The stool O & P test is also called the stool ova and parasites test or the ova and parasites collection. Examination of the stool for ova and parasites is done to diagnose parasitic infection of the intestines. Parasites can go through several different life stages depending on the unique characteristics of each type of parasite. For example, the parasite Entamoeba histolytica causes amebiasis, a parasitic intestinal infection that can cause diar-rhea and cramps. This disease is common in developing countries with poor sanitation or in the United States in institutions with poor hygiene practices. The stool of an infected person contains cysts of the parasite. These cysts have a protective covering and can survive outside the body in feces. If food or water is contaminated with such feces, another person can consume the cysts. Mature cysts that are ingested then turn into trophozoites that feed inside the large intestine. Some trophozoites then begin to encyst and create protective walls around their small, round center. These cysts are then expelled from the body in feces that can infect food or water, and the transmission process to another person is repeated. Stool O & P tests require the health care professional to identify parasites in a variety of structural forms.
The most common intestinal parasites in North America that cause infections are:
tapeworms: Diphyllobothrium latum, Taenia saginata, and Taenia solium
protozoa: Entamoeba histolytica (an amoeba), and Giardia lamblia (a flagellate)
Numerous other parasites are found in other parts of the world. These may be contracted by travelers to other countries. Patients with acquired immune deficiency syndrome (AIDS) or other immune system disorders are commonly infected with the parasites in the Microsporidia phylum, Cryptosporidium, and Isospora belli.
A stool O & P test may be performed in the physician's office or at an external laboratory. There are several commercial kits with instructions that patients can use at home to collect stool samples. These kits are comprised of sterile containers containing special chemical fixatives. The feces should be collected directly into the container and the patient should be careful not to contaminate the sample with urine, water, or other materials. Three specimens are collected, usually two or three days apart. However, as many as six specimens may be needed within 14 days to identify some organisms (like E. histolytica). A specimen held at room temperature should be examined within three hours. If testing is delayed, the sample may be refrigerated for two to three days or preserved. If a preservative is used, it must preserve all forms of the parasite (including eggs, or cysts and trophozoites, etc.) without interfering with the testing required for the stool sample. A commonly used preservative is the combination merthiolate-iodine-formalin (MIF).
In the laboratory, the stool sample is examined for a variety of parasitic forms. Some parasites are large enough to be seen without a microscope. For others, microscope slides are prepared with either fresh unstained stool or stool dyed with special stains. These preparations are viewed with a microscope to detect the presence of parasites or their eggs.
The recovery of ova or parasite forms depends upon the consistency of the stool sample, which suggests the parasitic stage is likely to be present. For example, if the stool specimen is soft or loose, it may be more likely to contain trophozoites. If the stool specimen is formed, then it may be more likely to contain cysts.
A stool examination usually requires three procedures: a direct wet mount, a concentration test, and a permanent smear. A direct wet mount requires preparing a slide with an appropriate fecal sample and then viewing the slide under a microscope for evidence of parasites. In a concentration by sedimentation test, chemicals (most often ethyl acetate and formalin) are used to separate the parasites from other fecal material (oils, fats, etc.). When a test tube containing the sample and these chemicals is centrifuged, the sedimentation on the bottom of the tube contains the parasite forms while the fats and other substances are closer to the top of the tube. The sediment is then appropriately processed and examined for parasite forms. A permanent smear is made by preparing a slide with a fecal sample and adding Gomori trichrome stain. When viewed with a microscope, the background appears blue-green while parasite forms stain blue-green and red. This test is required to identify trophozoites and is the most sensitive of the three tests.
Sometimes another method of examination must be used, as is the case for Cryptosporidium. Modified acid-fast staining must be used for this organism. When this stain is used, forms of the organism (oocysts) turn red.
Obtaining a specimen to identify pinworm (E. vermicularis) infection requires a different technique. Adult parasites lay eggs outside of the intestines on the skin folds of the anus. Eggs are usually not present in stool. Clear adhesive tape or a sticky swab or paddle is applied to the anus. Eggs then stick to the tape, swab, or paddle and can be examined microscopically. When adhesive tape is used, this technique is often called the "scotch tape method" of collection.
Linda D. Jones B.A., PBT (ASCP), The Gale Group Inc., Gale, Detroit,