The Ova and Parasites (O&P) test is a common laboratory procedure used to diagnose infections of the gastrointestinal (GI) tract caused by parasites. This microscopic examination of a stool sample specifically looks for parasitic organisms and their eggs, or ova, which are shed into the feces of an infected person. Intestinal parasitic infections are a global health concern. This test helps healthcare providers identify the specific pathogen responsible for digestive illness, which aids in guiding targeted treatment.
Why the Ova and Parasites Test is Ordered
A physician typically orders an O&P test when a patient presents with persistent or severe digestive symptoms that suggest a parasitic infection. These symptoms often include prolonged diarrhea, severe abdominal cramping, unexplained weight loss, nausea, and vomiting. The presence of blood or mucus in the stool, or stools that are unusually foul-smelling and greasy, also raises suspicion.
The decision to order the test is influenced by a patient’s recent history and risk factors for exposure. Patients who have recently traveled to areas where intestinal parasites are common, or those who have consumed untreated water, are considered at higher risk. Individuals with weakened immune systems, such as the very young, the elderly, or those with underlying diseases, are more likely to have testing ordered if symptoms are prolonged or severe.
Specimen Collection and Handling Procedures
The accuracy of the O&P test relies significantly on the proper collection and handling of the stool specimen by the patient. Patients are typically provided with a specialized kit containing containers with a preservative fluid, such as formalin or a polyvinyl alcohol (PVA) solution, to maintain the integrity of the parasites. It is generally recommended that the patient collect multiple samples, often three, over several days because parasites are shed intermittently and may not appear in a single stool.
The stool must be collected into a clean, dry container, such as a bedpan or a sheet of plastic wrap placed over the toilet bowl, to prevent contamination. It is important to ensure the sample does not mix with urine, toilet water, or toilet paper. Water can destroy the active forms of some parasites, and other contaminants can interfere with laboratory analysis. Once collected, a small portion of the stool is mixed with the preservative in the designated container up to the indicated fill line.
Mixing the sample thoroughly with the preservative fluid is necessary to stop the parasites’ life cycle and preserve their morphology for later microscopic examination. Certain medications and substances, including antacids, mineral oil, bismuth, and some antibiotics, can interfere with parasite detection. Patients may be advised to avoid these for a period before collection. The preserved specimens must then be returned to the laboratory promptly, following the specific instructions provided in the collection kit.
How the Laboratory Identifies Parasites
Once the specimen arrives at the laboratory, identifying parasites involves several specialized steps to enhance visibility. The laboratory technician first performs a macroscopic examination, noting the color, consistency, and presence of visible abnormalities like blood, mucus, or adult worms. This initial check provides clues about the type of infection before microscopic analysis begins.
The core of the O&P test involves a concentration technique, typically using formalin-ethyl acetate sedimentation, to separate the parasites from the fecal debris. This step concentrates ova, cysts (the dormant stage), and larvae into a smaller volume of sediment, making it easier for the technician to find them under the microscope. Although concentration may destroy the fragile, active feeding stage, known as trophozoites, it greatly increases the chance of detecting the more robust cysts and eggs.
Next, the technician prepares both a wet mount slide from the concentrate and a permanent stained smear. The permanent smear is stained using specialized dyes, such as trichrome or iron hematoxylin, which make the internal structures of protozoan cysts and trophozoites visible and distinct. The technician systematically examines the slides under high-power magnification, looking for the characteristic size, shape, and internal features of different parasites, such as Giardia or amoebas. Identification relies on recognizing the distinct morphological features of the ova, cysts, and trophozoites, allowing the lab to specify the genus and species of the detected organism.
Interpreting Positive and Negative Results
The results of an O&P test are typically reported as either negative (no parasites seen) or positive (indicating the presence of specific parasitic forms). A positive result confirms the diagnosis of an intestinal parasitic infection. The laboratory report will specify the type of parasite identified, such as Entamoeba histolytica or Giardia lamblia. This specific identification allows the healthcare provider to prescribe a targeted medication that is most effective against that particular pathogen.
A negative result suggests that no parasites or their eggs were found in the sample examined, but it does not completely rule out an infection. Some parasites are shed only intermittently, meaning they may not be present in the stool sample collected on a given day. If a patient’s symptoms persist despite a negative result, the provider may order additional O&P tests, sometimes up to three, to increase the probability of detection. If symptoms continue and the O&P remains negative, the physician may pursue other diagnostic tests to investigate alternative causes for the patient’s digestive issues.