A 32-year-old woman from Kerala presents with sudden onset of high-grade fever, severe joint pain (especially wrists, ankles, and knees), and a maculopapular rash on the trunk and limbs for 3 days. She also reports myalgia and headache. Clinical examination reveals arthralgia with mild swelling of the affected joints. What is the most appropriate investigation of choice to confirm the diagnosis at this stage of illness?

Explanation

## Diagnostic Approach to Chikungunya ### Clinical Context The patient presents with the classic triad of **acute chikungunya**: fever, arthralgia (especially polyarticular involving small joints), and rash. The 3-day duration places her in the **acute viraemic phase** (days 1–7 post-symptom onset). ### Investigation of Choice: RT-PCR **Key Point:** RT-PCR (reverse transcription polymerase chain reaction) is the gold standard for **early diagnosis** of chikungunya during the acute viraemic phase (first 5–7 days). **High-Yield:** RT-PCR detects viral RNA directly in serum and has: - **Highest sensitivity** (95–100%) in the first week - **Highest specificity** (>99%) - Rapid turnaround (4–6 hours) - Can differentiate from dengue and Zika co-infections ### Comparison of Serological Tests | Investigation | Timing | Sensitivity | Use Case | |---|---|---|---| | **RT-PCR (serum)** | Days 1–7 (acute phase) | 95–100% | **Acute diagnosis** | | **IgM ELISA** | Days 3–5 onwards (early immune response) | 80–90% by day 5 | Confirmation after day 5 | | **IgG ELISA** | Days 7–14 onwards (late/convalescent) | High after day 7 | Seroprevalence, past infection | | **Viral culture** | Days 1–3 (rarely used) | 30–50%, slow | Research only | ### Why RT-PCR is Superior in This Case 1. **Timing:** Patient is on day 3 of illness — still in the viraemic window. 2. **Rapid confirmation:** Allows immediate diagnosis without waiting for antibody development. 3. **Specificity:** Distinguishes chikungunya from co-circulating dengue and Zika (all three are endemic in Kerala). 4. **Clinical urgency:** Early diagnosis guides isolation and contact tracing. **Clinical Pearl:** In endemic areas with multiple arbovirus circulation (dengue, chikungunya, Zika), RT-PCR is the only investigation that can definitively identify the causative agent during acute illness. [cite:Park 26e Ch 20]