What is the initial test to order for diagnosing typhoid fever?

Initial Diagnostic Testing for Typhoid Fever

Blood culture is the initial test of choice for diagnosing typhoid fever, with the highest yield within the first week of symptom onset. 1

Diagnostic Algorithm

First-Line Testing

• Blood cultures should be performed in all patients with suspected enteric fever (typhoid fever) 1
  • Highest sensitivity when collected during the first week of illness 1
  • Multiple blood cultures (2-3) increase diagnostic yield 1
  • Larger blood volumes (15-20 mL for adults) improve sensitivity 1
  • Blood cultures have approximately 50% sensitivity compared to bone marrow culture 1

Additional Testing When Blood Culture is Negative

• Bone marrow culture has higher sensitivity (particularly valuable if antimicrobial agents have been administered) 1
• Additional cultures that may be beneficial:
  • Stool culture (becomes positive after the first week) 1
  • Duodenal fluid culture 1
  • Urine culture (becomes positive after the first week) 1

Tests to Avoid

• Serologic tests (including Widal test) should not be used for diagnosis of enteric fever due to poor performance characteristics 1, 2
• Newer rapid serological tests (Typhidot, Typhidot-M, Tubex) have shown mixed results and are not recommended as primary diagnostic tools 1

Clinical Considerations

When to Suspect Typhoid Fever

• Sustained fever in patients with:
  • Travel to endemic areas 1
  • Consumption of foods prepared by people with recent endemic exposure 1
  • Laboratory exposure to Salmonella Typhi 1
• Clinical presentation may include:
  • Headache, malaise, abdominal pain 1
  • Relative bradycardia 3
  • Constipation or diarrhea (diarrhea is uncommon in typhoid fever) 1

Laboratory Findings

• Complete blood count typically shows:
  • Leukopenia with relative lymphocytosis 3
  • Monocyte predominance 3
  • Thrombocytopenia 3

Emerging Diagnostic Methods

• Blood culture-PCR methods show promising results with:
  • Faster turnaround time (less than 8 hours vs. several days) 4, 5
  • Improved sensitivity compared to conventional blood culture 4, 5
  • Ability to detect bacteria after antibiotic administration 5
• Loop-mediated isothermal amplification (LAMP) techniques:
  • Higher sensitivity than conventional PCR 6
  • Can be performed without sophisticated equipment 6

Common Pitfalls

• Relying solely on serological tests like the Widal test, which lacks sensitivity and specificity 1, 2, 7
• Collecting insufficient blood volume for culture (adult samples should be 15-20 mL) 1, 8
• Delaying blood culture collection until after antibiotic administration, which reduces sensitivity 1
• Failing to consider typhoid fever in patients with fever returning from endemic areas 1