Diarrhoea: Causes, Tests & When to See a Doctor

What Is Diarrhoea?

Diarrhoea is loose, watery stools occurring more than three times a day. Most cases are acute (lasting a few days) and caused by infection; chronic diarrhoea lasting weeks needs further investigation.

Common Causes

• Viral infections (norovirus, rotavirus)
• Bacterial infections (E. coli, Salmonella)
• Food intolerances (lactose, gluten)
• Medications, especially antibiotics
• Irritable bowel syndrome (IBS)
• Inflammatory bowel disease (Crohn's, ulcerative colitis)
• Stress and anxiety

Tests Your Doctor May Order

Managing Diarrhoea at Home

• Drink oral rehydration solution or water with electrolytes
• Eat bland foods (bananas, rice, toast) as tolerated
• Avoid dairy, caffeine and fatty foods until recovered
• Avoid anti-diarrheal medication if fever or blood is present

Frequently Asked Questions

References

Sources cited on this page. PubMed links open the original abstract.

1. Guerrant RL, Van Gilder T, Steiner TS, et al; Infectious Diseases Society of America. Practice guidelines for the management of infectious diarrhea. Clin Infect Dis. 2001;32(3):331–351. PMID 11170940 · doi:10.1086/318514

What Blood Tests Can and Cannot Tell You About Diarrhoea

Acute diarrhoea lasting fewer than two weeks is most often self-limiting and infectious; blood tests are not routinely indicated unless the patient is systemically unwell, dehydrated, immunocompromised, or the diarrhoea is bloody. Chronic diarrhoea – defined as loose or increased-frequency stools persisting beyond four weeks – warrants a structured blood investigation to distinguish functional causes (such as irritable bowel syndrome) from organic disease (inflammatory bowel disease, coeliac disease, malabsorption, and endocrine causes).¹

The core panel for chronic diarrhoea includes full blood count (anaemia suggests blood loss or malabsorption), CRP and ESR (elevated in inflammatory bowel disease but typically normal in IBS), serum albumin (low in severe IBD or protein-losing enteropathy), thyroid function (hyperthyroidism accelerates gut motility and causes loose stools), and coeliac serology – tissue transglutaminase IgA (tTG-IgA) with total IgA to exclude IgA deficiency, which would give a false-negative coeliac test.

Specific Conditions Identified by Blood Tests

Coeliac disease affects approximately 1% of the population and frequently presents with diarrhoea, bloating, and fatigue. Tissue transglutaminase IgA (tTG-IgA) has a sensitivity of approximately 95% and specificity of 95% for coeliac disease when IgA levels are normal – making it the single most useful first-line blood test in suspected coeliac disease.² A positive result requires endoscopic biopsy (Marsh grading of duodenal mucosa) to confirm the diagnosis before starting a lifelong gluten-free diet.

Inflammatory bowel disease – Crohn's disease and ulcerative colitis – typically shows raised CRP, raised ESR, and low albumin in active flares. Faecal calprotectin (a stool test, not blood) is the most sensitive non-invasive marker of intestinal inflammation and helps distinguish IBD from IBS before colonoscopy. In Crohn's disease with small bowel involvement, B12 deficiency (impaired ileal absorption) and iron deficiency (from chronic blood loss) are common and require monitoring. Microscopic colitis, a significant cause of watery diarrhoea in middle-aged and older women, often produces a completely normal blood panel – diagnosis requires colonoscopic biopsy.

Electrolytes, Renal Function, and Dehydration

Prolonged diarrhoea – particularly in children, the elderly, or those with high-output secretory diarrhoea – depletes fluid and electrolytes in ways that blood tests quantify precisely. Sodium, potassium, chloride, and bicarbonate (urea and electrolytes panel) reveal the metabolic consequences: hypokalaemia is common and clinically significant (causing muscle weakness and cardiac arrhythmia risk at levels below 3.0 mmol/L); hypernatraemia develops when free water loss exceeds intake; metabolic acidosis with a normal anion gap (hyperchloraemic) is the typical pattern from bicarbonate loss in intestinal fluid.³ Creatinine and urea rise with prerenal dehydration. Measurement of these values guides the speed and composition of rehydration – oral or intravenous – in hospitalised patients.

Acute or chronic, and why the line is drawn at four weeks

Diarrhoea lasting under two weeks is nearly always infective and self-limiting, and investigation beyond checking hydration is rarely needed. Between two and four weeks it is called persistent, and a stool test becomes reasonable. Beyond four weeks it is chronic, and the likely causes change entirely: coeliac disease, inflammatory bowel disease, microscopic colitis, bile acid malabsorption, thyroid overactivity, pancreatic insufficiency, and medication side effects all become plausible, and irritable bowel syndrome is diagnosed only once the others have been considered.

Features that make a simple explanation less likely

• Blood or mucus in the stool, which is never a feature of irritable bowel syndrome and needs investigation.
• Waking at night to open the bowels, which distinguishes organic disease from functional causes fairly reliably.
• Unintentional weight loss, fever, or anaemia found on a blood test.
• Onset after age 50, or a family history of bowel cancer or inflammatory bowel disease.
• Pale, greasy stools that float and are hard to flush, suggesting fat malabsorption.

Medication and dietary causes that are easily missed

Metformin is a very common cause and often improves on the modified-release preparation. Magnesium-containing antacids and supplements, proton pump inhibitors, SSRIs, colchicine and many antibiotics all cause diarrhoea. Antibiotic-associated diarrhoea occasionally reflects Clostridioides difficile, which should be considered when it is severe, bloody, or follows a recent course of antibiotics or a hospital stay. On the dietary side, sugar alcohols such as sorbitol and xylitol in sugar-free gum and sweets are a recurring and easily overlooked culprit, as is a sudden increase in caffeine or fibre.

Rehydration and when to stop taking loperamide

Oral rehydration salts remain the mainstay for acute diarrhoea, because the glucose they contain drives sodium and therefore water absorption in a way plain water cannot. Loperamide is reasonable for short-term symptom control in uncomplicated cases, but should be avoided if there is fever or blood in the stool, where slowing the bowel can be harmful. Seek assessment if you cannot maintain fluids, pass very little urine, become confused or lightheaded on standing, or if symptoms persist beyond a week.