What blood test is used to diagnose rabies?

Diagnosis of Rabies: Blood Tests and Other Diagnostic Methods

There is no specific blood test used for the routine diagnosis of rabies in humans; instead, the gold standard for rabies diagnosis is the Direct Fluorescent Antibody (DFA) test performed on brain tissue. 1

Primary Diagnostic Methods for Rabies

Gold Standard Tests

• Direct Fluorescent Antibody (DFA) Test:

  • Performed on brain tissue specimens
  • Recommended by both CDC and WHO as the gold standard 1
  • Must be performed by qualified laboratories designated by health departments 2
  • Results can be available within 24 hours in emergency situations 1

• Direct Rapid Immunohistochemical Test (DRIT):

  • Used primarily in surveillance programs, not for human exposure cases 1
  • All positive results must be confirmed by DFA testing 1
  • Shows 100% correlation with DFA results in comparative studies 3

Molecular Testing Methods

• PCR-Based Tests:
  • Can detect rabies virus RNA in saliva, CSF, or tissue 1
  • Quantitative RT-PCR has shown comparable results to DFA testing 4
  • Can detect variants that might be missed by other methods 4

Specimen Collection and Handling

For Animal Testing

• Only the head or brain should be submitted (except for small animals like bats) 2
• Animals should be euthanized in a way that preserves brain integrity 2
• Specimens should be:
  • Stored and shipped under refrigeration, not frozen 2
  • Not chemically fixed (as this delays testing and may prevent reliable diagnosis) 2

For Human Diagnosis

For suspected human rabies cases, multiple specimens are collected:

• Full-thickness skin biopsy from the nape of the neck (sensitivity 50-94%, specificity approaching 100%) 2
• Brain tissue (if available post-mortem)
• Saliva for PCR testing
• CSF and serum for antibody detection 1

Detection of Rabies Antibodies

• Antibody Detection:
  • Can be performed on serum or CSF 1
  • Only useful in unvaccinated patients 1
  • Not reliable for determining the need for booster vaccinations 2
  • Cannot substitute for current vaccination in managing exposures 2

Common Pitfalls in Rabies Diagnosis

• Using unlicensed test kits 1
• Relying solely on rabies serology for diagnosis 1
• Improper specimen handling (freezing or chemical fixation) 1
• Misdiagnosis of paralytic rabies as Guillain-Barré syndrome 1
• Overlooking rabies in patients with altered mental status without obvious hydrophobia 1
• Failing to consider rabies in unexplained encephalitis cases, especially with travel history to endemic areas 1

Clinical Diagnosis

• Hydrophobia and aerophobia are pathognomonic for rabies 1
• Two main clinical forms:
  • Furious (encephalitic) rabies: characterized by hydrophobia, aerophobia, and episodes of generalized excitement (80% of cases) 1
  • Paralytic (dumb) rabies: characterized by progressive flaccid paralysis 1

Important Considerations

• Rabies is almost always fatal once clinical symptoms develop 1, 5
• Prevention through post-exposure prophylaxis is the only effective approach 1
• Proper specimen collection and handling are crucial for accurate diagnosis 2
• The Milwaukee protocol (therapeutic coma) has been shown to be ineffective and should no longer be used 5

Remember that early recognition and prevention through post-exposure prophylaxis remain the most effective approaches to managing potential rabies exposures, as the disease is virtually always fatal once clinical symptoms develop.