Diarrhea

How is the Cause of Diarrhea Diagnosed?
Diagnostic tests to find the cause of diarrhea may include the following:

• Medical History and Physical Examination
  The doctor will ask you about your eating habits and medication use and will examine you for signs of illness.
• Stool Culture
  A sample of stool is analyzed in a laboratory to check for bacteria, parasites, or other signs of disease and infection.
• Blood Tests
  Blood tests can be helpful in ruling out certain diseases.
• Fasting Tests
  To find out if a food intolerance or allergy is causing the diarrhea, the doctor may ask you to avoid lactose, carbohydrates, wheat, or other foods to see whether the diarrhea responds to a change in diet.
• Sigmoidoscopy
  For this test, the doctor uses a special instrument to look at the inside of the rectum and lower part of the colon.
• Colonoscopy
  This test is similar to a sigmoidoscopy, but it allows the doctor to view the entire colon.
• Imaging Tests
  These tests can rule out structural abnormalities as the cause of diarrhea.

References:
March 2007
NIH Publication No. 07-2749
digestive.niddk.nih.gov/ddiseases/pubs/diarrhea/