What are the guidelines for collecting a stool sample for analysis?

FIT Stool Sample Collection Guidelines

For fecal immunochemical testing (FIT), collect 2-3 stool samples at home according to manufacturer specifications, package with appropriate preservative agents, and never use a single sample obtained during digital rectal examination in the clinical setting. 1

Optimal Sample Collection Method

Sample Type and Quantity

• The optimal specimen is a diarrheal or unformed stool sample that takes the shape of the container 1
• For FIT specifically, an adequate stool sample must be obtained and packaged with appropriate preservative agents for shipping to the laboratory 1
• Collect 2-3 separate stool samples at home to complete testing, depending on manufacturer's recommendations 1

Critical Collection Pitfalls to Avoid

• A single stool sample gathered during a digital rectal exam in the clinical setting is NOT acceptable and should not be done 1
• This single-sample approach is explicitly rejected by major guidelines as it significantly reduces test sensitivity 1

Storage and Handling Requirements

Immediate Preservation

• Fresh stool is preferred for viral and protozoal agents, and C. difficile toxin detection 1
• Immediate storage at -20°C for short-term (weeks) or -80°C for longer-term conservation is recommended to prevent bacterial growth and cell lysis 1
• Avoid multiple freeze-thaw cycles as they significantly impact microbial cell integrity, resulting in degraded DNA and altered relative abundances 1

Practical Storage Approach

• Aliquot fresh stool samples before initial freezing to facilitate subsequent analyses and avoid repeated freeze-thaw cycles 1
• If immediate freezing is not possible, use manufacturer-specified preservative solutions (such as OMNIgene•GUT, RNAlater, or AquaStool) which maintain sample integrity 2

Testing Frequency and Follow-Up

Annual Screening Protocol

• If FIT is negative, it should be repeated annually 1
• Patients must understand that one-time testing is likely to be ineffective for colorectal cancer screening 1
• Positive tests are associated with increased risk of colon cancer and advanced neoplasia; colonoscopy should be recommended if results are positive 1

When NOT to Collect Additional Samples

• Follow-up testing is not recommended in most patients for case management following resolution of diarrhea 1
• Only a single stool specimen is generally required for infectious diarrhea diagnosis 1
• Multiple specimens do not increase yield for C. difficile testing 1

Alternative Collection Methods

Rectal Swabs as Backup

• If a timely diarrheal stool sample cannot be collected, a rectal swab may be used for bacterial infection detection, though this is suboptimal 1
• Viral and bacterial infectious agents are 4-6 fold more likely to be detected from stool samples (49% detection) than from rectal swabs (9% detection) 1
• Molecular techniques are generally more sensitive and less dependent than culture on specimen quality 1

Sensitivity Considerations

• Rectal swabs show overall sensitivity of 83.2% for syndromic molecular approaches, but only 65.6% for viruses and 57.1% for parasites compared to stool samples 3
• Mean threshold cycle values for most pathogens are higher (indicating lower pathogen load detection) in rectal swab specimens than in stool specimens 3