Diarrhoea is a very common complaint and may be the reason for a patient’s admission, or develop as a new problem during an inpatient stay. Diarrhoea related to inflammation of the stomach and small bowel is termed gastroenteritis, whilst inflammation of the large bowel is colitis. However, these are umbrella terms that describe syndromes, not causes.

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Clinical Definition

Bristol Stool Chart
  •  > 3 loose or liquid stools per 24 hours, and/or stool that weighs >200 g/day.
  • The former is a more practical measure and can be ascertained by the history.
  • It is imperative that nursing staff maintain a stool chart so this can be objectively monitored.
  • In certain cases, such as for patients with inflammatory bowel disease, it may wise to provide the patient with a stool chart they can complete which will usually ensure an accurate record.
  • The Bristol stool chart is validated for this purpose. Type 6/7 stools are considered to represent clinical diarrhoea.

Acute diarrhoea lasts < 14 days, persistent diarrhoea > 14 days and chronic diarrhoea > 4 weeks. Rightly so, the length should inform your differential diagnosis.

Clinical approach

In the very first instance, particularly for cases of acute diarrhoea, you must exclude infection as a possible cause. At the front door, this may be particularly relevant if a patient has a recent travel history or exposure to uncooked food(s). For inpatients on prolonged courses of antibiotics, you should consider the likelihood of Clostridium difficile colitis.

  • Duration of symptoms: (> 4 weeks is chronic). Is this new or different?
  • Frequency of bowel movements (> 3 loose stools per day). What’s “normal” for them?
  • Character of stools: Watery? Fatty?
  • Presence of blood and/or mucus in the stool (dysentery or PR bleeding)
  • Presence of nocturnal symptoms
  • Travel history
  • PMH: Thyrotoxicosis, hyperparathyroidism, diabetes mellitus, systemic sclerosis
  • Alcohol
  • Associated symptoms
  • Red flag symptoms:
    • Weight loss
    • Anorexia
    • Fevers, chills and rigors
    • Age > 55
  • Medication use
    • Laxatives, PPIs, NSAIDs
    • Antibiotics (N.B. broad-spectrum antibiotics risk of C. difficile colitis)
  • Family history of bowel cancer
  • Previous bowel surgery
  • History of immunosuppression/immunodeficiency (e.g. HIV positive, transplant recipients)

Key baseline investigations for diarrhoea include:

  • Stool culture & microscopy including C. difficile toxin – E. coli 0157:H7, Shigella, Salmonella and Campylobacter will be routinely tested for and are notifiable diseases
  • Bloods – order FBC, U&Es, LFTs, bone profile, CRP to guide electrolyte replacement & consideration for admission if high inflammatory markers (and also if rarely, they need antibiotics e.g. oral metronidazole)
  • Lactate to guide IV rehydration, electrolyte replacement and need for admission and rarely if antimicrobial therapy might be necessary (usually oral metronidazole)
Further Investigations

A more comprehensive set of investigations is listed below and should be considered together with senior input.

  • If anaemic: haematinics (ferritin, B12, folate) & coeliac serology
  • If young female/autoimmune history: coeliac serology & TFTs
  • If pancreatic insufficiency is suspected, faecal elastase
  • If inflammatory bowel disease suspected, faecal calprotectin. Any inflammation (e.g. infectious) or NSAIDs will cause raised calprotectin
  • Imaging
    • Abdominal X-ray if clinical suspicion of toxic megacolon
    • CT imaging & endoscopy (with or without biopsy) – sometimes if colitis or malignancy is suspected)

Key management principles

  1. Is the patient unwell? Do they need fluids?
  2. Are we causing the diarrhoea? (i.e. iatrogenic from medication)
  3. Any infection control precautions?
  4. Is there a serious cause? Is imaging required?
  5. Consider the potential causes (list of differentials & important tips)
Step 1) Is the patient unwell?
Step 2) Are we causing the diarrhoea?
  • Are they on any (new) medications that could be causing this?
  • Antibiotics (c diff), digoxin, allopurinol, magnesium supplements, metformin, NSAIDs, PPIs, SSRIs, thyroxine, vitamin c
Step 3) Any infection control precautions?
  • Ensure the nurse in charge (or at least the nurse in the bay) is aware to organise a side room and barrier nursing
  • Usually, all patients need this unless it is clearly related to a noninfectious cause such as constipation with overflow or induced by medications
  • Public health may need to be notified. It is worth checking if your lab does that automatically following the identification of the cause (Cholera, typhoid or paratyphoid fever, haemolytic uraemic syndrome (E. coli 0157), food poisoning and any infectious bloody diarrhoea)
Step 4) Do they require imaging?

