The Ova and Parasite (O&P) examination is a diagnostic laboratory test involving the microscopic analysis of a stool specimen. This procedure detects parasitic organisms, their eggs (ova), and their dormant forms known as cysts, within the gastrointestinal tract. Identifying these life stages helps clinicians diagnose parasitic infections that cause intestinal illness. The test determines the precise cause of digestive symptoms, which is necessary for selecting the appropriate antiparasitic treatment.
Clinical Indications for Testing
A healthcare provider typically orders an O&P test when a patient presents with persistent or unexplained gastrointestinal distress. The most common indication is diarrhea that lasts for several days or weeks, often accompanied by abdominal cramping or excessive gas. Symptoms such as nausea, vomiting, or the presence of mucus or blood in the stool may also prompt testing.
The likelihood of a parasitic infection increases with certain patient histories and exposures. Individuals who have recently traveled to areas with questionable sanitation should be tested if they develop symptoms. Exposure to contaminated water sources, such as untreated lakes or streams, is also a strong indicator for testing.
Patients with a compromised immune system, due to age or underlying medical conditions, are more susceptible to serious parasitic infections and are frequently tested. Unexplained weight loss, anemia, or persistent fatigue, combined with bowel changes, can also indicate a chronic parasitic presence. Since some parasites shed intermittently, the O&P test remains a foundational tool for investigating non-bacterial and non-viral causes of intestinal disease.
Preparing and Submitting the Sample
The accuracy of the O&P test relies heavily on the quality and timing of the sample collection, which the patient performs at home. Patients receive a kit with a vial pre-filled with a chemical preservative, such as formalin or sodium acetate-formalin (SAF), to stabilize parasitic forms. The stool must be passed into a clean, dry container, like a collection hat, to avoid contamination with water or urine. Contamination can destroy motile parasites (trophozoites), potentially causing a false negative result.
A small amount of the specimen is scooped into the preservative-filled vial up to the designated fill line. If the stool contains visible mucus, blood, or appears watery, the patient should prioritize sampling those areas, as parasites are often concentrated there. The preservative solution must be thoroughly mixed with the sample by shaking the sealed vial to ensure proper fixation.
To account for intermittent parasite shedding, providers often request a series of samples collected on different days to maximize detection. Patients are usually instructed to temporarily avoid certain substances for up to two weeks before collection. These include antibiotics, antacids, oily laxatives, and imaging contrast agents like barium, which can interfere with identification or suppress the parasites.
How the Laboratory Analyzes the Specimen
Once the preserved specimen arrives at the laboratory, it undergoes standardized procedures to prepare it for microscopic inspection. The first step is the concentration technique, which separates parasitic elements from the bulk fecal matter. The sample is mixed with a solution and centrifuged, causing ova and cysts to settle into a pellet. This process concentrates any organisms present, making them easier to find.
Next, a portion of the concentrated material is used to prepare a direct wet mount. A small drop is placed on a slide to allow the technologist to look for motile trophozoites, the actively feeding stage of protozoa like Entamoeba histolytica. The technician then prepares a permanent stained smear. This involves spreading a thin film of the sample onto a slide and treating it with a specialized stain. Staining highlights the internal structures of the ova, cysts, and trophozoites, which is necessary for definitive species identification under high-power magnification.
Interpreting Test Results
The results of an O&P test are typically reported as either negative or positive for parasitic organisms. A negative result indicates that no ova, cysts, or parasites were observed in the examined specimens. However, a single negative result does not rule out an infection, as the parasite may have been shedding in numbers too low for detection, necessitating additional testing if symptoms continue.
A positive result confirms the presence of a parasitic infection and includes the specific genus and species identified. Common examples detected include the protozoa Giardia lamblia and Cryptosporidium species, which frequently cause waterborne illness. The presence of hookworm eggs or tapeworm segments also constitutes a positive finding. The physician correlates the laboratory finding with the patient’s clinical symptoms and exposure history to confirm the diagnosis and initiate targeted antiparasitic medication.