Melaena is the passage of black, tarry stools. The stools have a characteristic and offensive smell due to the presence of blood that has been digested by intestinal enzymes and bacteria. The degradation of the blood also accounts for the dark colouration.
It usually implies a bleed at some point early in the gastrointestinal system proximal to the splenic flexure of the colon (usually the oesophagus, stomach or duodenum). There must be the loss of more than 60 ml of blood in the upper gastrointestinal tract before melaena occurs.
Most commonly, it is caused by acute or chronic peptic ulceration; less commonly, by right-sided colonic bleeding; and rarely, by small bowel bleeding.
Differential diagnosis of melaena includes dark stools due to ingestion of iron tablets, liquorice, charcoal or bismuth. Note that these substances tend to cause small well-formed non-tarry stools and there is no associated offensive smell.
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