Lab result explainedInflammationLast updated November 11, 2026

Ova and parasites test (O&P): what a stool parasite test looks for and what positive results mean

Ova and parasites test (O&P) · Stool test (often ordered with Giardia/Cryptosporidium antigen, stool culture or a GI PCR panel) · reported in Qualitative, by organism: 'no ova or parasites seen', or the organism and stage identified (cysts, trophozoites, oocysts, eggs, larvae), sometimes graded rare, few, moderate or many

An ova and parasites test (O&P) is a microscopic examination of stool for the eggs (ova), larvae, cysts and active forms of intestinal parasites. A technologist concentrates the sample and examines it as a wet mount and on a permanently stained slide, looking for single-celled protozoa such as Giardia, Cryptosporidium and Entamoeba histolytica and for the eggs of worms such as roundworm, hookworm, whipworm and tapeworms. A normal result reads 'no ova or parasites seen'. In the US it is ordered for diarrhea lasting more than a week or two, diarrhea after travel, camping or daycare exposure, unexplained weight loss or bloating, a high eosinophil count, or when a stool culture has come back negative.

Parasites are shed into stool intermittently, so a single sample detects only about 60 to 80 percent of infections. Labs traditionally ask for three samples collected on separate days, ideally two to three days apart, which raises detection above 90 percent. Many US labs now recommend a Giardia and Cryptosporidium antigen test or a PCR panel first, because these two are responsible for most parasitic diarrhea in people who have not traveled and the antigen and PCR tests find them more reliably than microscopy. The full O&P is then reserved for travelers, recent immigrants, people with weakened immunity and anyone with eosinophilia.

The result that most often confuses people is a report of a nonpathogenic organism, such as Entamoeba coli, Endolimax nana or Blastocystis, which the lab lists because it saw it, not because it is making you ill. The other common puzzle is a negative test when symptoms persist, which can reflect intermittent shedding, a parasite that needs a special stain or a different test (Cryptosporidium, Strongyloides, pinworm), or a non-parasitic cause such as post-infectious irritable bowel or lactose intolerance after a Giardia infection.

O&P at a glance

What it is: microscopy of concentrated and stained stool for protozoan cysts and trophozoites and for worm eggs and larvae; results are reported by organism, not as a number.
Normal: 'no ova or parasites seen' on each sample; nonpathogenic commensals (Entamoeba coli, Endolimax nana, Entamoeba hartmanni, Iodamoeba, Chilomastix) may be listed and do not need treatment.
Most common true positives in the US: Giardia duodenalis, Cryptosporidium, Cyclospora (produce outbreaks) and, after travel, Entamoeba histolytica and worm eggs.
Sensitivity: one sample finds roughly 60 to 80 percent of infections, three samples over 90 percent; Giardia/Cryptosporidium antigen tests and PCR panels are more sensitive than microscopy for those two.
Timing: wait 7 to 10 days after barium, bismuth (Pepto-Bismol), mineral oil or antacids and about 2 weeks after antibiotics, which all hide parasites.
Positive result: treatment is specific to the organism (for example, metronidazole, tinidazole or nitazoxanide for Giardia); household contacts and water or food sources are often checked.

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What O&P measures

The sample is first examined as a direct wet mount, in which motile trophozoites and larvae can be seen in fresh liquid stool, and then concentrated by formalin-ethyl acetate sedimentation, which pulls eggs, cysts and larvae out of the debris and lets the technologist screen a larger amount of stool. Iodine staining shows the internal structure of cysts, and the concentrated sediment is where most worm eggs are found and identified by their size, shape and shell. A separate smear is fixed and stained with trichrome or iron hematoxylin; this permanent stain is what allows the small protozoa, especially Dientamoeba fragilis and the amoebae, to be identified with confidence.

