What are the symptoms and diagnostic criteria of infectious mononucleosis?

Symptoms and Diagnostic Criteria of Infectious Mononucleosis

Infectious mononucleosis should be diagnosed when a patient presents with the classic triad of fever, tonsillar pharyngitis, and posterior cervical lymphadenopathy, confirmed by either a positive heterophile antibody test or, if negative, by EBV-specific viral capsid antigen antibody testing 1, 2.

Clinical Symptoms

The hallmark presentation includes:

Primary symptoms:

• Fever - persistent and often high-grade
• Tonsillar pharyngitis - severe sore throat with tonsillar enlargement
• Posterior cervical lymphadenopathy - enlarged lymph nodes, particularly in the posterior cervical chain 3, 2

Additional common findings:

• Profound fatigue - typically resolves within 3 months but may persist longer 2
• Splenomegaly - occurs in approximately 50% of cases 2
• Hepatomegaly - present in approximately 10% of cases 2
• Periorbital/palpebral edema - bilateral, occurs in one-third of patients 2
• Palatal petechiae - small hemorrhagic spots on the palate 4
• Skin rash - erythematous, maculopapular, widely scattered; occurs in 10-45% of cases 2

Important clinical pearl: The disease most commonly affects adolescents and young adults aged 15-24 years 2, 1.

Diagnostic Criteria

Laboratory Testing Algorithm

Step 1: Initial screening tests

• Complete blood count with differential looking for:
  • Lymphocytosis ≥50% of white blood cell differential 2
  • Atypical lymphocytes >10% of total lymphocyte count 2
  • Greater than 40% lymphocytes AND greater than 10% atypical lymphocytes strongly supports diagnosis 1

Step 2: Confirmatory antibody testing

• Heterophile antibody test (Monospot) - first-line confirmatory test
  • Sensitivity: 87%, Specificity: 91% 1
  • Critical caveat: False-negatives are common in children <5 years and during the first week of illness 1, 4
  • If positive with typical clinical presentation, diagnosis is confirmed 3

Step 3: If heterophile test is negative but clinical suspicion remains high

• EBV-specific viral capsid antigen (VCA) antibody testing - more sensitive and specific than heterophile testing 1
• Check for elevated liver enzymes, which increase suspicion for infectious mononucleosis even with negative heterophile test 1

When to Suspect Alternative Diagnoses

If heterophile antibody test remains negative, consider:

• Cytomegalovirus infection
• Toxoplasmosis
• Streptococcal pharyngitis
• Other viral infections 4

Key Diagnostic Pitfalls to Avoid

1. Don't rely solely on heterophile testing early in illness - false-negatives are common in the first week; repeat testing or proceed directly to EBV-specific antibodies if clinical suspicion is high 4, 1

2. Don't miss the diagnosis in young children - heterophile antibodies are less reliable in children under 5 years; use EBV-specific antibody testing instead 1

3. Don't overlook splenomegaly assessment - physical examination for splenomegaly is critical given the 0.1-0.5% risk of spontaneous splenic rupture 2

4. Recognize that atypical lymphocytosis of ≥20% OR atypical lymphocytosis ≥10% plus lymphocytosis ≥50% strongly supports the diagnosis even before antibody results return 4