An Ova and Parasites (O&P) stool test is a laboratory exam that looks for parasite eggs (ova) and parasites in a stool sample. Doctors usually order it when someone has ongoing diarrhea, stomach cramps, bloating, nausea, or travel-related digestive symptoms that could be caused by an intestinal parasite. The test does not measure a normal “level” in the blood; instead, it is typically reported as positive or negative, sometimes with the parasite species named if one is found.
What the Ova and Parasites stool test checks for
The O&P stool test helps detect intestinal parasites that may live in the gut and cause infection. According to the CDC, laboratory examiners look for parasite eggs, larvae, and forms of the parasite that are present in stool. Common parasites that may be identified include Giardia, Entamoeba histolytica, Cryptosporidium, hookworms, pinworms, roundworms, and others, depending on the test method and the laboratory’s expertise.
This test is often used when symptoms suggest a parasitic infection, especially after travel, unsafe water exposure, daycare exposure, or contact with contaminated food or soil. In some cases, a doctor may order a different stool test, such as a stool antigen test or molecular test, because those methods can be more sensitive for certain parasites, as the CDC and major clinical references note.
When doctors order an O&P stool test
Doctors may recommend an O&P stool test when symptoms last more than a few days or keep returning. Common reasons include:
- Persistent diarrhea
- Loose stools after travel
- Abdominal pain or cramping
- Bloating or excess gas
- Unexplained nausea
- Weight loss
- Greasy, foul-smelling stools
- Itching around the anus, especially in children
- A history of drinking untreated water or eating contaminated food
A clinician may also order the test if you have a weakened immune system, because parasitic infections can be more serious or harder to clear in some people. The Mayo Clinic and CDC both note that symptom pattern, travel history, and exposure risks help guide testing.
How the Ova and Parasites stool test is done
The test uses a fresh stool sample. Your healthcare team usually gives you a clean container and instructions for collection. In many cases, you collect the sample at home and return it to the lab soon after.
For some parasites, one stool sample may not be enough. Because parasites and their eggs may shed intermittently, many laboratories and clinicians ask for 2 to 3 stool samples collected on different days. The CDC and laboratory manuals note that multiple specimens can improve the chance of detection.
The lab technician examines the sample under a microscope. Depending on the laboratory, the sample may also undergo special staining, concentration, or other methods to make parasite forms easier to find. Some labs combine microscopic exam with antigen or molecular testing for better accuracy.
How to prepare for the O&P stool test
Preparation is usually simple, but instructions can vary by lab. Your doctor may ask you to:
- Avoid contaminating the sample with urine, toilet water, or toilet paper
- Label the container correctly
- Return the sample quickly
- Collect more than one sample if instructed
Tell your healthcare provider about any medicines or supplements you take. Certain treatments, such as antibiotics, antidiarrheal drugs, bismuth products, or antiparasitic medicines, can affect interpretation in some situations. Your doctor will tell you whether to pause any medicine before testing.
If you recently had a barium X-ray study, your doctor may delay stool testing, because barium can interfere with some laboratory methods.
What the results mean
An O&P stool test is usually reported as:
- Negative: No parasites or ova were found in the sample examined
- Positive: One or more parasites, ova, larvae, or cysts were found
- Indeterminate or limited: The sample was inadequate, or the findings were unclear
A negative result does not always rule out infection. Parasites may be missed if shedding is low, the wrong organism is being tested for, or the sample is collected at a time when the organism is not present in stool. The CDC and major lab references emphasize that test sensitivity depends on the parasite, the lab method, and the number of samples examined.
A positive result usually identifies the parasite, which helps your doctor choose treatment. If the report names a specific organism, that is more useful than a general “positive” result alone.
Ova and Parasites stool test versus other stool tests
Not every intestinal infection is best found with the O&P test. In many cases, doctors now use more targeted tests.
An O&P stool test is useful for looking broadly for many different parasites. However, stool antigen tests can be better for parasites such as Giardia or Cryptosporidium, and molecular tests may detect certain infections more accurately. According to the CDC and modern clinical practice, the best test depends on the suspected parasite and the clinical situation.
Your doctor may also order:
- Stool culture: Looks for bacterial infection
- C. difficile testing: Looks for Clostridioides difficile infection
- Fecal occult blood test: Looks for hidden blood
- Fecal calprotectin: Helps assess intestinal inflammation
- Stool antigen or PCR tests: Look for specific parasites or pathogens
Common parasites the O&P stool test may detect
The exact organisms found depend on where you live, where you traveled, and what the laboratory tests for. Examples include:
- Giardia lamblia, a common cause of watery diarrhea and gas
- Entamoeba histolytica, which can cause dysentery-like illness
- Cryptosporidium, which can cause watery diarrhea
- Blastocystis, whose clinical significance can vary
- Hookworms, roundworms, tapeworms, and pinworms in some settings
Some parasites do not appear well on routine microscopy. In those cases, your clinician may choose a different test. That is why test selection matters as much as test interpretation.
