Diagnosis

Your doctor will ask about your medical history, review the medications you take, conduct a physical exam and may order tests to determine what's causing your diarrhea. Possible tests include:

• Blood test. A complete blood count test, measurement of electrolytes and kidney function tests can help indicate the severity of your diarrhea.
• Stool test. Your doctor might recommend a stool test to see if a bacterium or parasite is causing your diarrhea.
• Hydrogen breath test. This type of test can help your doctor determine if you have a lactose intolerance. After you drink a liquid that contains high levels of lactose, your doctor measures the amount of hydrogen in your breath at regular intervals. Breathing out too much hydrogen indicates that you aren't fully digesting and absorbing lactose.
• Flexible sigmoidoscopy or colonoscopy. Using a thin, lighted tube that's inserted in your rectum, your doctor can see inside your colon. The device is also equipped with a tool that allows your doctor to take a small sample of tissue (biopsy) from your colon. Flexible sigmoidoscopy provides a view of the lower colon, while colonoscopy allows the doctor to see the entire colon.
• Upper endoscopy. Doctors use a long, thin tube with a camera on the end to examine your stomach and upper small intestine. They may remove a tissue sample (biopsy) for analysis in the laboratory.