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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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The evaluation of changes in stool color typically begins with a thorough physical examination and personal medical history. The doctor may ask about intake of alcohol, smoking, and other habits. Family history of any cancers, particularly of the liver or pancreas or bleeding problems may be helpful. A review of medications that may affect the color of stool, including over-the-counter (OTC) medications, also is important. Any changes in bowel habits (constipation, diarrhea, and change in frequency) or any pertinent symptoms (pain with or without eating, nausea, vomiting, weight loss, etc.) can provide valuable clues in evaluating the underlying causes of changes in stool color.

Diagnostic testing to find the cause of changes in stool color typically start with simple blood tests including complete blood count (CBC),blood  chemistries, liver enzymes (comprehensive metabolic panel or CMP or SMA 19), and blood clotting assays (tests of coagulation). These tests can demonstrate anemia, liver disease, gallbladder disease, or other underlying conditions that may be responsible for the changes in stool color. Pancreatic enzymes--amylase and lipase--also can be measured to determine if pancreatic disease may be present.

If bleeding from the stomach or intestines is suspected but the stool is not visibly black, red or maroon, occult stool blood testing  (a small amount of blood that does not cause the color of stool to change much) can be done. This test is achieved by testing the stool directly for blood with a dye (fecal occult blood test or FOBT). This test relies on a chemical reaction between a solution (called guaiac) and hemoglobin in a sample of stool. In the presence of hemoglobin, the drop of solution will turn the stool sample (smeared onto a special paper which reacts chemically with the solution) blue. This test is part of the recommendation for screening for colon cancer, although in clinical practice, it is often used to determine if any bleeding is occurring in the gastrointestinal system. In addition to the test using guaiac, there is an immunological test for blood in the stool that uses an antibody to hemoglobin to detect the blood.

Methods to evaluate a change in the color of stool are upper gastrointestinal endoscopy (esophago-gastro-duodenoscopy or EGD) and colonoscopy. These tests are done by gastroenterologists to look inside the esophagus and stomach (EGD) and the colon (colonoscopy) with a video camera to detect the source of the bleeding or other abnormality that may explain the change in stool color. If necessary, biopsies can be taken with these techniques.

More advanced endoscopic testing to look for obstruction of the biliary or pancreatic ducts is done by endoscopic retrograde cholangio-pancreatography or ERCP. This test is performed like an EGD except that during the test dye is injected into the biliary and pancreatic ducts to look with X-rays for obstruction of the ducts.

Other imaging studies sometimes are necessary in order to find the cause of the change in stool color. Computerized tomography (CT scan) is ordered frequently by physicians if the change in stool color is believed to be related to underlying cancer, pancreatic disease, or obstructive conditions of the bile ducts and gallbladder. Ultrasound of the abdomen is a frequently used, relatively inexpensive and reliable test to evaluate for gallstones or blockage of the gallbladder. Magnetic resonance imaging (MRI) of the abdomen sometimes is done to look more closely at any obstructive disease of the biliary or pancreatic ducts.

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