The Ova and Parasite (O&P) test is a common diagnostic tool used in medicine to identify infections of the gastrointestinal tract caused by microscopic parasites. This test involves a laboratory professional microscopically examining a stool sample for evidence of these organisms. The examination detects parasites and their reproductive forms, which exit the body in the feces. A positive result from an O&P test can help a healthcare provider determine the specific cause of a patient’s intestinal symptoms.
Specific Organisms Identified by the O&P Test
The O&P test is designed to find two major categories of intestinal parasites: single-celled protozoa and multicellular helminths (worms). Protozoa are organisms such as Giardia lamblia and Entamoeba histolytica, which are frequently identified. The test seeks out two distinct forms of these protozoa: the active, feeding stage called the trophozoite and the dormant, resistant stage known as the cyst. Detection of either stage confirms the presence of an infection.
Identifying the cyst form is often the most reliable method for diagnosing protozoan infections because cysts are more durable and are shed more consistently in the stool than the fragile trophozoites. Other protozoa commonly looked for include Cryptosporidium species and Cystoisospora belli. Specific methods like antigen testing may also be used alongside or instead of the traditional O&P for these organisms. The microscopic analysis distinguishes between pathogenic parasites and non-pathogenic ones.
The examination also targets helminths, which are larger, multi-cellular organisms like tapeworms, roundworms, and hookworms. For these infections, the O&P test primarily looks for the eggs (ova) that the adult worms produce inside the intestines. These ova possess distinct shapes and sizes that allow laboratory staff to identify the specific type of worm causing the infection.
While the ova are the most common finding, the test may occasionally reveal larvae (immature forms of the worms), or even segments of an adult worm. For instance, segments from a tapeworm, called proglottids, may be visible in the stool sample. The identification of the specific parasite is crucial because treatment protocols vary significantly depending on the organism found.
When Physicians Order the O&P Test
A physician will typically order an O&P test when a patient reports persistent gastrointestinal symptoms that suggest a parasitic infection. The most common indication is chronic or severe diarrhea that does not resolve on its own. Other patient-reported issues that prompt testing include persistent abdominal cramping, nausea, vomiting, and unexplained weight loss.
Symptoms such as bloody or mucus-filled stools, or foul-smelling, greasy stools that float, also raise suspicion for a parasitic cause. These symptoms are especially concerning in individuals who are very young, elderly, or have a compromised immune system, as they are at a higher risk for complications. The test is a method to establish the precise diagnosis, as similar symptoms can be caused by bacterial or viral infections.
A patient’s recent history and travel exposure are important considerations that influence a physician’s decision to order the test. Recent international travel, particularly to regions with less developed sanitation, is a significant risk factor for contracting intestinal parasites. Consuming untreated water, such as from a stream or lake during camping, also warrants testing.
Understanding the Sample Collection Process
Patients are provided with a collection kit and specific instructions to ensure the sample is collected correctly, as proper collection is necessary for accurate results. Before collecting the sample, patients are advised to avoid certain medications that can interfere with the test’s ability to detect parasites. These substances include antacids, oil-containing laxatives, bismuth compounds, and certain antibiotics.
The stool sample must be collected cleanly, without being mixed with urine or toilet bowl water, as these can destroy fragile parasitic forms. A special collection container or plastic wrap placed over the toilet is usually recommended to catch the specimen. Once collected, a portion of the sample is placed into vials containing a preservative solution, which stabilizes the organisms for transport to the laboratory.
Because parasites are shed intermittently, they may not be present in every bowel movement, which can lead to a false-negative result from a single collection. Healthcare providers often request that the patient collect multiple samples over several different days to increase the probability of detecting the organisms. The collected samples are then concentrated and stained in the laboratory for microscopic identification.