Acute diarrhoea (adult and child): when to test the stool?

Stool tests (fresh diarrhoeal stool, before any antibiotic)

• Stool culture (state: look for Shiga toxin-producing E. coli if bloody stools)
• Toxigenic Clostridioides difficile testing (antibiotic or hospital admission in the last 2 months, over 65 years)if neededAntibiotic or hospitalisation within the past 2 months, or age over 65: situations in which the NABM provides for the test, on prescription or at the initiative of the laboratory ^[2]; the IDSA includes it in the work-up of febrile or severe diarrhoea ^[3].
• Stool parasitology (return from travel with diarrhoea of 14 days or more, immunosuppression)if neededDiarrhoea of 14 days or more on return from travel, or immunosuppression ^[3]. Three samples collected 1 to 3 days apart ^[2].
• Cryptosporidium testing (immunosuppression; to be requested explicitly)if neededImmunosuppression, in particular AIDS with persistent diarrhoea ^[3]. Procedure distinct from the standard parasitological examination: state it on the prescription ^[2].
• Stool parasitology and Cryptosporidium testing (if diarrhoea of more than 14 days, without travel or immunosuppression)if needednon-French guidelineThe British standard S 7 (2026) includes Giardia and Cryptosporidium in the first-line work-up, and always in persistent diarrhoea, of more than 14 days ^[4]. The page reserves these tests for return from travel and immunosuppression, following the IDSA ^[3]. Cryptosporidium is a distinct procedure to be requested explicitly; three samples are preferable for the parasitological examination ^[2].
• Chlamydia trachomatis and gonococcus NAAT (PCR) on anal swab (if unprotected anal intercourse and bloody or mucous stools)if needednon-French guidelineThe British standard S 7 (2026) points out that Chlamydia, including lymphogranuloma venereum, and gonococcus can cause a sexually transmitted colitis in people having unprotected anal intercourse ^[4]. The French sources of the page do not address this case; the NABM provides for this test in proctitis ^[2].

Blood tests

• Blood cultures (sepsis, suspected typhoid fever, immunosuppression)if needednon-French guidelineSigns of sepsis, suspected typhoid fever or unexplained fever on return from an endemic area, systemic manifestations, immunosuppression, haemolytic anaemia, infant under 3 months ^[3].
• Serum electrolytesif neededReassess fluid and electrolyte balance when symptoms persist ^[3]. No urea: reimbursed only in acute kidney injury, dialysis or nutritional assessment of chronic kidney disease ^[2].
• Serum creatinine with GFR (dehydration, persistent symptoms)if neededReassess fluid and electrolyte balance when symptoms persist ^[3]. No urea: reimbursed only in acute kidney injury, dialysis or nutritional assessment of chronic kidney disease ^[2].
• FBC (high fever, blood in stools, frail patient)if needednon-French guidelineNot used to find the cause, but may be clinically useful ^[3]; essential with creatinine if a Shiga toxin-producing E. coli infection is confirmed (screening for HUS) ^[3].

• Gastroenteritis without signs of severity: no tests; oral rehydration solution and monitoring advice.

Stool tests (if one of the situations in the note)

• Stool culture (if bloody stools: request testing for Shiga toxin-producing E. coli)if neededDysenteric syndrome, septic state, return from overseas countries, immunosuppression or debilitated condition, food poisoning outbreak in a community setting, contacts of a shigellosis case ^[1]; to be considered if no improvement by day 7 ^[10]. Explicit STEC testing if bloody diarrhoea ^[3,6].
• Stool parasitology (return from travel, immunosuppression)if neededReturn from travel or immunosuppression ^[1,3]. Three samples collected 1 to 3 days apart ^[2].

Confirmed STEC infection, child not hospitalised (on specialist advice)

• FBC with search for schistocytesif needednon-French guidelineConfirmed Shiga toxin-producing E. coli infection: monitor haemoglobin and platelets, blood film looking for schistocytes if HUS is suspected ^[3]; seek specialist advice on the monitoring frequency ^[10].
• Serum electrolytesif neededConfirmed STEC infection: the renal involvement of HUS is screened for by creatinine and electrolytes ^[3]; warning signs of HUS: hospital emergency ^[11].
• Serum creatinine with GFRif neededConfirmed STEC infection: the renal involvement of HUS is screened for by creatinine and electrolytes ^[3]; warning signs of HUS: hospital emergency ^[11].
• Urine dipstick (haematuria, proteinuria)if neededRenal involvement of HUS: haematuria, proteinuria, oliguria ^[11].

Stool tests (one diarrhoeal stool)

• Toxigenic Clostridioides difficile testing
• Stool culture (if high fever, bloody or mucous stools)if needednon-French guidelineBloody stools (unusual in C. difficile infection), fever, severe pain or sepsis: also look for enteroinvasive bacteria ^[3].
• FBCif needednon-French guidelineNICE (NG199, 2021) classifies the severity of the infection on white cell count (severe above 15 G/L), serum creatinine rising by more than 50% from baseline, temperature and signs of severe colitis ^[13]. The French sources of the page do not address this point.
• Serum creatinineif needednon-French guidelineNICE (NG199, 2021) classifies the severity of the infection on white cell count (severe above 15 G/L), serum creatinine rising by more than 50% from baseline, temperature and signs of severe colitis ^[13]. The French sources of the page do not address this point.