What is the proper procedure for collecting a stool sample for cholera diagnosis?

Proper Collection of Stool Samples for Cholera Diagnosis

The optimal specimen for cholera diagnosis is a fresh diarrheal stool sample (one that takes the shape of the container) collected within the first 48 hours of illness, as this provides the highest diagnostic yield. 1

Collection Procedure

Timing and Sample Requirements

• Collect stool samples as soon as possible after onset of symptoms, ideally within the first 48 hours when vibrio excretion levels are highest (approximately 1 million particles/ml) 1
• Collect bulk samples (enough to fill a large stool cup) 1
• Only collect loose/liquid stools that assume the shape of their container 1
• Serial specimens from persons with acute, frequent, high-volume diarrhea are particularly useful 1

Collection Method

1. Use a clean, leak-proof container with a tight-fitting lid
2. Collect 25-50 ml (or at least 25g) of fresh diarrheal stool 1
3. Avoid contaminating the sample with urine or water
4. Properly label the container with:
   • Patient's unique identifier
   • Date and time of collection 1
   • Clear indication that cholera is suspected

Storage and Transport

• Keep specimens at 4°C (refrigerated), not frozen 1
• Transport to laboratory as soon as possible, preferably within 6 hours of collection 1
• If transport is delayed, use Cary-Blair transport medium 1
• Ensure containers are verified as leak-proof before transport 1

Important Considerations

What to Avoid

• Do not collect rectal swabs for cholera diagnosis as they have very low diagnostic yield 1
• Do not freeze specimens as this may destroy characteristic bacterial morphology 1
• Avoid small or formed stool specimens as they have lower diagnostic yield 1
• Do not delay collection awaiting results of other investigations 1

Laboratory Testing Methods

• Laboratory confirmation requires identification of V. cholerae by culture 1
• Rapid diagnostic tests (RDTs) can be used for initial screening but have moderate sensitivity (91%) and specificity (80%) 2
• PCR testing of stool specimens offers higher sensitivity than culture or enzyme immunoassays 3
• Alkaline peptone water (APW) enrichment before RDT testing improves specificity to 98% 2

Special Circumstances

• During outbreaks, collect specimens from at least 10 ill persons to maximize diagnostic yield 1
• For epidemiological investigations, paired serum specimens should also be collected:
  • Acute: during first week of symptoms
  • Convalescent: 3-6 weeks later 1

Pitfalls to Avoid

• Waiting too long to collect samples (after 48-72 hours, vibrio excretion decreases significantly) 1
• Using inadequate containers that might leak during transport
• Freezing samples (refrigerate instead) 1
• Relying solely on clinical diagnosis without laboratory confirmation
• Collecting only a single specimen when multiple specimens might increase detection sensitivity 1

By following these guidelines, healthcare providers can ensure proper collection of stool samples for accurate and timely diagnosis of cholera, which is essential for appropriate treatment and control measures.