Ova and Parasites (Stool) Testing: Understanding Your Report

Section Editor: Rodney E. Rohde PhD
June 5, 2026

An ova and parasite (O&P) test is a laboratory test that examines a stool (feces) sample under the microscope to detect parasites and their eggs. In the name of the test, “ova” means eggs, and “parasites” means the organisms themselves. A parasite is a living thing that lives in or on another living thing and takes its nourishment from it. The test is used mainly to find the cause of diarrhea or other digestive symptoms, especially after travel or a possible exposure.

This article explains how the ova and parasites test works, why more than one sample is often needed, and what the wording on your report means, so you can better understand a report you have received.

What is an ova and parasites test, and why is it done?

Parasites that infect the gut fall into two broad groups: protozoa, which are single-celled (such as Giardia, a common cause of diarrhea), and worms, which are larger. The test looks for these parasites and their eggs in the stool.

It is ordered when a parasitic infection of the gut is suspected, for example, with diarrhea that lasts more than a few days, after travel to areas where parasites are common, after drinking untreated water, or in a person with a weakened immune system. It is a separate test from a stool culture, which looks for disease-causing bacteria; the ova and parasites test looks specifically for parasites.

How the sample is collected, and why more than one

A fresh stool sample is collected, often into a container that holds a preservative. The most important practical point about this test is that more than one sample, usually taken on different days, is often needed. This is because parasites and their eggs are shed in the stool intermittently rather than steadily, so a single sample can miss an infection that several samples would detect.

A few things can interfere with the test, including recent imaging dyes such as barium, and certain medications such as antacids, antidiarrheal medicines, and some antibiotics. Your doctor may ask you to wait a period of time after these before the sample is collected.

The microscopic examination

In the classic ova and parasites test, the laboratory examines the stool under a microscope, often after special steps that concentrate and stain the sample to make anything present easier to see. The examiner looks for eggs (ova), as well as other forms of the parasites, such as cysts (dormant, protected forms) and the active organisms themselves. If a parasite is found, it can usually be identified by its appearance.

Other tests: antigen and molecular

Microscopy is not the only way to look for parasites.

Antigen tests — Detect pieces (antigens) of specific common parasites, such as Giardia, Cryptosporidium, and Entamoeba. For these particular parasites, antigen tests are often more sensitive than looking under the microscope.
Molecular (PCR) tests — Look for the genetic material of parasites, as described in the article on PCR, antigen, and antibody tests. Many laboratories now use panels that can detect multiple gut-related germs at once, including parasites, bacteria, and viruses.

A couple of related tests are done differently. Pinworm, a common cause of itching around the anus in children, is usually checked with a “tape test,” in which clear tape is pressed against the skin around the anus to collect eggs, rather than with a stool sample. And for some parasites that travel beyond the gut, a blood test may be used instead.

How to read your result

The wording on your report depends on which test was used.

No ova or parasites seen — None were found in the sample examined. Because parasites are shed on and off, a single negative result does not completely rule out an infection, and more samples or other tests may be done if your symptoms continue.
A named parasite identified — The report names the parasite, or its eggs, that were found, such as Giardia, identifying the cause of the infection.
Antigen test positive or negative — A specific parasite was, or was not, detected.
Molecular (PCR) test detected or not detected — The genetic material of one or more parasites was, or was not, found.

What happens after these tests

An ova and parasites test describes what was found and informs the decisions you and your healthcare team make together, rather than dictating them on its own.

When a parasite is found, treatment depends on which one it is. Specific antiparasitic medicines are used to treat many parasites, while some infections resolve on their own, particularly in people with a healthy immune system. Treating an infection can also help prevent spreading it to others, and good hand and food hygiene is important. Some parasitic infections are reported to public health authorities, especially when they may be linked to contaminated water or food. If your symptoms persist despite negative test results, your doctor may repeat the tests or look into other causes.

Questions to ask your doctor

Did my test find a parasite, and which one?
If no parasites were seen, could I still have one? Should I provide more samples?
How many samples should I give, and over how many days?
Could a recent medication or imaging dye have affected my result?
If a parasite was found, what does treatment involve?
Will this infection clear on its own, or does it need medicine?
How can I avoid spreading it to people at home?
Could my symptoms be caused by something other than a parasite?
Will my result be reported to public health?
Who should I contact if I am not improving?