## Dengue Diagnosis in Early Febrile Phase **Key Point:** On day 2 of fever, the patient is in the acute febrile phase. NS1 antigen is the most sensitive and practical test at this early stage, especially when rapid bedside diagnosis is needed. ### Diagnostic Accuracy by Day of Illness | Test | Day 1–2 | Day 3–5 | Day 6–10 | Sensitivity | Specificity | | --- | --- | --- | --- | --- | --- | | **NS1 antigen** | ★★★★★ | ★★★★ | ★★ | >95% (early) | >99% | | **Dengue IgM** | ★★ | ★★★★★ | ★★★★★ | <50% (day 2) | >98% | | **RT-PCR** | ★★★★★ | ★★★★ | ★ | 95–100% | 100% | | **IgG** | ★ | ★ | ★★★ | Low (acute) | High | **High-Yield:** NS1 antigen appears within 24 hours of symptom onset and peaks during the febrile phase (days 1–5). On day 2, it has >95% sensitivity and is the fastest, most practical test. **Clinical Pearl:** Rapid NS1 immunochromatographic tests (point-of-care) are available in most Indian hospitals and give results in <30 minutes, enabling immediate isolation and public health action. **Mnemonic: "NS1-FIRST" for early dengue** - **N**S1 antigen: First 5 days (highest sensitivity) - **S**ensitivity >95% in febrile phase - **1** = appears on day 1 - **F**ast results (rapid test available) - **I**mmediate isolation decision - **R**eliable for public health reporting - **S**pecific (>99%) - **T**iming: days 1–5 optimal ### Why NS1 Rapid Test Is Correct 1. **Timing:** Day 2 is peak sensitivity for NS1 antigen (>95%). 2. **Speed:** Rapid immunochromatographic test gives results in <30 minutes—essential for early isolation. 3. **Practicality:** Widely available in India, cost-effective, requires no specialized equipment. 4. **Public health:** Enables rapid case reporting and vector control measures.
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