RCGP SCA Algorithm — UK Primary Care
Acute diarrhoea is usually self-limiting infective gastroenteritis. The work of this consultation is to find the patient who is dehydrated, septic, bleeding, has C. difficile or STEC, or has a surgical or metabolic cause wearing a diarrhoea label. Chronic (4 weeks or more) diarrhoea follows the chronic diarrhoea algorithm.
Most deaths from gastroenteritis in adults are from dehydration and its complications in older, frail or comorbid people — not from the organism itself. Tachycardia can reflect fever, pain or anxiety, and skin turgor is unreliable in older adults, so judge the whole picture, including urine output and the ability to drink.
Loperamide and antibiotics in STEC may increase the risk of haemolytic uraemic syndrome, so bloody diarrhoea with severe abdominal pain is managed on specialist or Health Protection Team advice (UKHSA). C. difficile can be severe with few stools when ileus develops — a quiet bowel in an ill patient is not reassuring. In diabetes, SGLT2 inhibitors can cause DKA with near-normal glucose (MHRA), so check ketones rather than relying on the glucose.
Incubation and the dominant symptom narrow the organism: vomiting within hours of a meal suggests a toxin; bloody diarrhoea with fever suggests an invasive bacterium; prolonged watery diarrhoea after travel suggests a parasite. Most of the time the exact organism does not change management, so the history matters most for spotting the patient who needs a stool sample, a public-health action or a different diagnosis.
C. difficile risk persists for weeks after an antibiotic course and is increased by PPIs. A new diarrhoea in an older patient recently started on metformin, or after a laxative dose increase, needs a medication fix, not a stool sample.
The examination answers two questions: how dry and how septic is this person, and is there something in the abdomen that is not gastroenteritis? Diarrhoea is common in appendicitis, pelvic sepsis and mesenteric ischaemia, and "gastroenteritis" is a frequent label on delayed surgical diagnoses — particularly in older adults with pain out of proportion to the signs.
Overflow diarrhoea around impacted stool is a common and easily missed cause in frail, immobile or opioid-treated older people; giving loperamide makes it worse.
Most acute diarrhoea does not need a stool sample, but the indications above identify the patients in whom the result changes management (antibiotic choice, C. difficile treatment, IBD work-up) or public-health action (outbreak control, exclusion and clearance of food handlers and care workers). Positive laboratory results for notifiable organisms are reported by the laboratory, but clinicians must still notify suspected cases (UKHSA).
Calprotectin is raised by infective colitis, so testing during the acute episode generates false positives; test once the infection has settled if symptoms persist. Severe bloody diarrhoea meeting Truelove and Witts criteria is a medical emergency whatever the eventual cause.
Classifying by mechanism keeps the dangerous categories in view: the infective causes usually need fluids and time, the drug causes need a medication change, and the surgical, ischaemic and inflammatory causes need a different pathway altogether. In older adults the combination of sudden abdominal pain followed by bloody diarrhoea should prompt thought of ischaemic colitis, and leakage around impacted stool is the commonest cause of "diarrhoea" in the frail and immobile.
Registered medical practitioners have a statutory duty to notify suspected food poisoning and other notifiable diseases to the proper officer, on clinical suspicion and without waiting for laboratory confirmation (Health Protection (Notification) Regulations 2010). Early notification lets the HPT identify outbreaks, trace contaminated food and arrange clearance testing for people in risk groups.
Next steps: see Gastroenteritis · Sick day rules · Irritable bowel syndrome · Ulcerative colitis · Crohn's disease
Rehydration is the treatment; feeding should continue. Loperamide shortens watery diarrhoea in adults but should be avoided when there is blood, fever or a possible C. difficile or STEC infection, where slowing the bowel may worsen toxin-mediated disease. Antibiotics shorten some bacterial infections by about a day but drive resistance and C. difficile and may increase HUS risk in STEC, so they are kept for specific indications on microbiology advice.
Dehydration plus metformin, SGLT2 inhibitors, ACE inhibitors/ARBs, diuretics and NSAIDs is a common route to AKI, lactic acidosis or DKA; a temporary pause with a clear restart instruction prevents most of these admissions. The 48-hour exclusion rule reflects continued shedding after symptoms stop, especially with norovirus.
Norovirus is highly infectious, survives on surfaces and is not killed by alcohol gel; soap and water and bleach-based cleaning are the effective measures. Cryptosporidium oocysts resist chlorine, hence the longer swimming restriction (UKHSA).
Most acute infective diarrhoea settles within 7 days. Persistence beyond this prompts stool testing and reconsideration of C. difficile, parasites, IBD and drug causes. A clear, specific safety-net — especially about urine output and blood in the stool — is the main protection against delayed recognition of dehydration, HUS and colitis.