From the Guidelines
For diagnosing diarrhea, a diarrheal stool sample is the optimal specimen for laboratory diagnosis, as it provides greater fecal material and is less prone to environmental degradation compared to a rectal swab 1. When collecting stool samples, it is essential to follow proper procedures to ensure accurate test results.
- Collect a fresh stool sample that takes the shape of the container, as this is preferred for the identification of viral and protozoal agents, and C. difficile toxin 1.
- In general, only a single stool specimen is required, but culture of additional specimens may increase the sensitivity to detect bacterial pathogens in patients with persistent diarrhea 1.
- Use clean, leak-proof containers provided by your healthcare provider or laboratory, and deliver the samples to the lab within 2 hours of collection or refrigerate (not freeze) if there's a delay 1.
- For parasite testing, collect samples before taking any antibiotics or anti-diarrheal medications, as these can affect results 1.
- If bloody diarrhea is present, immediate collection is crucial as this may indicate serious infection, and some tests require special collection methods or preservatives, so follow specific instructions from your healthcare provider 1. Proper stool sample collection is essential because different pathogens require different testing methods, and the causative agent of diarrhea can only be identified through appropriate laboratory analysis, which guides effective treatment 1. Key considerations for stool testing include:
- Testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in people with diarrhea accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis 1.
- Testing for a broader set of bacterial, viral, and parasitic agents in the context of a possible outbreak of diarrheal illness or in immunocompromised individuals 1.
From the Research
Types of Stool Samples for Diagnosing Diarrhea
- A stool sample is the sample of choice for microbiological testing of enteric pathogens causing diarrhea, but a rectal swab can be a more practical alternative 2.
- Paired samples of stool and rectal swab specimens can be collected from patients with diarrhea to determine viral, bacterial, and protozoal pathogens using real-time multiplex PCR as well as standard stool culture 2.
- For acute diarrhea, stool tests are indicated when there is a strong pretest probability of infectious etiology or Clostridioides difficile infection 3.
- For chronic diarrhea, comprehensive stool biochemical analyses (osmolality, pH, electrolytes) can differentiate osmotic from secretory diarrhea and identify laxative abuse 3.
Collection and Testing of Stool Samples
- Stool samples can be tested using various methods, including culture, microscopy, and molecular approaches such as real-time PCR 2, 3.
- Enzyme immunoassay (EIA) is a commonly used test to diagnose Clostridium difficile infection (CDI) in stool samples, but it has limited sensitivity 4, 5.
- Real-time PCR detection of C. difficile toxin genes in stool samples has been shown to have higher sensitivity and specificity compared to EIA 5.
- Repeat stool testing using EIA does not significantly improve diagnostic yield for CDI 4.
Special Considerations
- For Clostridium difficile infection, a combination of tests, including culture, toxin testing, and detection of C. difficile antigen, should be used to establish a diagnosis 6.
- In pediatric settings, real-time PCR detection of C. difficile toxin genes in stool samples has been shown to be a rapid and cost-effective diagnostic modality 5.