Rectal Bleeding

What is rectal bleeding?

Blood lost from any part of the digestive tract, from oesophagus to the rectum (final part of the large intestine or colon) will eventually come out from the lower end.

The general rule of thumb is that the higher up the digestive tract the bleeding comes from, the more altered the blood will be. For e.g. bleeding from a peptic ulcer in the stomach or duodenum may show up as tarry, black, sticky, smelly stools called melaena. This is because the blood has been metabolised by gut bacteria along the way. However, in this article we will not discuss upper digestive tract bleeding, but will focus on bleeding that patients can identify, that is red, or reddish blood coming out from the rectum through the anus, which is more commonly from somewhere in the colon.

What does rectal bleeding look like?

Rectal bleeding is one of the most alarming symptoms that patients complain of. The appearance of bright red, fresh blood from the anus, whether alone or in relation to passing stools, usually sends patients rushing to see their doctor or go to an Emergency Department of a hospital. The quantity could be small, such as only staining toilet paper, or could be large, dripping into the toilet bowl, sometimes with red splashes all over the toilet bowl, which is an alarming sight for most people. If the bleeding is profuse, there could even be blood clots seen.

Blood from the right side of the colon (furthest away from the anus) is also subject to some digestion along the way out and when it appears it could be a darker red or maroon colour compared to fresh blood. Right sided bleeding could result in stools mixed with blood. Therefore the precise colour of the blood passed and whether there is associated stool could give a clue as to which part of the colon the bleeding is from.

What are the causes of rectal bleeding?

There are many causes of rectal bleeding, some more dangerous than others. Other symptoms associated with the rectal bleeding could give clues as to the diagnosis. Some of the more common causes of rectal bleeding are:

Anal fissures:

  • These are tears of the lining in the anal canal sometimes due to hard stools
  • There typically an intense sharp or tearing pain in the anus upon passing stools, which gives away the cause.
  • The blood is very fresh red.


  • Haemorrhoids are protrusions of the lining of the lower rectum containing blood vessels. They can be internal, can prolapse outside and go back inside, or remain prolapsed outside.
  • If the haemorrhoids are internal, bleeding could be otherwise asymptomatic.
  • If the haemorrhoids protrude outside, the patient may feel a lump at his anus.
  • Sometimes the haemorrhoids get inflamed and painful, especially if prolapsed outside and unable to reduce back inside.

Inflammatory bowel disease:

  • This is a chronic inflammatory disease of the digestive tract, either “Ulcerative Colitis” or “Crohn’s Disease”.
  • When the lower colon or rectum is involved, there could be inflammation and ulceration of the lining leading to bleeding.
  • There are often other symptoms such as general un-wellness, bloody diarrhoea, loss of weight or a heavy feeling in the rectum.


  • There are often out-pouches of the lining of the colon, sometimes more on the right side, sometimes more on the left side, or even both. These are due to years of high pressure inside the colon. These out-pouches are called diverticulosis.
  • They are generally benign, but sometimes one or more of these diverticula get inflamed and burst a blood vessel inside leading to bleeding. Inflammation of the diverticula is called diverticulitis.
  • There may be a history of previous right or left sided pain due to intermittent inflammation without bleeding. There may have been a previous diagnosis of diverticulosis from a colonoscopy.

Ischaemic colitis:

  • “Ischaemic” means lacking in blood supply. This is when a patient’s blood vessels (arteries) are diseased with build-up of plaques inside narrowing the passage of the vessels.
  • Beyond a critical point, the blood supply to the colon is inadequate, leading to an unhealthy lining and bleeding.
  • Patients may have prior diagnosis of other arterial disease such as ischaemic heart disease (angina, heart attack, heart failure etc.) which may give a hint.

Polyps or cancer:

  • Growths within the colon can bleed when the surface of the lesion is fragile.
  • Large polyps that are benign can bleed when “scratched” by passing stool.
  • Cancer of the colon and rectum has very fragile linings and often bleed. When the bleeding is significant, it is seen as rectal bleeding.
  • Advanced cancer may come with generalised symptoms such as weight loss, abdominal pain or abdominal distention.

There are of course many other causes which are less common, such as infective gastroenteritis, post radiation colitis, abnormal blood vessels (angiodysplasia or Dieulafoy lesions), liver cirrhosis with increased pressure of the digestive venous system, systemic diseases leading to ease of bleeding such as Dengue Fever and other causes of blood clotting disorders, use of anticoagulant medicines, etc.

How would a doctor investigate rectal bleeding?

The evaluation would start with taking a detailed history of the problem, together with a thorough clinical examination, which would include a finger rectal examination or a proctoscope (short tubular device to see inside the rectum).

The doctor must assess whether the amount of bleeding, either small amounts over a long period of time or a large volume all at once, has led to compromise of the patients’ blood volume with a fall of blood pressure and rise of pulse rate. If it is a chronic bleed, the patient may be pale from anaemia. If resuscitation is necessary, this is then a priority.

Assuming fluid resuscitation (intravenous injection of salt solution or blood transfusion) is not urgent, blood tests would need to be done to check for systemic disease that may be related to the bleeding.

Endoscopy is a mainstay of diagnostic testing. Gastroscopy is not usually done for rectal bleeding (it is more appropriate when the source is thought to be from the upper digestive tract), but colonoscopy is routine for rectal bleeding.

If the source of bleed is not readily apparent from colonoscopy, other modalities include angiography, where a radiologist puts a long narrow tube (catheter) into the artery at the groin and pushes it up to the arteries that supply the intestines and injects a special radio-opaque contrast that shows up where the bleeding is occurring. Another method is nuclear scintigraphy, where a labelled molecule is injected into the blood and a leak of blood is picked up by a scan.

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