What is the best test for diagnosing infectious mononucleosis (Mono)

Best Test for Diagnosing Infectious Mononucleosis

For most patients with suspected infectious mononucleosis, start with a complete blood count with differential and a rapid heterophile antibody test (Monospot), but if the heterophile test is negative and clinical suspicion remains high, proceed directly to EBV-specific antibody testing with VCA IgM, VCA IgG, and EBNA antibodies. 1, 2

Initial Testing Approach

First-Line Testing

• Order a complete blood count with differential looking for >40% lymphocytes and >10% atypical lymphocytes, which are characteristic findings in infectious mononucleosis 3
• Perform a rapid heterophile antibody test (Monospot) as the initial serologic test, which has 87% sensitivity and 91% specificity 3
• The heterophile antibody typically becomes detectable between days 6-10 after symptom onset and peaks during weeks 2-3 of illness 1

Critical Limitations of Heterophile Testing

• False-negative results occur in approximately 10% of patients and are especially common in children younger than 10 years 1, 2
• The test may be negative during the first week of illness even in true EBV infection 3
• False-positive results can occur with leukemia, pancreatic carcinoma, viral hepatitis, and CMV infection 1

When Heterophile Test is Negative

EBV-Specific Antibody Panel (The Definitive Test)

The Infectious Diseases Society of America explicitly states that Paul-Bunnell and Monospot tests are suboptimal for diagnosis, and recommends EBV-specific antibody testing when these are negative. 1

Order the following panel:

• VCA IgM antibodies - indicates acute/recent infection 1, 2
• VCA IgG antibodies - develops rapidly in acute infection 1, 2
• EBNA antibodies - critical for timing the infection 1, 2

Interpreting EBV Antibody Results

• Primary acute EBV infection is diagnosed when: VCA IgM is positive (with or without VCA IgG) AND EBNA antibodies are absent 1, 2
• Past infection (>6 weeks ago) is indicated when: EBNA antibodies are present, making EBV unlikely as the cause of current symptoms 1, 2
• EBNA antibodies develop 1-2 months after primary infection and persist for life 1
• Over 90% of normal adults have IgG antibodies to VCA and EBNA from past infection 1

Special Population Considerations

Children Under 10 Years (Especially Under 6 Years)

• Heterophile antibody tests have significantly higher false-negative rates in young children 2
• EBV-specific antibody testing combined with plasma EBV-DNA testing is more reliable in children aged <6 years 4
• Plasma EBV-DNA testing has 98.27% sensitivity and 91.13% specificity in pediatric populations 4

Early in Disease Course (First Week)

• Plasma EBV-DNA testing and peripheral lymphocyte counts are more useful in the first 7 days of illness 4
• VCA IgG avidity testing becomes more reliable after 7 days of disease 4
• Consider repeating heterophile testing if initially negative but obtained very early in illness 3

Immunocompromised Patients

• Quantitative EBV viral load testing by nucleic acid amplification (NAAT) in peripheral blood is recommended for suspected EBV-associated lymphoproliferative disease 2
• Use EDTA tube, room temperature transport within 2 hours 1
• Post-transplant patients require EBV DNA surveillance due to high risk of lymphoproliferative disease 1

Common Pitfalls to Avoid

• Do not rely solely on heterophile testing in children under 10 years - proceed directly to EBV-specific antibodies 1, 2
• Do not dismiss infectious mononucleosis based on a single negative heterophile test obtained in the first week of illness 3
• Do not order EBV testing from throat swabs - EBV can persist in throat secretions for weeks to months after infection and does not confirm acute infection 2
• Check liver enzymes - elevated transaminases increase clinical suspicion for infectious mononucleosis even with negative heterophile results 3
• Consider alternative diagnoses including CMV, adenovirus, HIV, and Toxoplasma gondii when evaluating mononucleosis-like illness 1, 2