Stool Color and What It Indicates

Black / Tarry Stool (Melena)

• Classically described as black, sticky, foul-smelling, tarry stool.
• Indicates upper GI bleeding (esophagus, stomach, duodenum) - blood is altered by gastric acid and gut bacteria as it transits the bowel, turning it black. Bleeding as little as 50-100 mL can produce melena - Frameworks for Internal Medicine.
• Slow-transit small bowel or right colon bleeding can also present as melena rather than red blood - ROSEN's Emergency Medicine.
• Non-bleeding causes: iron supplements, bismuth (Pepto-Bismol), or large amounts of black foods (blueberries, black licorice).

Red / Maroon Stool (Hematochezia)

• Passage of red or maroon blood per rectum, usually reflecting lower GI bleeding (colon, rectum, hemorrhoids, diverticulosis, colitis, malignancy) - Frameworks for Internal Medicine.
• Important caveat: about 10% of hematochezia is actually from a brisk upper GI bleed moving through the gut too fast to be altered into melena, so hematochezia with hemodynamic instability should still raise concern for an upper source - Tintinalli's Emergency Medicine.
• Non-bleeding causes: beets, red food dyes, tomato-based foods.

Pale / Clay-Colored / Gray Stool (Acholic Stool)

• Occurs when little or no bile pigment reaches the stool, typically from biliary obstruction (gallstone, pancreatic or bile duct tumor, biliary stricture) or severe hepatocellular disease impairing bile secretion.
• Onset is usually gradual as the obstructive process progresses.
• Often accompanies jaundice and dark urine (conjugated hyperbilirubinemia backing up into the blood/urine while failing to reach the gut).

Yellow / Greasy / Floating Stool (Steatorrhea)

• Suggests fat malabsorption, seen in pancreatic insufficiency (chronic pancreatitis, pancreatic cancer), celiac disease, or biliary disease impairing fat digestion.
• Stool is bulky, greasy, foul-smelling, and may float due to trapped fat/gas.

Green Stool

• Usually benign: rapid intestinal transit (bile doesn't have time to fully break down to brown), or ingestion of green vegetables/food dye/iron supplements.
• Can also occur with infectious diarrhea.

Occult (Not Visibly Colored) Blood

• Chronic, low-volume GI bleeding may not change visible stool color at all and is detected only by fecal occult blood testing, often presenting instead as iron-deficiency anemia - Goldman-Cecil Medicine.

When to Seek Care