What blood tests are used to diagnose rabies?

Blood Tests for Rabies Diagnosis

Blood tests are not the primary diagnostic method for rabies, as the disease is typically diagnosed through direct detection of the virus in brain tissue or through other specialized tests of skin, saliva, or cerebrospinal fluid.

Primary Diagnostic Methods for Rabies

Rabies diagnosis relies primarily on the following methods:

1. Direct Fluorescent Antibody Test (DFA/DFAT)

   • Gold standard test recommended by both WHO and OIE 1
   • Performed on fresh brain tissue (typically post-mortem)
   • Sensitivity of 93.58% and specificity of 95.90% 2
   • Not a blood test

2. Virus Isolation Methods

   • Mouse Inoculation Test (MIT)
   • Rabies Tissue Culture Infection Test (RTCIT)
   • Used for confirmation of DFAT results 2
   • Not blood tests

Blood-Based Testing for Rabies

When blood testing is performed in rabies contexts, it's primarily for:

1. Antibody Detection for Vaccination Response

• Rapid Fluorescent Focus Inhibition Test (RFFIT)

  • Measures virus neutralizing antibody titers
  • Used to monitor immune status in vaccinated individuals 3
  • Not used for diagnosis of active rabies infection
  • Specimens collected 1-2 weeks after vaccination should completely neutralize challenge virus at a 1:5 serum dilution 3

• Rabies Indirect Fluorescent Antibody (IFA) Test

  • Detects rabies-specific IgG and IgM antibodies in sera 4
  • Provides rapid results but doesn't provide virus-neutralizing antibody titer
  • Can be used to evaluate immune response in vaccinated individuals 4

2. Antemortem Testing in Suspected Cases

• Serum antibody testing has limited utility for diagnosis
• Detection of rabies antibodies in unvaccinated individuals suggests exposure
• However, antibodies typically appear late in disease course, limiting diagnostic value
• FDA-unapproved antibody tests should not be used as substitutes for RFFIT in suspected human rabies cases 3

Important Considerations

1. Limitations of Blood Testing

   • Antibody detection in blood is not reliable for early diagnosis
   • Antibodies typically develop late in disease progression
   • No "protective" titer is definitively known 3

2. Recommended Antemortem Testing for Suspected Rabies

   • Full-thickness skin biopsy from nape of neck with immunofluorescent antibody staining (50-94% sensitivity, ~100% specificity) 3
   • Saliva testing for viral RNA
   • CSF testing for antibodies

3. Post-Vaccination Monitoring

   • For high-risk individuals:
     • Continuous risk category: Check antibody titers every 6 months 3
     • Frequent risk category: Check antibody titers every 2 years 3
   • Booster dose recommended if titer falls below complete neutralization at 1:5 serum dilution 3

Pitfalls to Avoid

1. Relying solely on blood tests for diagnosis

   • Rabies is primarily diagnosed through direct viral detection methods
   • Blood antibody tests have limited utility for diagnosis of active infection

2. Using unapproved rapid test kits

   • No USDA-licensed rapid test kits are commercially available for rabies diagnosis
   • Unlicensed tests have unknown sensitivity/specificity and haven't been validated against standard methods 3

3. Delaying treatment while waiting for test results

   • Post-exposure prophylaxis decisions should not be delayed waiting for antibody test results
   • Several days are required to collect serum and determine test results 3