📘 UKSMI B 10 (2025): Faecal Specimen Investigation for Clostridioides difficile – What’s New?

Published: July 2025 | By FRCPathPrep.com Editorial Team

Essential reading for FRCPath Medical Microbiology Part 2 & lab policy updates

🧾 What is UKSMI B 10?

UKSMI B 10 is the national reference standard for the investigation of faecal specimens for Clostridioides difficile in England and Wales. Updated by UKHSA in March 2025, it outlines the diagnostic testing algorithm, infection control implications, and reporting standards for healthcare laboratories.

🆕 What’s New in the March 2025 Update?

Area	Update Summary (March 2025)
Testing Algorithm	Reinforces 2-step testing: GDH + toxin EIA followed by NAAT if discordant
NAAT Only Strategy	No longer recommended as standalone for diagnosis
Clinical Correlation	Diagnosis now requires positive toxin test or clinical features
Reporting Terminology	Updated to improve clarity; emphasizes "toxin positive" vs "NAAT positive/toxin negative"
Specimen Criteria	Only unformed stool (Bristol stool types 5–7) should be tested
Repeat Testing	Repeat samples within 7 days discouraged unless clinically justified
Infection Control	Emphasis on treating patients only with evidence of toxin production

🔬 Recommended Laboratory Testing Strategy (2025)

Step	Test	Purpose	Interpretation
1️⃣	GDH Antigen + Toxin EIA	Screening + direct toxin detection	Proceed to NAAT only if discordant
2️⃣	NAAT (PCR for toxin gene)	Confirmatory for discordant samples	Positive = toxigenic strain present, not necessarily disease

⚠️ A NAAT-positive but toxin-negative result may indicate colonization, not active disease.

📋 Revised Reporting Language (2025)

Old Language	Updated (2025)
"C. difficile detected"	"Toxigenic C. difficile detected by NAAT"
"C. difficile toxin detected"	"C. difficile toxin detected (active infection likely)"
"C. difficile PCR positive only"	"Toxin not detected; colonization possible"

💡 Reports must avoid implying active infection if toxin not detected.

❌ What Is No Longer Recommended?

• ❌ NAAT-only testing as sole diagnostic method

• ❌ Testing of formed stool

• ❌ Repeat testing within 7 days unless:

  • Clinical deterioration

  • Need for infection control re-evaluation

• ❌ Reflex testing without clinical consultation

📈 Clinical Implications

If Toxin Detected	Likely active infection → Treat + isolate
If NAAT+ but Toxin–	Colonization or early infection → Review clinically
If All Negative	C. difficile unlikely → Seek alternative diagnosis

🦠 Toxin vs Colonization — Know the Difference!

Feature	Toxin-Positive	NAAT-Positive, Toxin-Negative
Disease likelihood	High	Low/moderate – clinical correlation needed
Infection control	Full C. difficile precautions	Case-by-case basis
Treatment	Start metronidazole or vancomycin/fidaxomicin	Usually not required

🧠 FRCPath Part 2 Exam Tips

• Be able to describe the 3-test strategy: GDH + EIA + NAAT

• Interpret combinations of results and recommend clinical action

• Suggest infection control advice based on result profile

• Comment on why NAAT-only testing is misleading

• Handle case scenarios where:

  • Patient is on laxatives

  • Repeat specimen submitted within 72 hours

  • Patient is colonised but not symptomatic

📌 Summary

The 2025 UKSMI B 10 update refines the diagnostic approach to Clostridioides difficile by reinforcing the importance of detecting toxin production, clinical correlation, and avoiding overdiagnosis based on NAAT alone.

🚨 Accurate diagnosis = Better antimicrobial stewardship + Improved patient outcomes

📚 References

• UKHSA. UKSMI B 10 – Investigation of Faecal Specimens for Clostridioides difficile. March 2025.

• NICE C. difficile Guidelines

• ESCMID CDI Management Update 2024

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