Some parasites need extra steps. Cryptosporidium, Cyclospora and Cystoisospora are tiny and nearly invisible on routine stains and are detected with a modified acid-fast stain, a fluorescent antibody stain or an antigen test, which must be requested or is included only in some labs' routine O&P. Strongyloides larvae are found in fewer than a third of single routine samples and are better sought with agar plate culture or a blood antibody test. Pinworm eggs are laid on the skin around the anus rather than passed in stool, so pinworm is diagnosed with a tape test, not an O&P. Microscopy cannot distinguish the dangerous Entamoeba histolytica from the harmless look-alike Entamoeba dispar, so a report of 'E. histolytica/dispar' is followed by an antigen or PCR test.

Stool for O&P is collected into two vials of preservative (traditionally formalin and polyvinyl alcohol, now often a single-vial fixative) or examined fresh within an hour if trophozoites are sought. Several substances make the examination unreliable: barium from imaging, bismuth, mineral oil, kaolin, antacids and antidiarrheals coat or dilute the organisms for 7 to 10 days, and antibiotics suppress protozoa for about 2 weeks. The sample must not be mixed with urine or toilet water. A skilled microscopist takes 10 to 20 minutes per sample, and results take 1 to 3 days per specimen.

Normal range for O&P

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (Qualitative, by organism: 'no ova or parasites seen', or the organism and stage identified (cysts, trophozoites, oocysts, eggs, larvae), sometimes graded rare, few, moderate or many)	Notes
No ova or parasites seen	Negative	Normal. Because shedding is intermittent, one negative sample detects only 60 to 80 percent of infections; three negative samples on separate days make an infection unlikely.
Nonpathogenic commensals	Entamoeba coli, Endolimax nana, Entamoeba hartmanni, Iodamoeba bütschlii, Chilomastix mesnili	Harmless organisms that live in the gut without causing disease. They do not need treatment but do indicate that you swallowed something contaminated with stool, so a pathogen may be present too.
Blastocystis species	Positive, often with a quantity (few, moderate, many)	Found in 5 to 20 percent of healthy people; whether it causes symptoms is disputed. Treatment is considered only when symptoms persist and every other cause has been excluded.
Dientamoeba fragilis	Positive on permanent stain	A small protozoan associated with abdominal pain, loose stools and sometimes eosinophilia, particularly in children; treated when symptomatic.
Giardia duodenalis (G. lamblia, G. intestinalis)	Cysts or trophozoites seen, or antigen/PCR positive	The most common parasitic cause of diarrhea in the US, from untreated water, daycare and travel. Treated in anyone with symptoms.
Cryptosporidium, Cyclospora or Cystoisospora	Oocysts seen on modified acid-fast stain or antigen/PCR positive	Cryptosporidium spreads through swimming pools, lakes and farm animals and resists chlorine; Cyclospora comes on imported fresh produce. Both can be prolonged and severe in people with weakened immunity.
Entamoeba histolytica/dispar	Cysts or trophozoites seen; needs antigen or PCR to confirm E. histolytica	E. histolytica causes amoebic dysentery and liver abscess and is always treated; E. dispar is harmless. Trophozoites containing red blood cells favor E. histolytica.
Helminth eggs or larvae	Ascaris, hookworm, Trichuris (whipworm), Taenia (tapeworm), Hymenolepis nana, Schistosoma, Strongyloides	Mostly in travelers, recent immigrants and people exposed to soil or undercooked meat. Each is treated with a specific antiparasitic; Strongyloides must be treated before any steroid therapy.

Positive ova and parasites test

A positive O&P names the organism and the stage seen. The first question is whether it is a pathogen. Giardia, Cryptosporidium, Cyclospora, Cystoisospora, confirmed Entamoeba histolytica and any worm egg or larva are pathogens and are treated. Entamoeba coli, Endolimax nana, Entamoeba hartmanni, Iodamoeba bütschlii and Chilomastix mesnili are commensals that cause no disease and need no treatment; labs report them because they are evidence of fecal-oral exposure and because distinguishing them from pathogens is the point of the examination. Blastocystis and Dientamoeba fragilis sit in between: common, often harmless, and treated only when symptoms persist with no other explanation.