This is possibly due to altered anatomy, potential surgical cause or because they’re very unwell.

  • Do they have severe abdominal pain or guarding? Is their lactate raised suggesting dehydration, or colitis? If so is it ischaemic, inflammatory or infectious colitis?
  • Does the patient have altered anatomy due to IBD or previous gastrointestinal surgery? Is there a suspected surgical cause?
  • Could this be a malignancy?
  • Consider a dietician for additional measures such as a polymeric diet for IBD patients, hypotonic fluid restriction for stoma patients & to ensure adequate nutrition
  • A high-output stoma (> 1,500 mL/24 hours) should be managed in the same manner, i.e. stool samples should be sent, electrolytes replacement.
Step 5) Common or Serious causes

It can be helpful to differentiate the type of diarrhoea based on your history.

InflammatoryFever, pain, bleedingIBD, diverticulitis, radiation enteritis, viral or parasitic infections, colon cancer
SecretoryLarge volume even on fastingC diff
OsmoticTends to resolve on fastingLaxatives, Magnesium intake
Steathorreal or MalabsorptionGreasy, foul-smelling diarrhoea that floatsPancreatic insufficiency, coeliac, lactose intolerance, bile salt malabsorption

Constipation with overflow

Those with risk factors for constipation (opiates, poor diet or several days of not opening bowels) might be faecally loaded or impacted. A digital rectal examination feeling for hard stool with a suppository in hand might identify & treat this. Ensure you wear an apron and stand to the side. We have another article on managing constipation.

Infectious Diarrhoea

Most community-acquired causes of diarrhoea simply need rehydration. Avoid constipating agents & treat any spasmodic pain if required. Antibiotics are rarely necessary but may be considered in severe or bloody diarrhoea.

C difficile colitis

C diff is a gram-positive, anaerobic bacterium that’s toxins cause increased permeability and pseudomembrane formation. It tends to arise on days 4-9 of antibiotic therapy but may occur up to 8 weeks post-discontinuation of antibiotics (especially broad-spectrum penicillins and cephalosporins.)

Severity can be monitored clinically (pain, fever, tachycardia) and biochemically (white cell count and inflammatory markers). If clinically indicated, an abdominal X-ray should be ordered to exclude toxic megacolon (a surgical emergency).

Treatment must involve infection control input and antibiotics as guided by local guidelines (often vancomycin or metronidazole).

Flare-up of acute colitis

Patients with known IBD should still have an infectious cause considered. Their management should be guided by a gastroenterologist or IBD nurse. Imaging (endoscopic or radiological) may be helpful to look for collections, strictures or inflammation. The endoscopy is performed without bowel preparation (bowel prep would worsen diarrhoea & is unnecessary). For ulcerative colitis, Truelove & Witt’s criteria are used to assess severity.

Often the management in the acute phase is intravenous steroids e.g. IV hydrocortisone 100mg QDS with their usual medications on top or increased.


A common cause in the elderly, particularly those with constipation or obesity. Diverticulae are very common and usually asymptomatic, but they can get inflamed resulting in often left lower quadrant pain and raised inflammatory markers. Bleeding can sometimes feature. CT is helpful to confirm the diagnosis & screen for complications abscess, perforation, fistula, obstruction/stricture. Antibiotics are often used in addition to analgesia & hydration. Further reading is on Patient UK.


In patients with chronic pancreatitis or features to suggest coeliac disease, consider special investigations as outlined above. Further specialised investigations might be required for even rarer causes e.g. hydrogen breath testing for suspected small bowel overgrowth syndrome or SEH-CAT testing for suspected bile acid malabsorption.

Ischaemic colitis

Always bear this in mind in those with atrial fibrillation (AF), vascular risk factors or cocaine use. The pain is severe and out of proportion with the examination & the lactate can be very high. Imaging such as CT angiography can be helpful to make a diagnosis. Hydration, antibiotics & urgent surgical review are the mainstays of treatment. Mortality can be quite high. Further reading is on Patient UK.

Further Reading & References

Written by Dr Anahita Sharma (FY2) & Dr Sarah McBrinn (FY1)
Edits by Dr Akash Doshi (CT2)

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