Tests and diagnosis:

The cause of your diarrhea may be difficult to diagnose. Your doctor likely will take your medical history, ask you about recent activities, such as traveling, and perform a physical exam. A number of lab tests help diagnose parasitic diseases:

• Stool (fecal) exam. Also called an ova and parasite test, this test looks for parasites or their eggs (ova) that cause signs and symptoms, such as diarrhea and abdominal cramping and bloating. Your doctor may ask you for several stool samples, each from different days.

Your doctor may give you a special container with preservative fluid for your stool samples. If not, refrigerate your samples until you take them to your doctor's office or to the specific lab as per your physician's instructions.  Do not freeze the samples.

• Endoscopy. If you have diarrhea, but the fecal exam doesn't reveal the cause, your doctor may request this test. A doctor, usually a gastroenterologist, inserts a tube into your mouth or rectum to look for the cause of your symptoms. You'll be sedated for this test, and you'll need to fast beginning at midnight the night before the test.
• Blood tests. Although there are no blood tests that can detect Blastocystis hominis, your doctor may order blood tests to look for other causes of your signs and symptoms. That's because when your body is infected with a parasite, your immune system tries to fight the infection, which leaves antibodies or parasite antigens or both in your system. A lab can detect some parasites by examining a sample of your blood for the antibodies or antigens.
• Scans. Some parasitic diseases may cause swelling or scarring of internal organs. Your doctor may request an X-ray, magnetic resonance imaging (MRI) or a computerized tomography (CT) scan to detect the swelling or scarring.