What can affect the accuracy of the O&P stool test
Several factors can influence the result:
- The parasite may not be shed in that particular sample
- Only one stool sample may have been collected
- The lab may need special stains or concentration methods
- Some organisms are easier to find with antigen or molecular tests
- Recent treatment may lower detection
- A sample may be too small or contaminated
Because of these limits, doctors often interpret the O&P result together with symptoms, travel history, exam findings, and other lab results. A single negative test does not always end the evaluation if suspicion remains high.
What happens after a positive result
If the test is positive, your doctor will usually explain which parasite was found and what treatment is appropriate. Treatment often depends on the specific organism, your symptoms, your age, pregnancy status, and your overall health.
Some infections respond to prescription antiparasitic medicines. Others may improve with hydration and supportive care, while your doctor monitors for complications. In a few cases, especially with asymptomatic findings or organisms of uncertain significance, your clinician may recommend repeat testing or further evaluation before treatment.
If others in your household have similar symptoms, your doctor may suggest that they be evaluated too, especially if the infection is contagious or linked to a shared exposure.
What to expect if the result is negative
If the test is negative but symptoms continue, your doctor may consider:
- Repeating the O&P stool test with additional samples
- Ordering a stool antigen or PCR test
- Checking for bacterial, viral, or noninfectious causes of symptoms
- Evaluating for conditions such as irritable bowel syndrome, inflammatory bowel disease, celiac disease, or medication side effects
A negative result is helpful, but it is only one piece of the diagnostic picture.
Prevention of intestinal parasitic infection
You can lower your risk of many parasitic infections by using practical hygiene measures:
- Wash hands with soap and water before eating and after bathroom use
- Drink safe, treated water
- Avoid swallowing water from lakes, rivers, and pools
- Cook food thoroughly
- Wash fruits and vegetables well
- Use proper sanitation and safe diapering practices
- Follow travel health advice for high-risk destinations
The WHO and CDC both emphasize clean water, sanitation, food safety, and hand hygiene as core prevention strategies for intestinal parasites.
When to see a doctor
See a doctor promptly if you have diarrhea lasting more than a few days, especially after travel, camping, drinking untreated water, or exposure to someone with a known parasitic infection. You should also seek medical advice if you have abdominal pain, bloating, unexplained weight loss, greasy stools, blood in the stool, fever, or signs of dehydration such as dizziness, dry mouth, or very dark urine.
Seek urgent care if you cannot keep fluids down, feel faint, have severe abdominal pain, have a high fever, or notice signs of severe illness. If your doctor tells you that an O&P stool test is positive and you are pregnant, immunocompromised, or a young child, follow up promptly so the result can be interpreted and treated in context.
Frequently asked questions
Is an O&P stool test the same as a stool culture?
No. An O&P stool test looks for parasites and their eggs or cysts. A stool culture looks for certain bacteria. Doctors choose the test based on the most likely cause of symptoms.
How many stool samples are usually needed?
Often 2 to 3 samples collected on different days. Because parasites may not appear in every sample, multiple collections can improve the chance of detection, according to CDC laboratory guidance.
Do I need to fast before the test?
Usually no. Most people do not need to fast. Follow the instructions from your doctor or the laboratory, because collection rules can vary.
Can the test miss a parasite?
Yes. A negative result does not always rule out infection. Some parasites shed intermittently, and some are better detected with antigen or molecular tests.
How long does it take to get results?
Turnaround time varies by laboratory. Some results come back in a day or two, while more specialized testing may take longer.
If the test is positive, does that always mean I need treatment?
Not always. Your doctor will consider the specific parasite, your symptoms, your immune status, and whether the finding may represent true infection or an incidental result.
Glossary of key terms
- Ova: parasite eggs
- Parasites: organisms that live in or on another organism and may cause disease
- Stool: a bowel movement
- Cyst: a dormant, protective form of some parasites
- Larva: an immature stage of some worms
- Microscopy: looking at a sample under a microscope
- Antigen test: a test that looks for proteins from a germ
- PCR test: a molecular test that looks for genetic material from a germ
- Immunocompromised: having a weaker immune system than usual
- Dehydration: not having enough body fluid
Sources
- Mayo Clinic Laboratories — Ova and parasite testing
- NIH/PMC — Utility of Multiple-Stool-Specimen Ova and Parasite Examinations
- ASM Clinical Microbiology Reviews — Laboratory Diagnosis of Parasites from the Gastrointestinal Tract
Further reading
- Fecal Calprotectin: Understanding Test Results
- Stool Consistency: Understanding Normal and Abnormal Changes
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