What is the difference between tarry stools and black stools in terms of gastrointestinal (GI) bleeding?

Tarry Stools vs. Black Stools: Clinical Distinction

Tarry stools (melena) specifically indicate digested blood from the gastrointestinal tract and have a characteristic sticky, tar-like consistency, while black stools can result from various causes including dietary factors, medications, or blood—making melena a distinct clinical entity that always warrants evaluation for GI bleeding. 1, 2

Key Distinguishing Features

Melena (Tarry Stools)

• Defined as black, tarry, sticky stools with a distinctive foul odor that results from bacterial degradation of hemoglobin in the GI tract 1, 2
• Requires approximately 50-100 mL of blood in the upper GI tract and several hours of transit time for the characteristic appearance to develop 3
• The "tarry" descriptor is critical—the stool has an adhesive, sticky quality that distinguishes it from simple black coloration 1
• Most commonly originates from upper GI sources including peptic ulcers, gastroduodenal erosions, esophagitis, varices, and Mallory-Weiss tears 2

Black Stools (Non-Melenic)

• Can result from iron supplements, bismuth-containing medications (Pepto-Bismol), activated charcoal, or foods like black licorice or blueberries 3
• Lack the sticky, tar-like consistency and characteristic odor of melena
• Typically formed and do not have the adhesive quality when examined
• Do not represent digested blood and generally do not require urgent GI evaluation unless other concerning features are present

Clinical Significance and Localization

Source Prediction

• Melena strongly predicts upper GI bleeding with a positive predictive value of 0.95-0.98 when the characteristic black tarry appearance is present 4
• Darker blood mixed in stool suggests a more proximal bleeding source compared to bright red blood 5
• The British Society of Gastroenterology formally recognizes melena as a key indicator of upper GI hemorrhage requiring evaluation 1

Important Caveat

• Massive upper GI bleeding can present as bright red blood per rectum (hematochezia) rather than melena if transit time is rapid enough 5
• Conversely, slow bleeding from the right colon can occasionally produce dark stools, though typically not true melena 5

Evaluation Approach

When Melena is Present

• Immediate hemodynamic assessment including pulse, blood pressure, and perfusion status 6, 3
• Establish large-bore IV access and initiate resuscitation if unstable 6
• Esophagogastroduodenoscopy (EGD) within 24 hours is the diagnostic procedure of choice, with earlier timing for hemodynamically unstable patients 2, 6, 3
• Laboratory evaluation should include complete blood count, basic metabolic panel, coagulation studies, and type and crossmatch 3
• Transfuse to maintain hemoglobin above 7 g/dL (or 9 g/dL with massive bleeding or cardiovascular comorbidities) 5, 2

When Simple Black Stools are Present

• Obtain detailed medication and dietary history to identify non-bleeding causes
• If no clear benign explanation exists and patient has risk factors (anticoagulation, NSAID use, prior GI bleeding, older age), proceed with evaluation as for melena 3
• Do not assume black stools are benign without proper assessment, as coffee ground emesis and melena can indicate serious underlying pathology 6, 7

Common Pitfalls to Avoid

• Never rely solely on patient descriptions of stool color—objective assessment is more reliable, as patients use 23 different descriptors for stool blood color with frequent discrepancies 4
• Do not attribute fecal occult blood positivity to hemorrhoids without adequate colonic evaluation, as hemorrhoids alone do not cause positive guaiac tests 5
• In hemodynamically stable patients with coffee ground emesis or melena, evaluate for non-GI conditions (myocardial infarction, sepsis, pulmonary embolism, renal failure) even while managing the GI bleed 7
• Do not delay endoscopy beyond 24 hours in patients with melena, as early intervention reduces mortality 2, 3