What are the diagnostic tests for mononucleosis (mono)?

Diagnostic Testing for Mononucleosis

For patients with suspected mononucleosis, begin with a complete blood count with differential and a rapid heterophile antibody (Monospot) test, which together provide cost-effective initial diagnosis in most cases. 1, 2

Initial Laboratory Testing

Order these tests first:

• Complete blood count with differential looking specifically for greater than 40% lymphocytes AND greater than 10% atypical lymphocytes 2
• Rapid heterophile antibody test (Monospot) which has 87% sensitivity and 91% specificity 2

The combination of lymphocytosis (at least 50% lymphocytes) with at least 20% atypical lymphocytes, or at least 10% atypical lymphocytes, strongly supports the diagnosis 3

When the Monospot Test is Negative

If clinical suspicion remains high despite a negative heterophile test, proceed to EBV-specific antibody testing. 1

The heterophile test has important limitations:

• False-negatives occur in children younger than 10 years 1, 2
• False-negatives occur during the first week of illness 1, 2
• False-positives can occur with leukemia, pancreatic carcinoma, viral hepatitis, and CMV infection 1

EBV-Specific Serologic Testing

When heterophile testing is negative but suspicion persists, order a panel including:

• Viral Capsid Antigen (VCA) IgM and IgG antibodies 1
• Epstein-Barr Nuclear Antigen (EBNA) antibodies 1
• Early Antigen (EA) antibodies 1

Interpretation of EBV serology:

• Acute primary infection: VCA IgM positive (with or without VCA IgG) AND EBNA antibodies absent 1, 4
• Past infection: VCA IgG positive AND EBNA antibodies positive, with VCA IgM negative 1, 5

The presence of IgM antibody to EBV viral capsid antigen is the most valuable serologic finding during acute primary EBV infection 4

Alternative Diagnoses to Consider

If both heterophile and EBV testing are negative, test for:

• Cytomegalovirus (CMV) 1, 3
• HIV 1
• Toxoplasma gondii 1, 3
• Adenovirus 1
• Streptococcal pharyngitis 3

Special Populations Requiring Different Approaches

For immunocompromised patients, order more extensive testing:

• EBV viral load by nucleic acid amplification testing (NAAT) 1
• Testing for EBV-associated lymphoproliferative disease 1
• Quantitative PCR showing more than 10^2.5 copies/mg DNA in peripheral blood mononuclear cells suggests chronic active EBV 6

For children under 10 years: Skip the heterophile test and proceed directly to EBV-specific antibody testing, as heterophile tests are unreliable in this age group 1, 2

Additional Supportive Findings

Elevated liver enzymes increase clinical suspicion for infectious mononucleosis when the heterophile test is negative 2

The classic clinical triad consists of fever, pharyngitis, and cervical lymphadenopathy, with posterior cervical or auricular adenopathy being particularly suggestive 4, 2, 3