# Analysis of Suspected Measles Cases with Discrepant Measles-Specific IgM and rRT-PCR Test Results, Japan

## TL;DR

This study examines measles cases in Japan with conflicting test results to better understand how vaccination, prior infection, and other viruses affect diagnosis.

## Contribution

The study identifies factors influencing discrepant measles test results and suggests additional diagnostic approaches.

## Key findings

- 20 out of 24 IgM-positive, rRT-PCR-negative cases were in recently vaccinated children with low IgG avidity.
- 4 cases involved adults with unknown vaccination status and high IgG avidity.
- 62.5% of discrepant cases had nucleic acid from non-measles viruses.

## Abstract

We investigated clinically suspected measles cases that had discrepant real-time reverse transcription PCR (rRT-PCR) and measles-specific IgM test results to determine diagnoses. We performed rRT-PCR and measles-specific IgM testing on samples from 541 suspected measles cases. Of the 24 IgM-positive and rRT-PCR­–negative cases, 20 were among children who received a measles-containing vaccine within the previous 6 months; most had low IgG relative avidity indexes (RAIs). The other 4 cases were among adults who had an unknown previous measles history, unknown vaccination status, and high RAIs. We detected viral nucleic acid for viruses other than measles in 15 (62.5%) of the 24 cases with discrepant rRT-PCR and IgM test results. Measles vaccination, measles history, and contact history should be considered in suspected measles cases with discrepant rRT-PCR and IgM test results. If in doubt, measles IgG avidity and PCR testing for other febrile exanthematous viruses can help confirm or refute the diagnosis.