Red flag symptoms: Diarrhoea, investigations and when to refer

Treatment depends on the patient's age and the timeline of events.

If the patient works with food, this may affect their management plan (SPL)
If the patient works with food, this may affect their management plan (SPL)

Red flag symptoms

  • Symptoms >4 weeks
  • Bloody diarrhoea
  • Unintentional weight loss
  • Nocturnal diarrhoea
  • Reduced urine output
  • History of fever
  • History of foreign travel
  • History of eating out
  • Acute diarrhoea is an extremely common presentation in primary care, so it is important not to miss more serious pathology.

The majority of cases will be viral and self resolve. In these cases, advice about hydration, and reassurance are necessary. Advise patients when they can expect symptoms to resolve and provide appropriate safety netting. Patient information leaflets on gastroenteritis are valuable.

However a systematic approach is essential to ensure serious conditions are considered at initial assessment.

It may be useful to revert to an undergraduate diagnostic approach to cover the various differentials.


Acute diarrhoea

  • Infections - for example viral or bacterial
  • Recent antibiotic use
  • Exacerbation of an underlying chronic problem (IBS, IBD, diverticulosis)
  • Iatrogenic, for example, due to metformin (diarrhoea is a common side-effect on initiation)
  • Ischaemic colitis (ask about any history of AF)
  • Hepatitis A
  • HIV seroconversion illness

Chronic diarrhoea

  • Colorectal malignancy
  • IBD
  • IBS
  • Thyrotoxicosis
  • Autonomic neuropathy, such as diabetes
  • Laxative abuse
  • Diverticulosis
  • Iatrogenic, for example, NSAID-induced colitis, metformin
  • Bacterial overgrowth
  • Coeliac disease
  • Chronic pancreatitis
  • Alcohol dependency
  • HIV and AIDS
  • Malabsorption


First, establish exactly what the patient means by diarrhoea, which can mean different things to different people.

  • What has happened to the stool?
  • Establish the onset of symptoms, for example, is this acute or chronic diarrhoea?
  • What is normal for this patient with regard to bowel habit?
  • The patient's age and the timeline of events should guide your subsequent questions. Ask how often they are opening their bowels, and whether there has been a change in colour or smell of their stool.
  • Have they noticed any blood and if so, is this on the toilet paper or mixed in with the stool?
  • Have there been any nocturnal symptoms, for example, have they been woken up by the urge to defecate? (Nocturnal symptoms should alert the clinician to IBD)
  • Has there been any mucus associated with this change in bowel habit?
  • Have they experienced any abdominal pain?
  • Has there been any associated nausea or vomiting?
  • Have they noted any unintentional weight loss?
  • Has there been any recent foreign travel?
  • Has there been any recent change in diet and/or eating out?
  • Has there been any abdominal bloating?
  • Is the diarrhoea worse after food?

Ask about any history of constipation, and be alert to constipation with overflow. Additional questions include:

  • Is anyone else in the household feeling unwell?
  • Have they had any recent hospital admissions or episodes of antibiotic use? Be alert to the possibility of Clostridium difficile infection.
  • Have there been any changes to medication or addition of new prescribed or non-prescribed medication?
  • Has there been any associated fever?
  • Do they feel systemically unwell?

Ask whether the patient has had this problem before, and if so, whether it has been been investigated. What investigations were done? Were there any endoscopies or imaging?

Establish the patient's current occupation. Do they work with food or in healthcare? This may affect their management plan.

Do they smoke, and/or drink alcohol?

It is important to explore the patient's ideas, concerns and expectations around the symptom and why they have presented at this particular time. What does the patient feel may be the cause, and are they worried about a specific condition?


  • BMI
  • BP
  • Pulse for rate and rhythm
  • Hydration status - mucous membranes and skin turgor
  • Signs of jaundice
  • Stigmata of chronic liver disease
  • Temperature
  • Examine the abdomen - is there evidence of an acute abdomen?

Further examination may be prompted by certain features in the patient's history.


No investigations may be appropriate, depending on the onset of symptoms. Stop any culprit medications and reassess after an appropriate period of time. Symptoms lasting for more than two weeks are likely to need some basic investigations. These may include:

  • Blood tests such as FBC, U&Es, CRP, ESR, LFTs, TSH, HbA1c, coeliac screen (tissue transglutaminase) and consent to HIV testing (if appropriate)
  • Stool for culture and sensitivity, ensuring the laboratory is aware of any recent travel, eating out or antibiotic use. It may be appropriate to submit three samples on three consecutive days to ensure the best chance of detecting a pathogen
  • Faecal calprotectin level, to help to differentiate between inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Ensure the patient has not had NSAIDs in the preceding 4 weeks. This can lead to falsely elevated levels.
  • Faecal elastase if pancreatic insufficiency is suspected.

When to refer

  • If symptoms persist for more than four weeks
  • If IBD is suspected
  • If colorectal malignancy is suspected
  • Positive HIV test
  • Urgent admission if the patient is acutely dehydrated or is bleeding significantly
  • If basic tests are negative and symptoms persist

Dr Singh is a GP in Northumberland

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This is an updated version of an article that was first published in August 2014.

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