The O&P exam is used to diagnose the cause of prolonged diarrhea. It is ordered to determine whether there are parasites present in the gastrointestinal tract and, if so, to identify them. Since there are many other causes of diarrhea, the O&P is often ordered along with other tests, such as a stool culture, which identifies the presence of pathogenic bacteria in the stool. Classically, O&P exams were ordered in multiples, such as three samples from three different bowel movements, often on separate days. This was considered to be the best way to detect what might be small amounts of ova in the stool, providing a better chance of not missing an infection. Your doctor may order a giardia, cryptosporidium, or E. histolytica antigen test if he suspects that one of these parasites may be causing your diarrhea. These tests detect protein structures on the parasites and can identify an infection, even if no actual parasites or ova are seen in the stool. Since antigen tests only detect a few specific parasites, they are not replacements for the complete O&P, which will detect a wider variety of parasites.

O&P tests may also be ordered to monitor the effectiveness of treatment for a parasitic infection.

When you have prolonged diarrhea, abdominal pain, and/or blood and mucus in your stool. When you have symptoms and have recently drunk stream or lake water while camping, been exposed to someone who has a parasitic infection (like a family member), or traveled outside the U.S.

O&P tests may also be ordered when you have had a parasitic infection, to monitor the effectiveness of treatment.

If there are no ova or parasites seen, your diarrhea may be due to another cause. There may also be too few parasites to detect. Your doctor may want to order additional O&P tests and/or may order other tests to search for the cause of your symptoms.

If a parasite is identified, then you do have a parasitic infection. The type and duration of treatment will depend on what kinds of parasite(s) are found and on your general state of health. The number of parasites seen may give your doctor general information about how heavy or extensive your infection is.

If diarrhea lasts more than a few days, it may lead to dehydration and electrolyte imbalance, dangerous conditions in children and the elderly.

Drug therapies are usually used to treat giardia and E. histolytica infections. They may resolve themselves after several weeks, but they may also cycle, with symptoms subsiding and then worsening again. There is no effective treatment for cryptosporidium. In those with competent immune systems, crypto generally goes away after a few weeks. In those with compromised immune systems (such as those with HIV/AIDS, organ transplant, cancer, etc.), however, crypto may be dangerous, becoming chronic and causing wasting and malnutrition.

Parasitic infections are monitored on a community level. Other than travel-related cases, health officials want to try to determine where your infection came from so that they can address any potential public health concerns. For instance, if crypto or giardia is due to contaminated swimming pool water or community water supply, steps will need to be taken to prevent the spread of the infection. Parasites can be eliminated from the water supply with chlorine treatment and/or with adequate filtration.