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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.

This letter to the editor discusses the importance of test data in interpreting surveillance data for acute respiratory infections (ARI) in Saitama, Japan in 2020. The authors highlight the need to consider the number of tests performed and the positivity rate when analyzing trends in surveillance data. They provide examples of how test data can help interpret the number of cases and the burden of respiratory pathogens in different age groups. The authors also emphasize the importance of accounting for testing frequency in temporal interpretations of surveillance data. Overall, this letter emphasizes the significance of test data in improving data interpretation and informing public health responses. [Extracted from the article]