What is the likelihood of a false positive mono (mononucleosis) titer test given a negative EBV (Epstein-Barr Virus) antibody test result?

False Positive Mono Titer Test: Likelihood and Interpretation

A false positive mono titer (heterophile antibody) test is quite likely when EBV antibody testing is negative, as the mono test has documented false positive rates in various conditions including other viral infections.

Understanding Mono Test False Positives

The heterophile antibody test (Monospot) is commonly used as an initial screening tool for Epstein-Barr virus (EBV) infectious mononucleosis, but has important limitations:

• While generally specific (96-100%), false positives do occur 1
• False positives have been documented in multiple conditions:
  • Viral infections (cytomegalovirus, dengue fever, HIV, viral hepatitis)
  • Autoimmune conditions (systemic lupus erythematosus)
  • Malignancies (leukemia, lymphoma, pancreatic cancer)
  • Other conditions (rubella) 1, 2

Confirmatory Testing for EBV

When a mono test is positive but clinical suspicion remains uncertain, EBV-specific antibody testing is recommended by the Infectious Diseases Society of America 3:

• Standard EBV antibody panel should include:

  • Viral Capsid Antigen (VCA) IgM
  • Viral Capsid Antigen (VCA) IgG
  • Epstein-Barr Nuclear Antigen (EBNA) IgG 4

• Interpretation of EBV antibody results:

  • Acute primary infection: Positive VCA IgM and VCA IgG with negative EBNA IgG
  • Past infection (>6 weeks): Positive EBNA IgG (with or without VCA IgG) 4

Cross-Reactivity with Other Infections

Several documented cases show false positive mono tests with other viral infections:

• Cytomegalovirus (CMV) can cause false positive mono tests, as documented in a 75-year-old woman with negative EBV antibodies but positive CMV IgM/IgG 5
• Dengue fever has been reported to cause false positive mono tests despite negative EBV PCR and VCA IgM 1
• Other viral infections may cause polyclonal B-cell activation leading to increased antibody titers that cross-react with the heterophile test 2

Clinical Implications

When faced with a positive mono test but negative EBV antibodies:

1. Consider alternative diagnoses - particularly other viral infections that can cause mononucleosis-like syndromes:

   • Cytomegalovirus (CMV)
   • Human Immunodeficiency Virus (HIV)
   • Toxoplasma gondii
   • Adenovirus 3

2. Evaluate for timing issues - in very early infection, antibodies may not yet be detectable:

   • Consider repeating EBV antibody testing in 1-2 weeks if clinical suspicion remains high 6
   • Be aware that up to 10% of patients with infectious mononucleosis are heterophile negative 6

3. Check for cross-reactive conditions - particularly if the patient has:

   • Recent travel history (consider dengue, other arboviruses) 1
   • Risk factors for HIV or hepatitis
   • Symptoms inconsistent with classic EBV presentation

Conclusion

A positive mono test with negative EBV antibodies should be interpreted with caution. The likelihood of a false positive mono test is substantial, especially in the context of other viral infections or conditions known to cause cross-reactivity. Confirmatory EBV-specific antibody testing is essential for accurate diagnosis, and alternative diagnoses should be actively considered.

Healthcare providers should recognize that the heterophile antibody test, while useful as a screening tool, has limitations in specificity that necessitate confirmation with more specific EBV serologic testing when clinical presentation is atypical or antibody results are discordant.