Stool Testing

Expansion

Infective diarrhoea

• Culture for Campylobacter, Salmonella, Shigella and E. coli O157. Request O157 specifically if there is bloody diarrhoea or suspected haemolytic uraemic syndrome, and avoid antibiotics, which increase that risk
• Ova, cysts and parasites, three samples, for Giardia, amoebae and helminths, and requested specifically after travel or with persistent symptoms
• Viral PCR for norovirus and rotavirus, mainly for infection control
• Increasingly a multiplex PCR panel replaces the separate requests

Clostridioides difficile: a two stage algorithm.

1. Glutamate dehydrogenase (GDH) or PCR detects the organism, and is sensitive
2. Toxin immunoassay detects active disease

GDH positive with toxin positive means infection; GDH positive with toxin negative means carriage or a low level, and is usually not treated. Do not test formed stool, and do not test for cure, since toxin can persist for weeks.

Non-infective tests

• Faecal calprotectin: distinguishes inflammatory bowel disease from irritable bowel syndrome. Raised also by NSAIDs, infection, malignancy, coeliac disease and diverticulitis, and in the very young, so it is a triage tool rather than a diagnosis
• Faecal immunochemical test (FIT): detects human haemoglobin, used in bowel cancer screening and to triage symptomatic patients. A threshold of 10 micrograms per gram guides urgent referral, but a negative FIT does not override strong clinical suspicion
• Faecal elastase: low in pancreatic exocrine insufficiency
• Faecal fat, largely superseded, for steatorrhoea

Sample quality matters: the stool should take the shape of the container, samples should reach the laboratory promptly, and the clinical details on the form determine which tests are actually run.