A 6-year-old child presents with fever, sore throat, cervical lymphadenopathy, and hepatosplenomegaly. Atypical lymphocytes are seen on blood smear. Monospot test is negative. Which investigation is most appropriate to confirm the diagnosis of EBV infection?

Explanation

## Investigation of Choice for EBV Confirmation in Seronegative Patients ### Clinical Context: Why Monospot Failed **Key Point:** The Monospot (heterophile antibody) test is negative in this child, likely because: 1. **Age <4 years:** Heterophile antibodies develop inconsistently in young children; up to 80% of children <4 years with EBV may be Monospot-negative despite active infection. 2. **Early infection:** Heterophile antibodies appear 1–2 weeks after symptom onset; testing too early may be negative. In such cases, **EBV-specific serology (IgM and IgG)** is the confirmatory test of choice. ### Why EBV Serology Is Superior **High-Yield:** EBV-specific serology directly detects antibodies to viral antigens: - **EBV IgM:** Appears early (first week), indicates acute infection, disappears within 3–4 weeks. - **EBV IgG:** Appears in week 2–3, persists lifelong (indicates immunity). - **Interpretation:** IgM+ / IgG− = acute EBV; IgM− / IgG+ = past infection or immunity. **Mnemonic: EBV Serology Interpretation — "VICA"** - **V**iral capsid antigen (VCA) IgM → **acute** infection - **I**mmune to EBV if VCA IgG+ - **C**omplex: EBNA (Epstein–Barr nuclear antigen) IgG appears late (week 4–6), persists lifelong - **A**cute phase: IgM+, IgG− ### Why Other Options Are Suboptimal | Investigation | Limitation | |---|---| | **Viral culture** | Requires 2–4 weeks; EBV grows poorly in standard cell lines; not practical for diagnosis | | **EBV DNA PCR** | Sensitive but not routinely used for diagnosis of acute infection; reserved for immunocompromised patients or CNS disease | | **Repeat Monospot** | Will likely remain negative in this age group; specificity is high but sensitivity is poor in children <4 years | **Clinical Pearl:** The combination of fever, pharyngitis, cervical lymphadenopathy, hepatosplenomegaly, and atypical lymphocytes is classic for infectious mononucleosis. In a Monospot-negative child, EBV serology (VCA IgM/IgG) is the gold standard confirmatory test.