O and P stands for “ova and parasites,” a laboratory stool test that checks for intestinal parasites and their eggs. If your doctor ordered an O&P exam, a technician will examine your stool sample under a microscope looking for parasitic organisms or the eggs (ova) they leave behind. It’s one of the most common tests used to diagnose parasitic infections of the digestive tract.
What the Test Looks For
The O&P exam screens for two broad categories of organisms: protozoa (single-celled parasites) and helminths (worms). The most common parasites detected in the United States are Giardia, Cryptosporidium, and Entamoeba histolytica, which causes a condition called amebiasis. All three typically spread through contaminated water or food.
The test also picks up various types of worms, including pinworms and hookworms. While worm infections are less common in the U.S., they remain a major concern for people who have traveled to tropical or developing regions. Hookworms are notable because they can enter the body through the skin, not just through food or water, so travel history matters when your provider is deciding whether to order this test.
Why Your Doctor Orders It
An O&P test is typically ordered when you have persistent digestive symptoms that don’t have an obvious explanation: prolonged diarrhea (especially lasting more than a few days), cramping, bloating, nausea, or unexplained weight loss. Bloody or mucus-filled stools raise the suspicion further. Your provider is more likely to order the test if you’ve recently traveled internationally, drunk untreated water, or been exposed to someone with a confirmed parasitic infection.
It’s also commonly ordered for people with weakened immune systems, since parasitic infections can be more severe and harder to clear in those cases.
How to Collect the Sample
You’ll receive a collection kit that includes two vials: one containing formalin and one containing a preservative called PVA. Both vials need to be filled from the same bowel movement so the lab can run a complete exam. You need to collect the stool before it touches toilet water, so the standard approach is to place a clean piece of plastic wrap, a plastic bag, or even newspaper over the toilet seat opening to catch the sample. Urine should not mix with the stool.
Using the small spoon attached to each vial’s cap, add roughly three spoonfuls of hard stool (or five spoonfuls if it’s soft) into each vial. If the stool is firm, sample from both ends and the middle. If you notice any areas that look bloody, slimy, or watery, prioritize sampling from those spots, as parasites are more likely to be concentrated there. After adding the stool, mix it thoroughly with the liquid already in the vial, close the caps tightly, and invert each vial a few times. The sealed vials go back into the provided bag and do not need refrigeration.
Why Multiple Samples Are Needed
Most labs recommend collecting three separate samples on alternating days. Parasites don’t shed eggs continuously. They cycle in and out of active shedding, which means a single sample can easily miss an infection. Collecting every other day over the course of about a week increases the odds that at least one sample catches the organisms during an active shedding period. Submitting only one sample carries a meaningful risk of a false negative.
What Can Affect Your Results
Certain substances can interfere with the test and produce inaccurate results. Bismuth (the active ingredient in some upset-stomach medications), barium from recent imaging procedures, and certain antibiotics or anti-parasitic drugs can suppress the visibility of organisms in the sample. If you’ve taken any of these in the days before collection, let your provider know, as the lab may need to delay testing or request a new sample. Mineral oil and some antidiarrheal medications can also make it harder for technicians to identify parasites under the microscope.
How Long Results Take
Turnaround time for an O&P exam is typically 3 to 7 days from when the lab receives the specimen. In some cases it takes longer if the lab runs additional confirmatory testing on a positive finding. Your provider will contact you with results, though many labs also release them through online patient portals.
What Your Results Mean
A negative result means no parasites or eggs were seen in the sample. That’s reassuring, but because of the shedding cycles mentioned above, a single negative result doesn’t completely rule out infection. If your symptoms persist, your doctor may order additional rounds of testing or switch to antigen-based tests that detect specific parasites like Giardia and Cryptosporidium with higher sensitivity.
A positive result identifies the specific organism found, which guides treatment. Different parasites require different medications, so knowing exactly which one is present matters. Some positive results, particularly for organisms like Entamoeba histolytica, may prompt additional testing to distinguish between a true infection and a harmless lookalike species that doesn’t need treatment.