What is the gold standard diagnostic test for Shigella infections?

Gold Standard Diagnostic Test for Shigella Infections

Stool culture is the gold standard diagnostic test for Shigella infections, which should be performed alongside molecular methods like PCR or immunoassays for optimal detection.1

Diagnostic Approach for Shigella

Primary Diagnostic Method

• Stool culture on selective and differential media remains the gold standard for isolating and identifying Shigella species 1
• Culture is essential for obtaining a pure isolate needed for serotyping and molecular characterization, which are crucial for outbreak investigation and epidemiological surveillance 1
• Specimens should be collected early in the illness course, before antibiotic administration, for maximum sensitivity 1

Supplementary Diagnostic Methods

• Enzyme immunoassay (EIA) or polymerase chain reaction (PCR) should be used alongside culture for improved detection 1
• PCR directed against virulence-associated genes has shown superior detection rates compared to culture alone (19.8% vs. 7.1% positivity in one study) 2
• For PCR testing, samples should be subcultured to enrichment broth and incubated for 18-24 hours rather than testing directly on stool specimens 1

Specimen Collection and Handling

• Fresh stool specimens should be sent to the laboratory as soon as possible 1
• Specimens not processed immediately should be refrigerated and not held for >24 hours unpreserved or >48 hours in transport medium 1
• For specimens that cannot be immediately transported, a transport medium (e.g., Cary-Blair) should be used 1
• Rectal swabs may be used but contain less material; broth enrichment is recommended if rectal swabs are the only specimen available 1

Specific Testing Considerations

• Shigella dysenteriae type 1 infection is rare in the United States (<5 cases reported annually) but should be considered in returning travelers 1
• Molecular discrimination between Shigella and Escherichia coli is essential due to their genetic similarity 3
• In developing countries, S. flexneri is the most commonly isolated species (47.6% in one study), while in developed countries, S. sonnei may be more prevalent 3, 4

Pitfalls and Caveats

• Direct toxin testing of stool has lower sensitivity than testing of broth enrichment cultures 1
• EIA tests may yield false-positive results when other pathogens are present 1
• Non-typeable Shigella can account for a significant percentage of isolates (34.4% in one study), requiring molecular methods for definitive identification 3
• Antimicrobial resistance is increasingly common among Shigella isolates, making culture and susceptibility testing crucial for appropriate treatment 3, 4

Testing Algorithm

1. Collect fresh stool specimen early in illness course
2. Simultaneously perform:
   • Stool culture on selective and differential media 1
   • EIA or PCR testing on overnight enrichment broth cultures 1
3. Forward all isolates to public health laboratories for:
   • Confirmation
   • Serotyping
   • Molecular characterization (PFGE analysis)
   • Antimicrobial susceptibility testing 1

This comprehensive approach ensures maximum sensitivity for detecting Shigella infections while providing essential isolates for public health surveillance and outbreak investigation.