In an adult or adolescent with fever, sore throat, lymphadenopathy, fatigue, and atypical lymphocytes, how can I determine if a positive Epstein‑Barr virus (EBV) test reflects an active infection rather than past exposure?

Determining Active EBV Infection vs. Past Exposure

In a patient with fever, sore throat, lymphadenopathy, fatigue, and atypical lymphocytes, active EBV infection is confirmed by the presence of VCA-IgM antibodies with VCA-IgG but without EBNA-1 IgG antibodies, which distinguishes acute infection from past exposure. 1, 2, 3

Standard Serological Interpretation Algorithm

The three-marker approach (VCA-IgM, VCA-IgG, EBNA-1 IgG) allows interpretation of >95% of cases and provides clear differentiation between infection stages 1:

Active/Acute Infection Pattern

• VCA-IgM positive + VCA-IgG positive + EBNA-1 IgG negative = acute primary infection 1, 2, 3
• This pattern indicates infection occurring within the past 1-2 months, as EBNA-1 antibodies develop 1-2 months after primary infection 1

Past Infection Pattern

• VCA-IgM negative + VCA-IgG positive + EBNA-1 IgG positive = past infection 1, 3
• The presence of EBNA-1 antibodies indicates infection occurring more than 6 weeks prior, making EBV unlikely as the cause of current symptoms 2
• Over 90% of normal adults have IgG antibodies to VCA and EBNA antigens 1

When Initial Testing is Ambiguous

Heterophile Antibody Testing

• A positive Monospot test is diagnostic for EBV infection and no further EBV-specific testing is required 2
• However, heterophile antibodies have a ~10% false negative rate, particularly in children under 10 years 1, 2
• If the initial Monospot is negative but clinical suspicion remains high, repeat testing on a serum specimen obtained 7-10 days later when heterophile antibodies are more likely to be positive 2

Atypical Serological Patterns Requiring Additional Testing

Isolated VCA-IgG Pattern (VCA-IgG positive, VCA-IgM negative, EBNA-1 IgG negative):

• This pattern occurs in 7-11% of cases and can represent either acute infection with early VCA-IgM disappearance or past infection with EBNA-1 IgG loss 4, 5
• In adults, isolated VCA-IgG usually indicates past infection (81-100% of cases) 4
• In children <10 years, it may represent acute infection in up to 63% of cases 4
• Solution: Perform IgG avidity testing - low avidity confirms acute infection, high avidity indicates past infection 3, 6

All Three Markers Positive (VCA-IgM + VCA-IgG + EBNA-1 IgG):

• This pattern represents either recent infection (transitional phase) or reactivation 3, 5
• Occurs in approximately 2.7% of cases 5
• Solution: IgG avidity testing can differentiate - low avidity suggests recent primary infection, high avidity suggests reactivation or false-positive IgM 6

Advanced Testing for Complex Cases

IgG Avidity Testing

• Low avidity IgG confirms acute primary infection in 56.7% of VCA-IgM positive patients and 1.8% of VCA-IgM negative patients 6
• High avidity IgG in the presence of VCA-IgM indicates reactivation or false-positive IgM rather than acute infection 6
• Particularly useful in immunocompromised patients where 11 of 14 IgM-positive patients had high avidity, indicating reactivation rather than primary infection 6

Quantitative EBV PCR

• EBV DNA >10^2.5 copies/μg DNA in peripheral blood mononuclear cells indicates active infection 7, 8, 1, 2
• PCR is particularly valuable when serology is indeterminate or in immunocompromised patients 2, 9
• PCR increases diagnostic yield by >16% in primary infections when IgM VCA is positive but heterophile antibodies are absent 9
• However, only 3% of sera with elevated EA antibodies show positive PCR, questioning the utility of EA titers alone for diagnosing reactivation 9

Additional Markers for Specific Scenarios

• IgA antibodies to VCA and/or EA: Unusual in typical past infection; their presence suggests chronic active EBV infection (CAEBV) when combined with persistent symptoms 7, 1
• Early antigen (EA) antibodies: Positive EA with VCA-IgG ≥1:640 and EA ≥1:160 suggests possible CAEBV, though thresholds vary by laboratory 7, 1

Special Considerations for CNS Involvement

When evaluating suspected EBV-associated encephalitis:

• Both CSF PCR and serology (VCA-IgM/IgG and EBNA) should be performed 7, 2
• PCR testing can be associated with false-negative and false-positive results, with false-positives often occurring due to presence of EBV DNA in peripheral blood mononuclear cells 7
• Serology including VCA-IgM/IgG and EBNA is recommended in addition to CSF PCR 7

Critical Pitfalls to Avoid

• 5-10% of EBV-infected individuals never develop EBNA antibodies, making isolated VCA-IgG patterns possible in past infection 1, 4
• Laboratory variability: Antibody titers from different laboratories are not comparable due to subjective immunofluorescence testing methods and varying reagent quality 7, 1
• False-positive VCA-IgM can occur with leukemia, pancreatic carcinoma, CMV infection, and other viral hepatitis 1
• Immunocompromised patients may never develop EBNA-1 antibodies, making isolated VCA-IgG pattern common in this population 1
• Persistent symptoms beyond 10 days warrant evaluation for chronic active EBV infection (CAEBV) or hemophagocytic lymphohistiocytosis (HLH), not just repeat serology 8, 2

When to Suspect Chronic Active EBV Infection

If symptoms persist >3 months with fever, lymphadenopathy, and hepatosplenomegaly that cannot be explained by other conditions 1:

• Check quantitative EBV PCR on peripheral blood mononuclear cells 8, 1
• Look for markedly elevated VCA-IgG (≥1:640) and EA-IgG (≥1:160) 7, 1
• Consider cytokine analysis if HLH is suspected 8
• Bone marrow examination may be necessary if HLH is suspected 8