A 6-year-old boy from Delhi presents with fever for 4 days, myalgia, headache, and a maculopapular rash over trunk and extremities. On examination, he is febrile (38.5°C), has mild hepatomegaly, and a positive tourniquet test. Platelet count is 95,000/μL. Which investigation is most appropriate to confirm dengue infection at this stage?

## Diagnosis of Dengue in Children: Timing of Investigations **Key Point:** The choice of confirmatory test depends on the day of illness. NS1 antigen is the earliest and most reliable marker in the acute phase (days 1–5 of fever). ### Investigation Timing in Dengue | Investigation | Optimal Timing | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **NS1 antigen** | Days 1–5 of fever | 95–100% | 99% | **Gold standard for acute phase** | | **IgM antibody** | Days 4–5 onwards (peaks by day 10) | 80–90% | 95% | Useful after day 5; unreliable early | | **RT-PCR** | Days 1–3 of fever | 95% | 100% | Reference standard but expensive, not routine | | **HAI test** | Days 7–14 (convalescent phase) | Variable | 90% | Retrospective diagnosis only | **High-Yield:** In this child presenting on day 4 of fever, NS1 antigen detection is the **investigation of choice** because: 1. It is positive from day 1 through day 5 of illness 2. It has the highest sensitivity and specificity in the acute phase 3. It is rapid, cost-effective, and widely available in India 4. IgM antibodies are only beginning to appear and may be falsely negative at this stage **Clinical Pearl:** NS1 antigen is a non-structural protein released during viral replication and appears in serum before antibodies develop. This makes it the earliest and most reliable marker for acute dengue diagnosis. ### Diagnostic Algorithm for Dengue ```mermaid flowchart TD A[Suspected dengue fever]:::outcome --> B{Day of illness?}:::decision B -->|Days 1-5| C[NS1 antigen ELISA]:::action B -->|Days 4-14| D[IgM antibody ELISA]:::action B -->|Days 1-3 only| E[RT-PCR dengue RNA]:::action B -->|Day 7+ convalescent| F[HAI test]:::action C --> G[Positive = Acute dengue]:::outcome D --> H[Positive = Dengue infection]:::outcome E --> I[Gold standard if available]:::outcome F --> J[Retrospective confirmation]:::outcome ``` **Mnemonic:** **NIRD** — **N**S1 (days 1–5), **I**gM (days 4–14), **R**T-PCR (days 1–3), **D**ouble serum (HAI, convalescent)