What Does a Standard Stool Sample Test For?
A standard stool sample primarily tests for infectious causes of diarrhea, including bacterial pathogens (via culture), Clostridioides difficile toxin, and parasites (via ova and parasite examination), though the specific components vary by clinical context and laboratory protocols. 1
Core Components of Standard Stool Testing
Bacterial Pathogens
- Expanded stool culture detects common bacterial causes including Salmonella, Campylobacter, Yersinia, Aeromonas, and Plesiomonas 2
- Traditional culture methods remain standard, though newer multiplex molecular panels can detect multiple bacteria simultaneously within hours 3
- A single diarrheal stool specimen is generally sufficient for bacterial detection, though additional specimens may increase sensitivity in persistent diarrhea 1
Clostridioides difficile Testing
- Separate testing is required for C. difficile infection, as it is not detected by routine stool culture 2
- Testing should only be performed on diarrheal (unformed) stools, not formed specimens, as asymptomatic colonization is common 1
- Modern testing typically uses glutamate dehydrogenase (GDH) screening followed by toxin confirmation or nucleic acid amplification tests 4
Parasitic Infections
- Microscopic examination for ova and parasites traditionally requires 3 stool samples collected on different days to account for intermittent shedding 2
- The most commonly detected parasites are Giardia, Cryptosporidium, Cyclospora, and Entamoeba histolytica 2
- Newer molecular tests (enzyme immunoassay or PCR) show 100% sensitivity for Cryptosporidium and Giardia with >98% specificity, and may replace traditional microscopy 2
What Standard Stool Testing Does NOT Include
Tests Requiring Separate Orders
- Fecal fat measurement for malabsorption is not part of routine testing and requires a 3-day collection, though this test has significant limitations and is increasingly discouraged 5, 6
- Fecal calprotectin for inflammatory bowel disease requires specific ordering 1
- Fecal elastase for pancreatic insufficiency is a specialized test 5
- Bile acid malabsorption testing (SeHCAT) is not available in the United States 2
Tests That Should NOT Be Ordered
- Fecal leukocyte examination and lactoferrin testing should not be used to establish infectious diarrhea causes, as they perform poorly and degrade during transport 1
- These tests cannot differentiate between infectious and non-infectious inflammatory conditions 1
Important Clinical Considerations
Specimen Quality Matters
- A diarrheal stool sample (one that takes the shape of the container) is optimal for all infectious testing 1
- Rectal swabs may be used for bacterial detection if timely stool cannot be collected, but are 4-6 fold less sensitive than stool samples for detecting viral and bacterial pathogens 1
- Fresh stool is preferred for viral, protozoal, and C. difficile toxin detection 1
Common Pitfalls to Avoid
- Do not test formed stools for C. difficile as this detects colonization rather than infection 1
- Avoid testing children under 2 years for C. difficile as asymptomatic colonization rates reach 70% in this age group 1
- Single-sample digital rectal exam stool testing is not acceptable and should not be performed 7
- The estimated cost per significant organism identified by traditional ova and parasite examination is $1,836, making empiric treatment reasonable when pretest probability is high 2
Evolving Technology
- Multiplex molecular panels can detect up to 20 pathogens in as little as 1 hour, but their role in routine practice is still being defined 2, 3
- These panels have higher detection rates but may lead to overdiagnosis of colonization versus true infection, particularly in immunocompromised patients 3, 8