Introduction to haematochezia and melaena

30 Introduction to haematochezia and melaena


Blood may be present in diarrhoea of either large or small intestinal origin; however, sometimes the passing of blood is the predominant presenting clinical feature. Melaena refers to a dark tarry appearance to the stool which is caused by the presence of digested blood. The dark colour is due to the oxidation of haemoglobin and the tarry appearance is due to bacterial breakdown of haemoglobin. In dogs 300 to 500 mg/kg of haemoglobin must be present in the upper gastrointestinal (GI) tract before melaena is visually present, so its absence does not rule out GI bleeding.


In humans, blood must be present in the GI tract for at least 8 hours to turn a black colour, so a slow colonic transit time can cause melaenic stools from bleeding from a colonic area. Generally though, melaena is due to bleeding from the proximal GI tract or swallowed blood from a nasal, oral, oesophageal, pharyngeal or respiratory tract lesion. The use of salicylates, bismuth or charcoal can colour the faeces black as well.


Haematochezia is the presence of bright red blood in the faeces. If fresh blood is present on the outside of the faeces it is more likely to be from the distal colon or rectum.


Confirming the presence of melaena should be performed using a faecal occult blood test after the patient has been fed a meat-free diet for 3 days. In patients with undiagnosed anaemia, a faecal occult blood test should also be performed, as smaller amounts of GI bleeding can result in an apparently normal coloured stool.


Differential diagnoses for melaena include:


• Swallowed blood, e.g. from oro-nasal lesions

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Sep 22, 2016 | Posted by in SMALL ANIMAL | Comments Off on Introduction to haematochezia and melaena

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