Preparation for Flexible Sigmoidoscopy

The colon and rectum must be completely empty for flexible sigmoidoscopy to be thorough and safe, so the physician will probably tell you to drink only clear liquids for 12 to 24 hours beforehand. A liquid diet means fat-free bouillon or broth, gelatin, strained fruit juice, water, plain coffee, plain tea, or diet soda. The night before or right before the procedure, you may also be given an enema, which is a liquid solution that washes out the intestines. Your physician may give you other special instructions.

Fact sheets on other diagnostic tests:
Colonoscopy ERCP Liver Biopsy	Lower GI Series Upper Endoscopy Upper GI Series

source: http://digestive.niddk.nih.gov/ddiseases/pubs/sigmoidoscopy/index.htm