For a pathogen, treatment is specific to the organism, usually a short course of an antiparasitic such as metronidazole, tinidazole or nitazoxanide for Giardia, nitazoxanide for Cryptosporidium, trimethoprim-sulfamethoxazole for Cyclospora, metronidazole followed by a luminal agent for E. histolytica, and albendazole, mebendazole, praziquantel or ivermectin for the various worms. Symptoms usually settle within a week of treatment. A follow-up stool test is not routinely needed for Giardia if symptoms resolve, but is done for E. histolytica and after treating worms in some settings.

Household contacts, sexual partners and children in the same daycare may be tested or treated, because Giardia, Cryptosporidium, Entamoeba and Hymenolepis nana spread person to person. Giardia, Cryptosporidium and Cyclospora are reportable in most states, so you may hear from the health department about water or food sources. Lingering bloating, loose stools or milk intolerance for weeks after successful treatment is common, especially after Giardia, and does not mean the parasite is still there.

Common causes

Giardia duodenalis
Swallowed cysts from untreated stream or well water, swimming in lakes, daycare settings, travel and sexual contact. Incubation 1 to 2 weeks; causes foul-smelling greasy diarrhea, bloating, gas, cramps, nausea and weight loss that can last weeks. About 15,000 cases are reported in the US each year; the true number is far higher.
Cryptosporidium
Oocysts survive chlorinated pools and water parks, and spread from calves, lambs and infected people. Causes profuse watery diarrhea for 1 to 2 weeks in healthy people and chronic, life-threatening diarrhea in people with advanced HIV or transplant immunosuppression. The leading cause of US waterborne outbreaks.
Cyclospora cayetanensis
Imported fresh produce, classically raspberries, basil, cilantro and bagged salads, with US outbreaks most summers. Causes watery diarrhea, marked fatigue, loss of appetite and weight loss lasting weeks if untreated; it resists chlorine washing.
Entamoeba histolytica
Mostly in travelers to and immigrants from Central and South America, South Asia and Africa, and in institutional settings. Ranges from no symptoms to bloody dysentery with fever and to liver abscess weeks or months later. Only 1 in 10 people carrying E. histolytica becomes ill, but all confirmed carriers are treated because of that risk.
Dientamoeba fragilis and Blastocystis
Both are common and of disputed significance. Dientamoeba is linked to abdominal pain and loose stools in children and sometimes eosinophilia. Blastocystis is found in many healthy people; treatment helps some with persistent symptoms and no other cause, but trials are small and inconsistent.
Soil-transmitted worms: Ascaris, hookworm, Trichuris
Acquired from soil contaminated with human feces in tropical and subtropical regions. Light infections are silent; heavier ones cause abdominal pain, iron-deficiency anemia (hookworm, whipworm), malnutrition in children and, with Ascaris, a cough as larvae pass through the lungs. Eosinophils are often raised.
Strongyloides stercoralis
Enters through skin from contaminated soil and can persist for decades by reinfecting its host. Usually causes mild rash, abdominal symptoms and eosinophilia, but corticosteroids or other immunosuppression can trigger a fatal hyperinfection, so people who grew up in endemic areas are screened before steroids. Routine O&P misses most cases; serology is preferred.
Tapeworms: Taenia saginata, Taenia solium, Hymenolepis nana, Diphyllobothrium
From undercooked beef (T. saginata), pork (T. solium, whose eggs can also cause cysticercosis), person-to-person spread (H. nana, common in children) and raw freshwater fish (Diphyllobothrium, which causes vitamin B12 deficiency). People often notice segments in stool.
Schistosoma and liver flukes
Freshwater exposure in Africa, the Middle East, Asia and South America (schistosomiasis) or raw freshwater fish and watercress (Clonorchis, Opisthorchis, Fasciola). Eggs appear in stool months after exposure; chronic infection damages the liver, bladder or bile ducts.

