What is the stool antigen test to look for Staphylococcus (Staph) aureus?

Staphylococcus aureus Stool Antigen Testing

There is no specific stool antigen test designed for routine detection of Staphylococcus aureus in clinical stool specimens. Instead, specialized procedures for toxin detection are used when S. aureus food poisoning is suspected 1.

Diagnostic Methods for S. aureus in Stool

Primary Diagnostic Approach

• S. aureus is not routinely tested for in stool specimens submitted for enteric pathogen detection 1
• When S. aureus enterocolitis is suspected, the following findings support the diagnosis:
  • Presence of gram-positive cocci in clusters on gram stain of stool specimens 2
  • Isolation of S. aureus as the dominant or sole flora in stool culture 2

Toxin Detection Methods

• For suspected S. aureus food poisoning, specialized procedures for toxin detection are required 1
• Staphylococcal enterotoxins (SE) are typically detected in:
  • Food samples rather than stool specimens 1, 3
  • Broth cultures from isolated colonies when clinical specimens are tested 2

Laboratory Methods for Enterotoxin Detection

• Enzyme immunoassay (EIA) methods are most commonly used for detecting staphylococcal enterotoxins 4, 5
• Commercial test systems include:
  • VIDAS Staph Enterotoxin tests for detection of classical (SEA-SEE) and newer (SEG, SEH, SEI) enterotoxins 4
  • Sandwich ELISA methods with detection limits around 0.5 ng enterotoxin per ml 5
• Real-time PCR assays can detect genes encoding staphylococcal enterotoxins (SEA, SEB, SEC1, SED) with higher sensitivity than protein detection methods 6

Clinical Considerations

When to Consider S. aureus Testing

• Consider S. aureus as a potential cause in:
  • Antibiotic-associated diarrhea, especially with large-volume, cholera-like presentation 2
  • Food poisoning outbreaks with rapid onset (2-6 hours) of symptoms 3
  • Immunocompromised patients or infants with enterocolitis 2

Important Clinical Distinctions

• S. aureus enterocolitis differs from C. difficile infection by presenting with:
  • Large-volume, cholera-like diarrhea 2
  • More rapid onset of symptoms after exposure 3
  • Shorter duration of illness (typically 24-48 hours) 3

Testing Limitations and Pitfalls

• Protein A produced by S. aureus can interfere with immunoassays and produce false-positive reactions in sandwich ELISA tests 5
• Direct toxin testing in stool has lower sensitivity than testing of broth enrichment cultures 7
• S. aureus is not included in routine stool culture protocols for enteric pathogens, which typically focus on Salmonella, Shigella, Campylobacter, and E. coli O157:H7 1
• A specific request for S. aureus culture and toxin testing must be made when this pathogen is suspected 1, 2

Summary

Unlike other enteric pathogens such as Shiga toxin-producing E. coli or Shigella that have specific stool antigen tests, S. aureus detection in stool relies on specialized culture techniques and toxin detection methods that are not routinely performed unless specifically requested 1, 2. The diagnosis of staphylococcal food poisoning is typically made based on clinical presentation and detection of enterotoxins in implicated food rather than in stool specimens 3, 4.