What stool test is recommended for diagnosing typhoid (Salmonella Typhi) and paratyphoid (Salmonella Paratyphi) fever?

Diagnostic Tests for Typhoid and Paratyphoid Fever

Blood culture is the primary recommended test for diagnosing typhoid and paratyphoid fever, supplemented by stool, bone marrow, and duodenal fluid cultures when indicated. 1, 2

Primary Diagnostic Methods

Blood Culture

• First-line test: 2-3 blood cultures of 20mL each for adults (lower volumes for children)
• Sensitivity: 40-80% (higher with modern methods)
• Should be collected before antimicrobial administration
• Continuously monitored blood culture systems improve detection time and sensitivity
• Blood cultures may be drawn simultaneously 1, 2

Additional Recommended Cultures

• Bone marrow culture: Gold standard with higher sensitivity (particularly valuable if antimicrobials have been administered) 1, 2
• Stool culture: Beneficial for detecting enteric fever pathogens 1
• Duodenal fluid culture: May be beneficial for diagnosis 1, 2
• Urine culture: May be positive after the first week of symptoms (sensitivity 0-58%) 2

Molecular Diagnostic Methods

• Culture-independent methods: Panel-based multiplex molecular diagnostics from stool and blood specimens are recommended when enteric fever is suspected 1
• Nucleic acid amplification tests (NAATs): May be useful for rapid detection, though sensitivity for blood specimens is limited 1

Tests to Avoid

• Serologic tests (including Widal test): Not recommended due to poor performance characteristics 1, 2, 3
• Multiple stool specimens for the same pathogen: Does not increase yield 1

Specimen Collection Guidelines

• Blood: Collect before antimicrobial administration
• Stool: Diarrheal stool sample taking the shape of the container is optimal 2
• Direct plating of blood buffy coat: Can be used when resources for culture are scarce (50% reduction in sensitivity compared to standard blood culture) 4

Clinical Considerations

• Consider typhoid/paratyphoid in patients with:
  • Sustained fever (39-40°C)
  • Headache, lethargy, malaise
  • Relative bradycardia
  • Abdominal pain
  • Hepatosplenomegaly 2
• Diarrhea is uncommon in typhoid fever but may occur 1

Common Pitfalls

1. Relying solely on serologic tests: Serologic tests like the Widal test have poor sensitivity and specificity 1, 2, 3
2. Failing to collect blood cultures before starting antibiotics: Reduces diagnostic yield significantly 2
3. Not considering typhoid in patients without diarrhea: Diarrhea is uncommon in typhoid fever 1
4. Inadequate blood volume: Using insufficient blood volume reduces sensitivity 1, 2

Diagnostic Algorithm

1. First step: Collect 2-3 blood cultures (20mL each for adults) before antibiotics
2. If high suspicion but negative blood cultures: Consider bone marrow culture
3. Additional testing: Collect stool and duodenal fluid cultures
4. When resources limited: Direct plating of blood buffy coat can be considered
5. Molecular testing: Consider panel-based molecular diagnostics if available

The diagnosis of typhoid and paratyphoid fever remains challenging, with blood culture being the most practical first-line test despite its limitations. Bone marrow culture, while more invasive, provides the highest diagnostic yield, especially after antimicrobial administration. Modern molecular methods show promise but are not yet widely available in endemic regions.