What initial investigations should I consider for a patient presenting with chronic diarrhoea lasting more than four weeks?

Consider requesting blood tests including a full blood count, urea and electrolytes, liver function tests (including albumin), calcium, vitamin B12 and red blood cell folate, iron status (ferritin), thyroid function tests, ESR, and CRP in all patients with chronic diarrhoea ¹.

Test for coeliac disease using immunoglobulin A (IgA) and IgA tissue transglutaminase (tTG) or IgA endomysial antibody (EMA), ensuring the patient is eating gluten at the time of testing ¹.

Consider sending stool samples for routine microbiology investigation and examination for ova, cysts, and parasites, especially if infectious causes are suspected or there is a history of foreign travel ¹.

Request specific testing for Clostridioides difficile if there is recent antibiotic or PPI use, hospital admission, or recurrence of symptoms ¹.

Perform faecal calprotectin testing to help differentiate between irritable bowel syndrome and inflammatory bowel disease in people under 40 years if cancer is not suspected ¹.

Further investigations such as colonoscopy or imaging may be considered based on clinical assessment and red flag features ¹.