In a patient with a positive heterophile (Monospot) test, negative Epstein‑Barr virus IgG and IgM serology, palpitations and lymphadenopathy that improve after intravenous saline, what is the most likely diagnosis and recommended management?

Diagnosis: False-Positive Heterophile Test with Dehydration

This patient most likely has a false-positive heterophile (Monospot) test, and the primary issue is dehydration causing palpitations, not acute EBV infectious mononucleosis. The negative EBV IgG and IgM serology definitively rules out both acute and past EBV infection, and the dramatic improvement with IV saline points to volume depletion as the cause of palpitations 1.

Understanding the Discordant Laboratory Results

The key to this case is recognizing that heterophile antibody tests have well-documented false-positive results that occur in several conditions 1:

• Leukemia
• Pancreatic carcinoma
• Viral hepatitis
• CMV infection
• Other viral illnesses

The IDSA/ASM guidelines explicitly state that when heterophile tests are positive but clinical suspicion warrants confirmation, EBV antibody testing for IgG and IgM to viral capsid antigen (VCA) and Epstein-Barr nuclear antigen (EBNA) are recommended 1. In this case, both IgG and IgM are negative, which means:

• No acute EBV infection (would show VCA IgM positive)
• No past EBV infection (would show VCA IgG and EBNA positive)
• The heterophile test is falsely positive

Clinical Reasoning for Dehydration

The improvement with IV saline is the critical clinical clue. Palpitations that resolve with fluid resuscitation suggest:

• Hypovolemia causing compensatory tachycardia
• Orthostatic changes
• Electrolyte disturbances

This is inconsistent with true infectious mononucleosis, where symptoms persist despite hydration.

Recommended Diagnostic Approach

Immediate steps:

1. Disregard the positive heterophile test - it is a false positive given negative EBV serology

2. Evaluate for alternative causes of lymphadenopathy:

   • Complete blood count with differential (looking for atypical lymphocytes, which would be absent in false-positive heterophile)
   • CMV IgM and IgG (CMV is a common cause of heterophile-negative mononucleosis-like illness) 1, 2
   • HIV testing (acute HIV can present with lymphadenopathy and mononucleosis-like syndrome) 2
   • Toxoplasma gondii serology 2
   • Throat culture for Streptococcus pyogenes 2

3. Assess hydration status:

   • Orthostatic vital signs
   • Basic metabolic panel
   • Urinalysis for specific gravity

Management Recommendations

Primary management:

• Continue supportive care with adequate hydration (oral or IV as needed)
• Monitor for resolution of palpitations with fluid repletion
• Investigate lymphadenopathy based on additional testing above

What NOT to do:

• Do not treat for EBV infectious mononucleosis
• Do not restrict activity for "splenic rupture risk" (no actual EBV infection present)
• Antivirals are not indicated and not effective for EBV anyway 3

Common Pitfalls to Avoid

Critical mistake: Accepting a positive heterophile test without confirmatory EBV serology when the clinical picture doesn't fit. The heterophile test has a 25% false-negative rate in the first week of true IM 4, but also has false-positives that must be recognized 1.

The specificity of monospot testing is lower than traditionally taught - one study in collegiate athletes showed specificity of only 90.6% with a positive predictive value of just 36.4% 5. This means many positive monospot tests are false positives.

Remember: Negative EBV IgG and IgM together completely exclude EBV as the cause, regardless of heterophile test results 1, 6.