Diagnosis of Infectious Mononucleosis
Order a complete blood count with differential and a rapid heterophile antibody (Monospot) test as your initial diagnostic approach for suspected infectious mononucleosis. 1
Initial Laboratory Testing
- Obtain a complete blood count with differential looking specifically for absolute lymphocytosis with atypical lymphocytes, which is present in most cases of EBV infectious mononucleosis 1
- Lymphocytosis ≥50% of the white blood cell differential and atypical lymphocytosis ≥10% of total lymphocyte count strongly support the diagnosis 1, 2, 3
- Order a rapid heterophile antibody (Monospot) test as the first-line serologic screen, which has a sensitivity of 87% and specificity of 91% 1, 2
Critical Timing Considerations for Heterophile Testing
The heterophile test has important limitations based on timing and patient age that you must understand to avoid misdiagnosis:
- The test typically becomes positive between days 6 and 10 after symptom onset, so false-negative results are common during the first week of illness 1, 4
- Overall false-negative rate is approximately 10%, but this increases substantially in specific populations 1, 5
- Children younger than 10 years have particularly high false-negative rates, making the heterophile test less reliable in this age group 1, 5
- A single negative heterophile result obtained in the first week should not be used to exclude mononucleosis 1
When to Proceed to EBV-Specific Serology
If clinical suspicion remains high after an initial negative heterophile test, either repeat the heterophile assay after 7-10 days OR proceed directly to EBV-specific serology. 1
When ordering EBV serology, obtain all three antibodies together as a panel:
- IgM antibodies to viral capsid antigen (VCA)
- IgG antibodies to VCA
- Antibodies to Epstein-Barr nuclear antigen (EBNA) 1
Interpreting EBV Serology
- Acute primary infection is confirmed by the presence of VCA IgM (with or without VCA IgG) in the absence of EBNA antibodies 1, 6
- Past infection (>6 weeks ago) is indicated by the presence of EBNA antibodies, which effectively rules out acute mononucleosis 1
- EBNA antibodies appear 1-2 months after primary infection and persist for life 1
Supportive Laboratory Findings
- Liver function tests (AST, ALT, bilirubin) are elevated in roughly 90% of infectious mononucleosis cases and can reinforce the diagnosis when heterophile testing is negative 1, 7, 2
- Mild transaminase elevations are typical, and the presence of elevated liver enzymes increases clinical suspicion for infectious mononucleosis in the setting of a negative heterophile antibody test 7, 2
Differential Diagnosis Testing
When both heterophile and EBV testing are negative or equivocal, consider testing for other causes of mononucleosis-like illness:
- Cytomegalovirus (CMV) infection
- Human immunodeficiency virus (HIV) infection (particularly important in adolescents and young adults)
- Toxoplasma gondii infection
- Adenovirus infection
- Streptococcal pharyngitis (which may coexist with EBV infectious mononucleosis) 1, 2, 8
Common Pitfalls and False-Positive Results
Be aware that false-positive heterophile antibody results may occur in patients with:
Clinical Features Supporting Diagnosis
While laboratory confirmation is essential, recognize these key clinical features:
- Classic triad: fever, pharyngitis (tonsillar), and cervical lymphadenopathy (particularly posterior cervical) 2, 3
- Profound fatigue that may be protracted but typically resolves within three months 7, 3
- Splenomegaly occurs in approximately 50% of cases and hepatomegaly in 10% 5, 3
- Maculopapular rash in 10-45% of cases, particularly common in patients treated with antibiotics 5, 3
- Fever can reach as high as 40°C (104°F) 4, 5