Physicians often rely on a patient’s report of symptoms in diagnosing the cause of and determining treatment for diarrhea. A medical history will include questions about the duration and severity of symptoms, eating habits and current medications. A physical examination will allow a physician to look for signs of illness that may be causing the diarrhea.

Most people with diarrhea will not need extensive testing. When diarrhea goes away by itself, it is not usually necessary to submit to various tests to determine its cause. When tests are used, they may include:

• Stool tests. Stool is tested for bacteria, parasites or other signs of infection that may be causing the diarrhea.
  
  
• Blood tests. Blood is drawn and tested for evidence of disease that may be causing the diarrhea.
  
  
• Fasting tests. Patients are asked to avoid certain products to determine whether an allergy or food intolerance may be responsible for the diarrhea. Bowel habits are observed to identify whether a change in diet affects the diarrhea. Physicians commonly ask patients to avoid dairy products, carbohydrates and wheat, although other foods may be included. Milk may be purchased with added lactase, and yogurt is usually well tolerated.
  
  
• Endoscopy. A flexible tube (with attached light and camera) is inserted into a patient’s digestive tract to identify any abnormalities that may be causing the diarrhea. Biopsies and color photographs may be taken during this procedure. The tube is inserted via the anus and usually through the entire colon in a colonoscopy. A sigmoidoscopy is similar, but only involves the lower part of the colon (the sigmoid colon).