What is the recommended diagnostic test for brucellosis (Brucella infection)?

Diagnostic Testing for Brucellosis

The recommended diagnostic approach for brucellosis is serological testing with an ELISA screening followed by a confirmatory agglutination test, with blood cultures performed simultaneously to establish a definitive diagnosis. 1

Primary Diagnostic Methods

Serological Testing

• In non-endemic areas:

  • Initial ELISA screening for IgM and IgG antibodies
  • Followed by confirmatory standard tube agglutination test (SAT) 1
  • Positive titer cutoff: ≥1:160 1, 2

• In endemic areas:

  • Serum agglutination test (SAT) or microagglutination test (MAT) may be used directly 1, 3
  • Strongly positive Brucella serology (titers ≥1:160) may be sufficient for diagnosis without additional testing 4

Blood Culture

• Should be performed simultaneously with serological testing 2
• Sensitivity ranges from 15-70% depending on:
  • Laboratory practices
  • Bacterial load in blood
  • Culture methods used 2
• Definitive diagnosis requires isolation of Brucella from blood, bone marrow, or tissue samples 2

Special Considerations

Bone Marrow Culture

• Considered the gold standard for diagnosis of brucellosis 2
• Higher sensitivity than blood culture due to higher concentration of Brucella in reticuloendothelial system
• Should be considered when blood cultures are negative but clinical suspicion remains high

PCR Testing

• Rapid molecular method with high sensitivity and specificity 5, 6
• Particularly useful in early stages of infection before antibody development
• Can detect Brucella DNA in serum samples 5, 6
• May yield positive results when cultures are negative, especially if antibiotics have been started

Diagnostic Algorithm

1. Initial Assessment:

   • Obtain detailed history (animal contact, consumption of unpasteurized dairy, travel to endemic regions)
   • Evaluate for compatible clinical presentation (fever, arthralgia, hepatosplenomegaly)

2. First-line Testing:

   • Blood cultures (at least 2 sets)
   • Serological testing:
     • ELISA for IgM and IgG (in non-endemic areas)
     • SAT or MAT (in endemic areas)

3. Interpretation:

   • Positive diagnosis if:
     • Positive blood culture for Brucella species, OR
     • Serological titer ≥1:160 with compatible clinical picture 1, 2

4. Additional Testing (if initial tests inconclusive):

   • Bone marrow culture (if blood cultures negative)
   • PCR testing of blood/serum
   • Repeat serological testing in 2-4 weeks if initial results are negative but clinical suspicion remains high

Common Pitfalls and Caveats

• False-negative serology: May occur early in infection, with "blocking" antibodies, or due to the "prozone" phenomenon 2

  • Solution: Repeat testing or use alternative methods like ELISA or Coombs' test

• False-positive serology: Can occur with cross-reacting antibodies from other infections

  • Solution: Confirmatory testing with more specific methods

• Culture limitations: Brucella is slow-growing and requires special culture conditions

  • Solution: Inform laboratory of clinical suspicion for brucella to ensure appropriate culture techniques

• Vertebral osteomyelitis (NVO): In patients with suspected brucellar NVO:

  • Image-guided aspiration biopsy is not necessary if blood cultures are positive for Brucella or if serology is strongly positive in endemic areas 4, 1
  • MRI is mandatory when spinal involvement is suspected 1

The microagglutination test (MAT) has shown excellent correlation with ELISA, with sensitivity and specificity of 93.3% and 96.7% for IgG and 96.7% and 98.3% for IgM, respectively 3. This test requires less time, antigen, and serum than the standard tube agglutination test, making it a practical alternative in resource-limited settings.