The Ova and Parasite (O&P) test is a common diagnostic procedure used to investigate a suspected gastrointestinal infection caused by a parasite. The test involves the microscopic examination of a patient’s stool sample to visually identify the presence of parasitic organisms or their characteristic byproducts. By looking for these specific forms, healthcare providers can determine the cause of chronic or severe digestive symptoms. This method remains a standard approach for diagnosing enteric parasitic diseases.
Defining the Ova and Parasite Test
The O&P test diagnoses infections caused by organisms that inhabit the digestive system. A physician typically orders this test when a patient presents with symptoms suggesting a parasitic infection, such as prolonged diarrhea, persistent abdominal cramping, nausea, or unexplained weight loss. These organisms live and reproduce within the intestinal tract, disrupting normal digestive function.
The test looks for parasites in their various life cycle stages shed into the stool. These stages include ova, which are the eggs of helminths (worms), and cysts, the hardy, protective forms of protozoa. The test can also detect trophozoites, the active, feeding forms of protozoa, although these rapidly disintegrate after the stool is passed. Identifying these specific structures helps confirm the infection and determines the exact type of parasite involved.
Specific Pathogens Identified
The O&P test identifies two main categories of organisms: single-celled protozoa and multicellular helminths. Protozoa multiply within the human host and are detected as cysts or trophozoites. Common protozoa identified include Giardia lamblia, a frequent cause of waterborne illness, Cryptosporidium species, and Entamoeba histolytica, which causes amebiasis.
Helminths (worms) are larger parasites that do not multiply inside the host but pass their ova into the stool. The test detects the ova of various roundworms, such as Ascaris lumbricoides and hookworms, and flatworms like tapeworms. The microscopic examination distinguishes these organisms by noting their unique shapes, sizes, and internal structures. While the O&P test screens for a wide variety of pathogens, specialized antigen tests may be ordered for common parasites like Giardia and Cryptosporidium to enhance detection sensitivity.
Accurate Specimen Collection
Proper collection of the stool sample is necessary because parasitic organisms are often shed intermittently and can be difficult to find in a single sample. Healthcare providers frequently recommend a serial collection process, which involves collecting three specimens over a period of up to ten days. This strategy significantly increases the chance of detecting the parasite or its life stages.
To ensure the integrity of the sample, patients must use a clean, sterile container and prevent contamination with toilet water or urine. Both water and urine can destroy the fragile trophozoites, making microscopic identification impossible. Certain medications and substances can also interfere with the test results by obscuring the parasites or temporarily suppressing their release.
Patients are usually advised to avoid substances like antibiotics, antidiarrheal drugs, antacids, mineral oil, and bismuth compounds for up to two weeks before collection. After collection, the sample must be transported to the laboratory promptly, often within two hours for fresh samples, or immediately mixed with a preservative provided in the collection kit. This immediate preservation helps maintain the morphology of the parasites until they can be examined by a technician.
Interpreting Test Outcomes
Test results are typically reported as either negative or positive for specific organisms. A negative result means that no ova, cysts, or parasites were observed during the microscopic examination. However, a negative finding does not entirely rule out an infection, as the parasite load might be too low or the organism may be shed irregularly, leading to a false negative result.
A positive result confirms the presence of an infection and identifies the specific genus, species, and life stage of the parasite found in the stool. The laboratory report provides the healthcare provider with the necessary information to select the appropriate treatment, as therapy is specific to the type of parasite identified. Once a parasitic infection is confirmed, the healthcare provider correlates the findings with the patient’s symptoms, travel history, and other clinical factors to begin targeted anti-parasitic treatment.