Red flag symptoms: Diarrhoea

The red flag symptoms that indicate potentially serious causes of diarrhoea in adults.

(Photo: Tharakorn Arunothai/EyeEm/Getty Images)
(Photo: Tharakorn Arunothai/EyeEm/Getty Images)

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 will 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.

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
  • Significant abdominal pain

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


Acute diarrhoea

  • Infections - for example viral or bacterial; COVID-19 can also present with diarrhoea
  • Recent antibiotic use (there is an increased risk of C. difficile)
  • Exacerbation of an underlying chronic problem such as irritable bowel syndrome (IBS), inflammatory bowel disease (IBD) or diverticulosis
  • Iatrogenic, for example, due to metformin (diarrhoea is a common side-effect on initiation)
  • Ischaemic colitis (ask about any history of atrial fibrillation)
  • Hepatitis A
  • HIV seroconversion illness
  • Autonomic neuropathy secondary to diabetes

Chronic diarrhoea

  • Colorectal malignancy
  • IBD
  • IBS
  • Thyrotoxicosis
  • Other endocrine causes such as phaeochromocytoma
  • Autonomic neuropathy secondary to diabetes
  • Laxative abuse
  • Diverticulosis
  • Iatrogenic, for example, NSAID-induced colitis, metformin
  • Bacterial overgrowth
  • Coeliac disease
  • Chronic pancreatitis
  • Alcohol dependency
  • HIV and AIDS
  • Malabsorption
  • Bile acid 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 Bristol stool chart may be useful here.

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 inflammatory bowel disease.)
  • 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. This is more common in those with severe frailty, or in the care home setting.

More questions to ask

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 C. 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?
  • If they have acute diarrhoea, screen for other symptoms of COVID-19 infection.

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

  • 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?



  • Face-to-face examination should include:
  • 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?
  • Rectal examination

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

Telephone assessment
If assessing remotely via telephone, establish:

  • How do they sound? Is there any sign of pain?
  • If abdominal pain is present, then can the patient feel their abdomen? if tender, where is the tenderness - above or below the umbilicus? When they press it, is it worse on pressing in or letting go?

Video assessment
If assessing via video, establish:

  • How they look - when considering hydration status, do their eyes look sunken and how does their tongue look?
  • If they have abdominal pain, do they need to lie still ? Can they assess rebound or guarding following instructions from you?

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.
  • Consider a faecal immunochemical test (FIT) test to help exclude lower gastrointestinal malignancy. Faecal immunochemical testing has been recognised as a sensitive test for excluding lower GI malignancy and identified in recent NICE guidance.1,2 It should be used when it is unclear what may be causing the patient’s lower GI symptoms. it should not be used if blood is already present. A positive FIT test if used appropriately should prompt a two-week wait LGI referral. This test is now readily available in primary care.3

When to refer

  • If symptoms persist for more than four weeks with no clear diagnosis established in primary care
  • 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
  • Positive FIT test
  • Raised calprotectin

Dr Singh is a GP in Northumberland


  1. Northern Cancer Alliance. GI and colorectal pathway. [Accessed 9 Sept 2020]
  2. NICE. Quantitative faecal immunochemical tests to guide referral for colorectal cancer in primary care. DG30. 26 July 2017.
  3. Hazell T. Guidance update: latest NICE guidelines on recognition and referral of suspected cancer. MIMS Learning, 25 March 2016.

Click here to complete an interactive version article and claim a certificate on MIMS Learning

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

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