What Causes Blood in the Stool?

Blood in the stool can appear in various forms, from a few drops of bright red blood coming from the back passage through to a stool that is entirely black and tarry in appearance.

Bleeding may arise from common causes such as haemorrhoids or an anal fissure, but it can sometimes be a sign of conditions that need to be investigated, such as bowel polyps, inflammatory bowel disease or cancer of the large bowel.

For this reason it is not right to put blood seen in the stool down to haemorrhoids automatically.

Does the colour of the blood matter?

Yes. The colour of the blood can give important clues about the source of the bleeding.

Bright red blood

Bright red blood seen on the lavatory paper, on the surface of the stool or in the pan generally arises from the last part of the large bowel, the rectum or the anal region.

Common causes include:

• Haemorrhoids
• Anal fissure
• Diseases of the rectum and colon

The blood being bright red does not, however, show that there is no serious disease.

Dark red or maroon-coloured blood

Blood mixed in with the stool, or appearing maroon in colour, can be seen in bleeding arising from the higher parts of the large bowel and, in some situations, from the small bowel.

Black, tar-coloured stools

A black, shiny, sticky and foul-smelling stool is called melaena.

This is mostly seen in bleeding arising from the upper digestive tract — the gullet, the stomach or the duodenum.

Conditions such as gastric or duodenal ulcer can cause it.

Melaena is an important finding in terms of gastrointestinal bleeding and requires medical assessment.

What are the common causes of blood in the stool?

Haemorrhoids

Haemorrhoids can cause bright red bleeding, particularly during or after opening the bowels.

The blood is mostly noticed on the surface of the stool, on the lavatory paper or in the pan.

The presence of haemorrhoids in a person does not, however, mean that every bleed seen must be coming from them.

Anal fissure

This is a small tear developing at the anus.

It can appear after hard stools and constipation in particular. There is generally a sharp or tearing pain while opening the bowels, with a small amount of bright red blood.

Diverticular bleeding

Diverticula in the large bowel can sometimes bleed.

The bleeding may be painless and in some patients the amount can be considerable.

Colon polyps

Some colon polyps can cause bleeding. A significant proportion of polyps may, however, be present without giving any symptom at all.

This is why screening for colorectal cancer is important in people of the appropriate age and risk group.

Colorectal cancer

Cancers of the large bowel and rectum can cause blood to appear in the stool.

Where the bleeding is accompanied by:

• A new change in bowel habit
• Unexplained weight loss
• Anaemia
• Abdominal pain
• Weakness

in particular, assessment should not be delayed.

Blood in the stool does not by itself mean cancer, but this possibility has to be excluded where required.

Can inflammatory bowel diseases cause bleeding?

Yes.

Bloody diarrhoea can be seen in ulcerative colitis and Crohn’s disease.

The bleeding may be accompanied by:

• Frequent opening of the bowels
• Diarrhoea
• Mucus
• Abdominal pain
• Urgency
• Fever
• Weight loss

Can blood be seen in bowel infections?

Certain bacterial bowel infections can give rise to bloody diarrhoea.

Where bloody diarrhoea of sudden onset, fever and abdominal pain are present together in particular, the possibility of infection should be assessed.

Does blood in the stool matter in people taking blood thinners?

Yes.

In people taking medicines that can increase the risk of bleeding — those called anticoagulants or antiplatelets — gastrointestinal bleeding can become more pronounced.

The bleeding should not, however, be attributed to the medicine alone. The source of the bleeding may need to be found.

Blood-thinning medicines should not be stopped without a doctor’s advice.

Is a red stool always blood?

No.

Certain foods such as beetroot, and certain food colourings, can make the stool look red.

Similarly, iron preparations and certain medicines can darken the stool.

Where the person is not sure whether or not this is blood, however, it is safer to have it assessed.

When is blood in the stool an emergency?

Urgent medical assessment may be required if any of the following is present:

• Bleeding of a large amount
• Bleeding that continues or recurs
• Black, tar-coloured stools
• Vomiting blood, or vomit that looks like coffee grounds
• Dizziness or fainting
• Cold sweating
• Palpitations
• Breathlessness
• Marked weakness
• Severe abdominal pain

Where the bleeding is accompanied by fainting, low blood pressure or a marked deterioration in general condition in particular, emergency help should be sought.

How is blood in the stool investigated?

The doctor first assesses:

• The colour of the blood
• Its amount
• Whether or not it is mixed in with the stool
• How long it has been going on
• Whether there is constipation or diarrhoea
• Whether abdominal or anal pain accompanies it
• The medicines being taken

Depending on the patient’s condition:

• Physical examination and assessment of the anal region
• A blood count and other blood tests
• Anoscopy or rectal assessment
• Colonoscopy
• Gastroscopy
• Computed tomography and other imaging where required

may be used.

Which investigation is needed is decided by assessing the character of the bleeding together with the patient’s clinical condition.

Is colonoscopy needed for every case of bleeding?

Colonoscopy is not carried out automatically in every patient.

Colonoscopy may nevertheless be required, taking into account the patient’s age, the character of the bleeding, changes in bowel habit, anaemia, family history and other alarm symptoms.

In rectal bleeding that is unexplained or recurrent in particular, it is important to establish the cause.

Remember

Blood in the stool is not a disease; it is a symptom.

Haemorrhoids and anal fissure are common causes. It is not right, however, to put blood in the stool down to haemorrhoids alone without an examination.

In bleeding that is recurrent, of unknown cause, or accompanied by other alarm symptoms, the source must be investigated.