What is the initial lab test for diagnosing mononucleosis?

Initial Laboratory Test for Diagnosing Mononucleosis

The Monospot test (heterophile antibody test) is the recommended initial laboratory test for diagnosing infectious mononucleosis due to its rapid results, cost-effectiveness, and good diagnostic accuracy. 1

Diagnostic Algorithm for Infectious Mononucleosis

• Begin with a Monospot test as the first-line diagnostic tool for suspected infectious mononucleosis, as it detects heterophile antibodies that develop during EBV infection 1, 2
• Simultaneously order a complete blood count (CBC) with differential to assess for lymphocytosis (>40% lymphocytes) and atypical lymphocytes (>10%), which support the diagnosis 2, 3
• If the Monospot test is positive, consider the diagnosis of EBV infection confirmed, and no further EBV-specific testing is required 1, 4
• If the Monospot test is negative but clinical suspicion remains high, proceed with EBV-specific serologic testing using the same sample 1, 4

EBV-Specific Serologic Testing

• When Monospot is negative but mononucleosis is still suspected, test for IgG and IgM antibodies to viral capsid antigen (VCA) and antibodies to Epstein-Barr nuclear antigen (EBNA) 1, 4
• Recent primary EBV infection is indicated by positive VCA IgM (with or without VCA IgG) and negative EBNA antibodies 1, 4, 5
• Past infection is indicated by the presence of EBNA antibodies, which typically appear 6-12 weeks after infection 4, 5
• Elevated liver enzymes in a patient with negative heterophile antibody test but clinical features of mononucleosis increases suspicion for EBV infection 2

Special Considerations

• The Monospot test has a sensitivity of approximately 87% and specificity of 91%, making it a reliable initial test for most patients 2, 6
• False-negative Monospot results occur in approximately 10% of cases, most commonly in children younger than 10 years and in adults during the first week of illness 1, 6
• False-positive Monospot results may occur in patients with leukemia, pancreatic carcinoma, viral hepatitis, or CMV infection 1
• In children under 10 years, EBV-specific antibody testing is more important due to higher false-negative rates of heterophile antibody tests 4

Common Pitfalls to Avoid

• Relying solely on a negative Monospot test to rule out infectious mononucleosis, especially in children or early in the course of illness 1, 6
• Failing to consider EBV-specific serologic testing when clinical suspicion is high despite a negative Monospot 1, 4
• Not recognizing that heterophile antibodies may take time to develop, potentially requiring repeat testing if initial results are negative but symptoms persist 1, 6
• Overlooking that approximately 9% of patients with infectious mononucleosis may initially have no detectable EBV-specific antibodies, requiring follow-up testing 7

Interpretation of Results

• The diagnosis of infectious mononucleosis can be made when IgG-VCA, IgM-VCA antibodies are present and EBNA antibodies are absent 5
• Hematological criteria alone (lymphocyte count ≥50% and atypical lymphocyte count ≥10%) have a sensitivity of only 39%, though specificity is high at 99% 7
• In patients with negative initial testing but persistent symptoms, consider repeat testing as some patients show delayed emergence of EBV-specific antibodies 7