Symptoms that can go with it

Diarrhea lasting more than 1 to 2 weeks, often greasy, foul-smelling and floating with Giardia
Bloating, excessive gas, belching with a sulfur taste and cramping, worse after eating
Weight loss, fatigue and loss of appetite out of proportion to the diarrhea
Bloody, mucoid stools with fever and cramping in amoebic dysentery
Nausea, and new intolerance of milk and dairy after the infection
Itching around the anus at night (pinworm, which needs a tape test, or tapeworm segments)
Cough, wheeze, rash or a raised eosinophil count during the tissue-migration phase of worm infections
No symptoms at all: many parasites are found incidentally, and some carriers never become ill

Read it alongside

Giardia and Cryptosporidium stool antigen or PCR — more sensitive than microscopy for the two most common US parasites; often ordered first.
Stool culture or GI PCR panel — bacterial causes (Campylobacter, Salmonella, Shigella, STEC) and viruses produce similar symptoms and are more common.
Absolute eosinophil count — raised in tissue-invading worm infections (Strongyloides, hookworm, Ascaris, schistosomiasis) but usually normal in protozoal infections such as Giardia.
Hemoglobin, ferritin and iron saturation — iron-deficiency anemia points to hookworm or whipworm, or to chronic blood loss from another cause.
Vitamin B12 — low with fish tapeworm (Diphyllobothrium) and with chronic Giardia or bacterial overgrowth.
Fecal calprotectin — high values with a negative O&P suggest inflammatory bowel disease; Entamoeba histolytica colitis also raises it.
Strongyloides and Schistosoma antibody tests — blood tests that are far more sensitive than stool microscopy for these two and are used before immunosuppressive treatment.
Albumin and weight — reflect the nutritional cost of a prolonged infection.

What usually happens next

If one sample is negative and symptoms continue, expect to be asked for two more samples on separate days, or for a Giardia/Cryptosporidium antigen test or PCR panel, which may already have been run.
A report of E. histolytica/dispar is followed by a stool antigen or PCR test to tell the two apart before any treatment is decided.
If the test finds a pathogen, treatment is matched to the organism; ask whether household members or sexual partners should be tested, and whether you need to stay away from pools (two weeks after Cryptosporidium diarrhea stops) or food handling.
If a commensal or Blastocystis alone is reported, the usual advice is that it does not explain severe or prolonged symptoms, and attention turns to other causes: celiac disease, bile acid diarrhea, inflammatory bowel disease, lactose intolerance or post-infectious irritable bowel syndrome.
After travel to the tropics, immigration from an endemic area, or before planned steroid or immunosuppressive treatment, ask about Strongyloides serology, because stool microscopy misses most cases.
If diarrhea has lasted more than 4 weeks with negative stool tests, colonoscopy with biopsies is often the next step, and can also find parasites that stool tests missed.

Diet, lifestyle and natural levers with evidence

What can move O&P once the cause is known. Nothing here replaces finding the cause.

Safe water: filtering, boiling and avoiding swallowing recreational water

Strong evidence

No diet or supplement clears an established parasitic infection, but exposure is highly preventable. Giardia and Cryptosporidium cysts are removed by filters rated 'absolute 1 micron' or certified to NSF standard 53 or 58 for cyst reduction, and killed by boiling water for 1 minute (3 minutes above 6,500 feet); iodine and chlorine tablets do not reliably kill Cryptosporidium. A Cochrane review of household water treatment found it reduced diarrhea by about a quarter to a half in settings without safe supplies. Not swallowing pool, lake or river water, and keeping children with diarrhea out of pools, prevents most Cryptosporidium outbreaks.

Hand washing, food washing and safe food choices when traveling

Strong evidence

Hand washing with soap reduces diarrheal illness by about 30 percent in trials, and alcohol gel is less effective against Giardia and Cryptosporidium cysts. Cyclospora resists chlorine, so washing imported herbs and berries helps less than cooking them during an outbreak; the CDC posts current produce alerts. When traveling, food that is cooked and served hot, fruit you peel yourself, and bottled or boiled water prevent most protozoal and worm infections; shoes on soil prevent hookworm and Strongyloides.

