False-Positive Mononucleosis Test: Understanding EBV Serology

A positive heterophile antibody test (Monospot) or EBV-specific antibodies without clinical mono is common and has specific explanations.

Why False Positives Occur

Scenario	Mechanism
Recent EBV infection (past 6–12 mo)	IgM persists; heterophile antibodies can linger months after symptomatic mono resolves.
Chronic/reactivated EBV	Immunosuppression (HIV, transplant, chemo) → viral replication → antibody rise without acute mono syndrome.
Cross-reactivity	Heterophile test: false + in lymphoma, SLE, rubella, CMV, toxoplasmosis, some drugs.
Asymptomatic primary infection	~50% of childhood EBV infections are subclinical; seroconversion without mono.

Interpreting EBV Serology

Marker	Acute	Past	Reactivation
VCA-IgM	+	–	± (low/transient)
VCA-IgG	+ (rising)	+	+ (high)
EBNA-IgG	–	+ (appears 2–4 mo)	+
EA-D-IgG	+	–	+

Clinical Approach

1. If asymptomatic + positive test: likely past infection. No treatment needed.
2. If symptomatic: correlate with VCA-IgM, EBNA. Heterophile alone insufficient.
3. If immunosuppressed + rising EBV DNA PCR: preemptive therapy (rituximab, antivirals) for PTLD risk.

EBV seropositivity ≠ acute mono. 90% of adults are EBV+. Clinical context is everything.