The stool ova & parasites (O & P) test involves examination of a stool (feces) sample for the presence of intestinal parasites. The distinct types of parasites differ with regard to their structures, life stages, and transmission forms. A parasite may be a worm that has a mature form, an immature form (larvae), and eggs (ova). A parasite may be a protozoa with an adult form that lives in the intestines (trophozoite) and a round, encapsulated tranmissiable form (cyst). Stool analyses examine all parasitic forms that may be present in the sample.
The ova and parasites test is performed to identify intestinal parasites and their eggs or cysts in patients with symptoms of gastrointestinal infection. Patients may have no symptoms, or may experience diarrhea, blood in the stools, and other gastrointestinal distress. Stool O & P testing is usually ordered along with tests for the bacterium Clostridium difficile as well as a stool culture since overlapping symptoms may result from bacterial or parasitic infections. Identification of a particular parasite indicates the cause of the patient's disease and determines the medication needed to treat it.
Health care providers should always use proper infection control procedures when handling stool samples since they are potentially infectious material.
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:
- roundworms: Ascaris lumbricoides
- hookworms: Necator americanus
- pinworms: Enterobius vermicularis
- whipworm: Trichuris trichiura
- 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.
Amoeba type of protozoa (one-celled animal) that can move or change its shape by extending projections of its cytoplasm.
Bismuth substance used in medicines to treat diarrhea, nausea, and indigestion.
Cryptosporidium type of parasitic protozoa.
Centrifugedpun via a centrifuge. A centrifuge is a specialized machine used to spin a test tube at a very fast rotation in order to separate the particles inside the tube into different layers according to their density.
Fecesaterial excreted by the intestines.
Flagellate microorganism that uses flagella (hair-like projections) to move.
Gastrointestinalhe digestive tract; the stomach and intestines.
Isospora belli type of parasitic protozoa.
Microsporidia phylum of small parasitic protozoans.
Parasiten organism that lives on or inside another living organism (host), causing damage to the host.
Protozoane-celled eukaryotic organisms belonging to the kingdom Protista.
Immunological testing of stool is a faster diagnostic tool and does not require knowledge of the structures and life stages of parasites. Fresh or fresh-frozen stool is diluted, filtered, and added to a commercial device containing antibodies that will react if several specific parasite antigens are present in the stool sample. If the antigen of the parasite is present, a purple color is produced. However, this type of testing can only be conducted with unpreserved stool and can only assist in identifying a few common parasites.
Insurance coverage for stool ova and parasites may vary among different insurance plans. This test usually is covered if ordered by a physician approved by the patient's insurance plan, and if it is done at an approved laboratory. However, since insurance plans vary greatly, patients should contact their insurance company with regard to specifics.
The physician, or other healthcare provider, will ask the patient for a complete medical history, and perform a physical examination to determine possible causes of the gastrointestinal symptoms. Information about the patient's diet, any medications taken, and recent travel may provide clues to the identity of possible infectious parasites.
Patients should avoid taking any medications or treatments containing mineral oil, castor oil, or bismuth, magnesium or other antidiarrheal medicines, or antibiotics for up to 10 days before collecting the specimen.
The patient should avoid taking preparations that interfere with specimens for the duration of time the specimen collection is required.
There are no complications associated with a patient providing a stool sample for stool O & P testing.
Normally, parasites and eggs are not found in stools. Some parasites are not pathogenic (for example, Endolimax nana and Iodamoeba butschlii), which means they do not cause disease. If these are found, no treatment is necessary. The presence of any pathogenic parasite indicates an intestinal parasitic infection. Depending on the parasite identified, other tests may be required to determine if the parasite has invaded other parts of the body. Some parasites travel from the intestines to other parts of the body and may already have caused damage to other tissues by the time a diagnosis is made. For example, the roundworm Ascaris penetrates the intestinal wall and can cause inflammation in the abdomen. It can also migrate to the lungs and cause pneumonia. This kind of injury can occur weeks before the roundworm eggs appear in the stool.
Other types of damage caused by intestinal parasites include anemia due to hemorrhage caused by hookworms, and anemia caused by depletion of vitamin B12 due to infection with the tapeworm Diphyllobothrium latum.
When a parasite is identified, the patient can be treated with the appropriate medications to eliminate the parasite.
Health care team roles
A physician orders a stool O & P test. Stool samples may be collected by a physician, nurse, physician assistant, or other trained health care professionals. Laboratory professionals (usually called clinical laboratory scientists or medical technologists) who perform microscopic tests for stool ova and parasites have received specialized training in preparing, handling, and examining the samples. These professionals have been trained to look for specific characteristics of parasite forms that will lead to accurate diagnosis and treatment for the patient.
Health care providers should teach the patient how to use the collection kit, how many samples will be required, and how to keep the samples free from contamination. Usually patients should be instructed to take the stool sample in the morning before bathing or taking a shower. Patients should be taught how to avoid re-infection based on how the parasite is contracted when a definite diagnosis is made. For example, patients with pinworms should practice sound personal hygiene in the future such as washing hands after using the restroom and before eating, and wearing clean undergarments daily. Patients with tapeworms should avoid eating specific raw or undercooked meat or fish in the future. Health care providers should also stress that patients follow the full duration of treatment as required to eliminate the parasite.
Fischbach, Frances. "Diagnosis of Parasitic Disease." In A Manual of Laboratory & Diagnostic Tests. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2000, pp. 51621.
Forbes, Betty A., Daniel F. Sahm, and Alice S. Weissfeld. "Laboratory Methods for Diagnosis of Parasitic Infections." In Bailey & Scott's Diagnostic Microbiology. 10th ed. St. Louis: Mosby, 1998, pp. 78487.
Kee, Joyce LeFever. "Ova and Parasites (O and P) (Feces)." In Laboratory & Diagnostic Tests with Nursing Implications. 5th ed. Stamford, CT: Appleton & Lange, 1999, pp. 32021.
Tierney, L.M., S.J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 1998. Stamford, CT: Appleton & Lange, 1998.
The American Society for Microbiology. 1752 N St. N.W., Washington, DC 20036. (202) 737-3600. <<a href="http://www.asmusa.org">www.asmusa.org>.
Centers for Disease Control and Prevention. National Center for Infectious Disease. Division of Parasitic Diseases. 1600 Clifton Road NE, Atlanta, GA 30333. (800) 311-3435. <<a href="http://www.cdc.gov/ncidod/dpd">www.cdc.gov/ncidod/dpd>.
"Infectious Diseases; Parasitic Infections." In The Merck Manual of Diagnosis and Therapy. 17th ed. (internet Edition), <<a href="http://www.merck.com/pubs/mmanual">http://www.merck.com/pubs/mmanual>, 1999.
Linda D. Jones, B.A., PBT (ASCP)
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