Temporary dairy restriction after Giardia

Moderate evidence

Giardia flattens the intestinal villi and strips the enzyme lactase, so 20 to 40 percent of people have lactose intolerance for several weeks after treatment, which is often mistaken for treatment failure. Avoiding milk and soft dairy (hard cheese and yogurt are usually tolerated) for 2 to 4 weeks and then reintroducing it relieves the lingering bloating and loose stools in most people. In the 2004 Giardia outbreak in Bergen, Norway, irritable bowel symptoms were still present in nearly half of those infected three years later compared with 14 percent of controls, so persistent symptoms after a cleared infection are common and are managed as post-infectious IBS.

Saccharomyces boulardii alongside standard Giardia treatment

Preliminary evidence

One randomized trial of 65 adults with giardiasis compared metronidazole alone with metronidazole plus Saccharomyces boulardii 250 mg twice daily for 10 days. At 4 weeks, Giardia cysts were still present in about 17 percent of the metronidazole-only group and none of the combination group. It is a single small study and the yeast has no effect on its own; it is an add-on, not an alternative, to prescribed treatment.

Avoid in people with central venous catheters, severe immune suppression or critical illness because of reported bloodstream yeast infections.

Herbal 'parasite cleanses' (wormwood, black walnut, clove, oregano oil)

Preliminary evidence

Despite heavy marketing, there are no controlled human trials showing that these products eliminate Giardia, Entamoeba, Cryptosporidium or intestinal worms, and the 'parasites' people report passing after them are typically mucus and shed intestinal lining. A few laboratory and small pilot studies of wormwood extracts exist but have not been confirmed in stool-verified trials. If a parasite has been identified, the prescription drug for it has documented clearance rates of 80 to 95 percent; if none has been identified, a cleanse treats nothing.

Wormwood contains thujone, which can cause seizures at high doses; high-dose black walnut and clove oil can irritate the gut and liver; none should be used in pregnancy. Do not delay or replace prescribed treatment with these products.

When to see a clinician promptly

Bloody diarrhea with fever, or severe right-upper abdominal pain with fever and sweats after travel: possible amoebic dysentery or liver abscess, which need prompt treatment.
Diarrhea with signs of dehydration (dizziness, very little urine, confusion) or in an infant, pregnant woman, older adult or anyone immunosuppressed, especially if Cryptosporidium is suspected.
Diarrhea lasting more than 2 weeks, or any diarrhea with weight loss of more than a few pounds, night-time stools or blood.
Anyone from or who has lived in a Strongyloides-endemic region who is about to start steroids, chemotherapy or other immunosuppression: screening is needed first because hyperinfection can be fatal.
Passing visible worms or segments in stool, or a persistent cough, wheeze and rash with a high eosinophil count after tropical travel.
Persistent symptoms after a negative test: it often needs repeat samples, a different test, or a search for a non-parasitic cause rather than reassurance alone.

Frequently asked questions

What does 'no ova or parasites seen' mean?

It means the technologist found no parasite eggs, cysts, larvae or active forms in that sample. It is a negative result for that specimen. Because parasites are shed intermittently, one negative sample rules out only about 60 to 80 percent of infections; three negative samples collected on different days bring that above 90 percent. If you have ongoing diarrhea, a Giardia/Cryptosporidium antigen test or a PCR panel is usually the next step because they are more sensitive than microscopy for the common parasites.

Why do I need three stool samples for an O&P?

Giardia, Entamoeba and many worms release eggs or cysts in bursts rather than continuously, so a sample taken on a quiet day can be clear even in a real infection. Collecting three samples on separate days, ideally 2 to 3 days apart and within 10 days, roughly halves the miss rate. Some labs accept one sample if an antigen or PCR test for Giardia and Cryptosporidium is run alongside, since those tests are not affected by shedding to the same degree.

Is Blastocystis hominis in stool something to worry about?

