A 28-year-old woman from Delhi presents on day 5 of fever with severe myalgia, headache, and a maculopapular rash over trunk and limbs. She has mild thrombocytopenia (platelet count 95,000/μL) and elevated transaminases. Dengue is suspected. Which investigation is most appropriate to confirm dengue infection at this stage?

Explanation

## Investigation Timing in Dengue **Key Point:** The timing of symptom onset determines which diagnostic test is most appropriate and has the highest sensitivity. ### Diagnostic Window by Day of Illness | Investigation | Optimal Timing | Sensitivity | Notes | |---|---|---|---| | **NS1 Antigen** | Days 1–5 (peak days 2–3) | 60–90% | Detects viral protein; earliest marker | | **RT-PCR** | Days 1–5 (peak day 3) | 80–95% | Gold standard but expensive; limited availability | | **IgM ELISA** | Days 4–5 onwards (peak days 7–10) | 80–90% at day 5+ | Antibody response; becomes positive from day 4 | | **IgG ELISA** | Days 7+ (peaks at 2–3 weeks) | High after day 7 | Indicates past infection or secondary dengue | **High-Yield:** On **day 5 of fever**, the patient is at the transition between viremia and antibody phase. IgM ELISA becomes reliably positive from day 4–5 onwards, making it the most appropriate confirmatory test at this stage. ### Clinical Pearl **NS1 antigen** would have been optimal on days 2–3, but by day 5, it is declining and may become negative. IgM ELISA is the standard confirmatory test for dengue in the second week of illness and is widely available in India. **Mnemonic:** **VIREMIA → ANTIBODY** — NS1 peaks early (days 2–3), IgM peaks later (days 7–10), so at day 5, IgM ELISA is the transition-phase test of choice. [cite:Harrison 21e Ch 197]