IM16.8-12 | Diarrheal Disorder Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

CBC in diarrhoea: leucopenia + relative lymphocytosis → enteric fever; leucocytosis + neutrophilia → bacterial invasion; eosinophilia → helminthic parasitosis; microcytic anaemia → iron deficiency/chronic blood loss; macrocytic → B12/folate deficiency (malabsorption).

Stool examination: (1) Physical: colour/consistency/blood/mucus. (2) Saline wet prep: RBCs, pus cells (faecal leucocytes = invasion), motile trophozoites (E. histolytica = erythrophagocytosis; Giardia = pear-shaped, tumbling). (3) Iodine prep: cysts — E. histolytica (4 nuclei, rounded chromatoid bodies), Giardia (4 nuclei, median bodies), E. coli (8 nuclei — non-pathogenic). (4) Modified Ziehl-Neelsen: Cryptosporidium (small pink oocysts).

Hanging-drop for V. cholerae: fresh liquid stool + hollow-ground slide → darting motility of comma-shaped bacilli; confirm with immobilisation test (anti-O1 antiserum); culture on TCBS agar (yellow colonies).

Stool culture indications: dysentery + fever; diarrhoea >7 days not resolving; outbreak (public health); post-antibiotic (C. diff toxin — NOT routine culture); immunocompromised; suspected enteric fever week 2–3. Blood culture: enteric fever week 1 (gold standard); NTS in HIV/sickle cell; systemic sepsis with diarrhoea.

Chronic diarrhoea advanced investigations: anti-tTG IgA + total IgA → coeliac (confirm: duodenal biopsy); faecal calprotectin → IBD vs IBS (normal <50 µg/g); colonoscopy + biopsy → IBD/CRC/microscopic colitis (normal macroscopy does NOT exclude microscopic colitis); CT enterography → Crohn's small bowel extent/fistulae; 72-hour faecal fat → quantify malabsorption; hydrogen breath test → lactose intolerance/SIBO.

REFLECT

Think about the three patients from the opening of this module — the 7-year-old girl with watery diarrhoea, the 42-year-old man with bloody stools and fever, and the 28-year-old with fatty malabsorptive stools. Each requires a completely different investigation strategy. The girl needs a dehydration assessment and basic rehydration first, with stool microscopy for parasites if she does not improve; culture is only needed if the picture suggests bacterial dysentery. The man with bloody stools needs stool microscopy immediately (for E. histolytica trophozoites — fresh specimen) and a decision about stool culture based on severity and epidemiology. The young woman with malabsorptive symptoms needs anti-tTG IgA, three stool samples for Giardia cysts, and ultimately upper GI endoscopy. What the three share is the underlying principle: the investigation follows the clinical hypothesis, not a fixed protocol. As you move into clinical practice, what will distinguish you is not the ability to recall which tests to order, but the habit of forming a specific diagnostic question before reaching for the request form.