Severe upper gastrointestinal bleeding (the patient will usually be shocked and, in contrast to a lower GI bleed, plasma urea will be disproportionately raised).

Diverticular disease (the commonest cause particularly in the elderly, usually with no pre-existing symptoms).

Ulcerative colitis (almost invariably associated with diarrhoea and a pre-existing history; inflammatory markers may be normal in left-sided disease).

Left-sided colonic cancer (usually low-grade but recurrent bleeding; should always be actively excluded as a cause).

Post-polypectomy bleeding (delayed bleeding occurs on average 5–7 days post-procedure; risk factors include: polyps >10 mm, age >65, cardiovascular or renal disease, use of anti-coagulant or antiplatelet agents).

Ischaemic colitis (older adult with risk factors for arterial thromboembolism (atherosclerosis or vasculitis) or acute hypotension, e.g. post cardiac arrest; there is usually associated abdominal pain).

Angiodysplasia (venous bleeding and therefore less severe).

Radiation colitis (often following radiotherapy for cancer of prostate; can cause acute bleeding shortly after treatment and also delayed bleeding, typically within two years).

Rectal varices (these are relatively common in all cases of portal hypertension but seldom cause significant bleeding; they should be distinguished from haemorrhoids).

Haemorrhoids (will still require endoscopic evaluation to exclude a more serious cause).