antigen, should be used for laboratory diagnosis of these organisms.  Many labs offer an “ova and parasite screen” that consists of an immunoassay to detect only these 2 organisms. Some laboratories offer an immunoassay for E histolytica, but this test must be performed on unpreserved sample.
The routine ova and parasite examination does not detect microsporidia or coccidia (Cyclospora cayetanensis, Cryptosporidium spp, Cystoisospora belli). Special stains (modified acid-fast for coccidian and modified trichrome for microsporidia) must be done to detect these organisms, so the physician must indicate to the lab that these pathogens are suspected. 
A recent study from the University of Utah proposed a diagnostic algorithm for evaluating patients with persistent diarrhea or gastrointestinal illness for parasitic infection.  Their recommendations include the following:
Nonimmunocompromised patients with no travel to parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp and G lamblia by immunoassay. If negative and symptoms persist, repeat immunoassay and perform complete ova and parasite examination.
Nonimmunocompromised patients with a travel history to parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. If these evaluations are negative and symptoms persist, repeat immunoassay and perform complete ova and parasite examination. Consider serology testing based on exposure.
Nonimmunocompromised patients hailing from parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. Serology for S stercoralis and other parasites as geographically indicated. Complete ova and parasite examination 3 times. If negative and symptoms persist, repeat immunoassay and perform modified acid fast stain for coccidia.
Immunocompromised patients: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. Perform modified acid fast for coccidia, and special stains for microsporidia, as well as serology for S stercoralis and complete ova and parasite examination 3 times. If negative and symptoms persist, repeat immunoassay and perform modified acid fast stain for coccidia.
If the cause is not found, consider referring to specialist for evaluation of infectious and noninfectious causes.