Ova, cysts and parasites (OCP) test

You do not need to do anything special before collecting a sample. Your GP or laboratory will give you a container and instructions. 

Ideally three stool specimens collected over no more than a 10-day period. It is usually recommended that specimens are collected every other day. Collect a fresh stool sample into the clean container provided. The sample must not be contaminated with urine or water. Label the container clearly with your name, and the date and time of collection — the laboratory needs this information to process your sample correctly. 

You may be asked to give up to three samples on different days. This is because many parasites are shed in the stool only occasionally, so a single sample can sometimes miss an infection. Ideally, collect one sample every other day over about a week. 

What the test measures 

The OCP test looks for parasites — tiny organisms that live inside the body and can cause illness. There are two main types that infect the gut: 

• Protozoa — microscopic single-celled organisms such as Giardia intestinalis (also known as G. duodenalis / G. lamblia) and Cryptosporidium. These spread through contaminated water or food, or person-to-person contact. 
• Helminths — worms such as roundworms, hookworms, tapeworms, and threadworms. These usually spread through contaminated soil, food, or water. 

In the laboratory, a thin smear of your stool is placed on a glass slide and examined under a microscope, although microscopy alone has limited sensitivity for some parasites. The laboratory scientist looks for the parasite itself, its eggs (ova), or protective resting forms (cysts). Different parasites have distinctive shapes and sizes that allow them to be identified. 

Many UK laboratories test OCP samples for Giardia and Cryptosporidium using a rapid antigen test, in addition to microscopy. If your doctor suspects a specific parasite, they will include this information on the request form so the laboratory can use the most appropriate method. 

What your results mean 

If parasites are found (positive result) 

A positive result means the laboratory found one or more parasites, eggs, or cysts in your stool. The report will name the parasite identified. Your doctor will work with you to choose the right treatment, which depends on which parasite was found. 

Common parasites found in the UK include: 

• Giardia lamblia — causes Giardia infection (giardiasis). Symptoms include prolonged diarrhoea, bloating, and stomach cramps. It is treated with a course of antibiotics, most commonly metronidazole. 
• Cryptosporidium — causes cryptosporidiosis, often picked up from contaminated water or farm animals. Most healthy people recover without treatment, though it can be serious in people with a weakened immune system. 
• Entamoeba histolytica — causes amoebic dysentery. This is usually picked up abroad. It can cause bloody diarrhoea and, in some cases, affects the liver. 
• Roundworms, hookworms and tapeworms — usually picked up in countries where sanitation is limited. Treatment is with antiparasitic medicines such as mebendazole or albendazole. 

If no parasites are found (negative result) 

A negative result means no parasites were seen in the sample. This is reassuring, but it does not always rule out an infection completely. Parasites are sometimes shed in very small amounts, or only at certain times, so a single sample can miss them. 

If your symptoms continue, your doctor may ask you to give further samples. They may also arrange other tests, such as a stool culture for bacterial infections, or specific antigen tests for Giardia or Cryptosporidium. 

What happens after your test depends on what the laboratory finds. If a parasite is identified, your GP will usually prescribe treatment. The type of medicine depends on the parasite — some infections are treated with a short course of antibiotics, while others clear up on their own with rest and fluids. 

While you are unwell, drink plenty of fluids to avoid dehydration. Do not take anti-diarrhoeal medicines such as loperamide unless your doctor tells you it is safe to do so, as these can sometimes make parasitic infections worse. 

Good hygiene is essential to stop the infection spreading. Wash your hands thoroughly with soap and water after using the toilet, before preparing food, and before eating. Stay away from swimming pools until at least two weeks after your diarrhoea has stopped. 

Parasites are not always present in every stool sample, even when an infection is present. Some parasites are shed only in small amounts or at irregular intervals. This is why multiple samples on different days give a more reliable result than a single sample. 

Certain medicines can interfere with the test. These include antibiotics, antacids containing bismuth, oily laxatives, and antidiarrhoeal medicines. Barium used in X‑ray procedures can also reduce the quality of the result. Tell your doctor about any medicines you are taking before you collect your sample. 

The sample must reach the laboratory quickly. Delays in transit, or contamination of the sample with urine or water, can affect the result. If your laboratory has provided a special preservative container, use it as instructed. 

The OCP test is often requested alongside a stool culture, which checks for harmful bacteria such as Salmonella, Campylobacter, or STEC (E. coli). Your doctor may also request specific antigen tests for Giardia or Cryptosporidium, which can sometimes detect these parasites more reliably than microscopy alone. 

If threadworm (pinworm) is suspected — for example, in a child with itching around the bottom — a perianal swab is a better test than an OCP, as threadworm eggs are usually found around the anus rather than in the stool. Your GP can advise on how to collect this. 

If Schistosoma infection is suspected (usually following travel to sub-Saharan Africa), urine collected between 10am and 2pm may be tested alongside stool.

See also: Stool culture; Shiga toxin-producing Escherichia coli (STEC) test 

Parasitic gut infections are caused by organisms that live and feed in the digestive tract. They range from microscopic protozoa such as Giardia to larger worms. Many are caught abroad, but some — including Cryptosporidium and Giardia — are also found in the UK. For more information, visit NHS: Diarrhoea and vomiting.