Diagnostic Testing for Mononucleosis
The Monospot test is the recommended first-line diagnostic test for infectious mononucleosis, but when clinical suspicion remains high despite a negative result, EBV-specific serologic testing should be performed. 1
Initial Diagnostic Testing
- The Monospot test detects heterophile antibodies that develop during EBV infection, making it a useful first-line diagnostic tool 1
- A positive Monospot test is diagnostic for EBV infection and no further EBV-specific testing is required 1
- False-negative Monospot results occur in approximately 10% of cases, most commonly in children younger than 10 years 1
- False-positive results may occur in patients with leukemia, pancreatic carcinoma, viral hepatitis, or CMV infection 1
When Monospot is Negative
- When the Monospot test is negative but clinical suspicion remains high, perform EBV-specific serologic testing using the same sample 1, 2
- Test for IgG and IgM antibodies to viral capsid antigen (VCA) and antibodies to Epstein-Barr nuclear antigen (EBNA) 1, 3
- Recent primary EBV infection is indicated by positive VCA IgM (with or without VCA IgG) and negative EBNA antibodies 1, 4
- Past infection is indicated by positive VCA IgG and positive EBNA antibodies 5
Complete Diagnostic Panel
- The most valuable serologic finding for acute infection is the presence of IgM antibody to EBV viral capsid antigen 3
- The diagnosis of infectious mononucleosis can be made when IgG-VCA, IgM-VCA, and anti-D (early antigen) antibodies are present while EBNA antibodies are absent 4
- EBNA antibodies appear later in the course of infection and, together with IgG-VCA antibodies, persist indefinitely 4
Blood Count Analysis
- An atypical lymphocytosis of at least 20% or atypical lymphocytosis of at least 10% plus lymphocytosis of at least 50% strongly supports the diagnosis 6
- However, relying solely on hematological criteria has a sensitivity of only 39%, though specificity is 99% 7
Testing for Alternative Diagnoses
- When EBV tests are negative, consider testing for other causes of mononucleosis-like illnesses 2:
- Cytomegalovirus (CMV)
- Human herpesvirus 6 (HHV-6)
- HIV
- Adenovirus
- Herpes simplex virus
- Streptococcus pyogenes
- Toxoplasma gondii
Common Pitfalls to Avoid
- Relying solely on a negative Monospot test to rule out infectious mononucleosis, especially in children 1
- Failing to consider EBV-specific serologic testing when clinical suspicion is high despite a negative Monospot 1
- Not recognizing that heterophile antibodies may take time to develop, potentially requiring repeat testing 1, 7
- Overlooking the possibility of false-negative results early in the course of infection 6