Melena, characterized by black tarry stools, can occur with as little as 50 cc of blood in the stomach. How do we know this? We need to go back to clinical experiments involving oral administration of citrated blood in human subjects back in 1930’s and 40’s. 1-3 One study was performed on a group of “healthy medical students” who drank their own blood!3
Melena suggests an upper GI bleeding source where there is more time for enzymatic breakdown to transform blood to melena. Although gastric acid may also contribute to its formation, it does not appear to be a pre-requisite to melena as blood inserted into the small bowel or cecum can also produce melenic stools if it stays there long enough. Melena is dependent primarily on the length of transit time of blood in the GI tract, such that very rapid movement of 1 liter of blood from upper GI tract may lead to bright red blood per rectum, not melena, within 4 hours.2,4
Don’t get melena confused with other causes of dark stools such as oral iron supplementation and bismuth-containing medications (eg, Peptobismol®). In addition to its tarry texture, melena also has a characteristic pungent odor.
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References
- Schiff L, Stevens R, Shaprio N, et al. Observations on the oral administration of citrated blood in man. Am J Med Sci 1942;203:409-12.
- Srygley FD, Gerardo CJ, Tran T, et al. Does this patient have a severe upper gastrointestinal bleed. JAMA 2012;307:1072-79. https://jamanetwork.com/journals/jama/article-abstract/1105075?redirect=true
- Daniel WA, Egan S. The quantity of blood required to produce a tarry stool. JAMA 1939;113:2232.
- Wilson ID. Hematemesis, melena, and hematochezia. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The history, physical, and laboratory examinations. 3rd edition. Boston: Butterworths:1990. Chapter 85. Available from: https://www.ncbi.nlm.nih.gov/books/NBK411/
Contributed in part by Brad Lander, MD, Mass General Hospital, Boston, MA.