Usually not. Blastocystis is found in 5 to 20 percent of healthy people in the US and more in warmer countries, and most carriers have no symptoms. Whether it ever causes illness is still argued; it may matter in some people with persistent bloating and loose stools once everything else has been excluded, and a trial of treatment is sometimes offered then. It should not stop your clinician from looking for other explanations for your symptoms.

What is the best stool test for Giardia?

A stool antigen test (enzyme immunoassay or direct fluorescent antibody) or a PCR panel, both of which detect more than 90 to 95 percent of Giardia infections on a single sample, compared with roughly 50 to 70 percent for one microscopic O&P. Microscopy still has a role because it can find organisms that the antigen test does not target, so the two are often done together for prolonged or travel-related diarrhea.

Can an O&P test detect pinworms?

Rarely. Pinworms lay their eggs on the skin around the anus at night rather than in the stool, so they are seldom found in an O&P. The right test is a tape test: a piece of clear adhesive tape or a commercial paddle pressed against the anal skin first thing in the morning before washing, on three consecutive mornings, and examined under the microscope. Adult worms are sometimes seen directly on the skin or in the toilet.

What should I avoid before a stool parasite test?

Barium from an X-ray or CT, bismuth products such as Pepto-Bismol, mineral oil, kaolin, antacids and anti-diarrheal medicines all obscure parasites for 7 to 10 days, and antibiotics, antimalarials and tetracyclines suppress protozoa for about 2 weeks, so the lab will ask you to wait. Do not let the stool touch toilet water or urine. You do not need to fast or change your diet.

How long do O&P results take?

Usually 1 to 3 days per sample once it reaches the lab, because each specimen has to be concentrated, stained and read by a trained microscopist, and the permanent stain takes time to prepare. If three samples are submitted over a week, the final report follows the last one. Antigen tests and PCR panels for Giardia and Cryptosporidium typically come back the same or the next day.

I was treated for Giardia but still have bloating and diarrhea. Is it still there?

Often not. Treatment clears Giardia in 80 to 95 percent of people, but temporary lactose intolerance lasting several weeks affects 20 to 40 percent afterward, and post-infectious irritable bowel syndrome is common for months. If symptoms persist beyond 2 to 4 weeks after treatment, a repeat antigen or PCR test will show whether the parasite is still present; if it is, a second course or a different drug is used. If it is negative, a short dairy-free trial and time usually resolve things.

References

Ova and parasite test. MedlinePlus, US National Library of Medicine (2023)View source
DPDx: Laboratory identification of parasites of public health concern. Diagnostic procedures: stool specimens. Centers for Disease Control and Prevention (2024)View source
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Shane AL, Mody RK, Crump JA, et al.. 2017 Infectious Diseases Society of America clinical practice guidelines for the diagnosis and management of infectious diarrhea. Clinical Infectious Diseases (2017)
Hiatt RA, Markell EK, Ng E. How many stool examinations are necessary to detect pathogenic intestinal protozoa?. American Journal of Tropical Medicine and Hygiene (1995)
Rosenblatt JE. Clinical importance of adequately performed stool ova and parasite examinations. Clinical Infectious Diseases (2006)
Minetti C, Chalmers RM, Beeching NJ, Probert C, Lamden K. Giardiasis. BMJ (2016)
Wensaas KA, Langeland N, Hanevik K, Mørch K, Eide GE, Rortveit G. Irritable bowel syndrome and chronic fatigue 3 years after acute giardiasis: historic cohort study. Gut (2012)
Besirbellioglu BA, Ulcay A, Can M, et al.. Saccharomyces boulardii and infection due to Giardia lamblia. Scandinavian Journal of Infectious Diseases (2006)
Clasen TF, Alexander KT, Sinclair D, et al.. Interventions to improve water quality for preventing diarrhoea. Cochrane Database of Systematic Reviews (2015)
Parasites - Giardia. Centers for Disease Control and Prevention (2024)
Giardiasis. Merck Manual Professional Version (2024)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.