A 3-year-old child from rural Maharashtra presents with fever, cough, coryza, and conjunctivitis for 3 days. On examination, Koplik spots are noted on the buccal mucosa. The child has not received any MMR vaccine. What is the investigation of choice to confirm measles at this stage?

Explanation

## Diagnostic Approach to Measles ### Clinical Context The patient presents during the **prodromal phase** (fever, cough, coryza, conjunctivitis) with **Koplik spots** — pathognomonic intraoral lesions appearing 2–3 days before rash. Confirmation at this stage requires detection of viral nucleic acid or antigen, not antibodies. ### Investigation Comparison | Investigation | Timing | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **RT-PCR (nasopharyngeal)** | Prodrome to early rash | 95–100% | 99–100% | **Gold standard; earliest detection** | | Viral culture | Prodrome to early rash | 60–80% | 100% | Slower; requires 5–10 days | | Serum IgM | Rash onset onwards | 90% | 95% | Appears after rash; late confirmation | | Serum IgG | Post-rash (2+ weeks) | High | High | Indicates past infection/immunity | ### Why RT-PCR Is Optimal **Key Point:** RT-PCR from nasopharyngeal secretions is the **investigation of choice** for measles confirmation during the prodromal and early rash phases because it detects viral RNA with highest sensitivity and specificity before antibody responses develop. **High-Yield:** Measles virus is most abundant in respiratory secretions during the first 3–4 days of illness (prodrome and early rash). This is the **optimal window** for molecular detection. **Clinical Pearl:** Koplik spots are pathognomonic but clinical diagnosis alone is insufficient for public health surveillance; laboratory confirmation is mandatory in India under the Integrated Disease Surveillance Programme (IDSP). ### Specimen Collection - Nasopharyngeal swab or aspirate (preferred over throat swab) - Collect during prodrome or within 3 days of rash onset - Store at 2–8°C if delayed transport **Mnemonic: VIRAL TIMING** - **V**iral nucleic acid: Prodrome to early rash (RT-PCR best) - **I**mmunoglobulin M: Rash onset onwards - **R**eal-time PCR: Gold standard - **A**ntibody detection: Late phase - **L** = Late (IgG: weeks later) [cite:Park 26e Ch 7]