Laboratory tests - culture and toxin testing

Role

Stool culture and toxin testing identify bacterial enteric pathogens and their toxins - complementary to (and in some cases more specific than) faecal microscopy for diagnosing infectious diarrhoea, particularly for organisms not reliably detected by microscopy alone
Used selectively in infectious diarrhoea workup, guided by clinical features (bloody diarrhoea, severity, duration, recent antibiotic use, travel history, and public health/outbreak context) rather than routinely for every episode of self-limiting diarrhoea

Stool culture identifies viable bacterial pathogens (Salmonella, Shigella, Campylobacter, pathogenic E. coli strains) by growing them on selective media - takes days and requires viable organisms in the sample
Toxin testing (e.g. C. difficile toxin EIA, or increasingly molecular/PCR-based toxin gene detection) identifies the toxin itself or the toxin-producing genes, relevant particularly for C. difficile where toxin (not just organism carriage) determines clinical significance - a patient can be colonised with toxigenic C. difficile without active toxin-mediated disease

*Bloody diarrhoea, severe/prolonged diarrhoea, immunocompromised patients, recent antibiotic exposure (prompting C. difficile testing specifically), and outbreak/public health investigation* are the main drivers for culture/toxin testing rather than routine use in every diarrhoeal episode
Molecular (PCR-based) multiplex panels are increasingly used, offering faster turnaround and simultaneous detection of multiple pathogens (bacterial, viral, parasitic) compared with traditional culture, though interpretation requires awareness that PCR detects genetic material and can be positive with non-viable organisms or in asymptomatic carriage for some targets

Positive results should be interpreted alongside the clinical picture - not every positive result (particularly molecular panel detections at low levels, or C. difficile toxin gene PCR without toxin EIA confirmation) necessarily indicates the organism is the cause of the current illness, particularly for organisms with a recognised asymptomatic carriage state
Notifiable pathogens (e.g. Salmonella, Shigella) trigger public health reporting regardless of clinical severity

Clostridioides difficile infection (toxin-mediated significance), Salmonella/Shigella/Campylobacter enteritis, traveller's diarrhoea, outbreak investigation
The distinction between C. difficile colonisation (organism/toxin gene present, no active toxin-mediated disease) and infection (active toxin production causing clinical disease) is a frequently tested, clinically important nuance - toxin EIA positivity (not just PCR/toxin gene detection) better correlates with clinically significant, toxin-mediated disease

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