Measles testing volume and positivity rates continued to increase over the past year amid outbreaks, according to findings presented at IDWeek 2026.
“This increase may be driven by higher awareness of infections and continued spread among unvaccinated individuals,” wrote presenting author Charles M. Walworth, MD, associate vice president of medical affairs and education at Monogram Biosciences/Labcorp in Laguna Beach, California, and colleagues.
The United States has experienced a rise in measles infections since 2024, when 285 were confirmed. According to the authors, approximately 2,200 infections were confirmed in 2025, followed by more than 1,700 by mid-April 2026, with new outbreaks reported across 33 jurisdictions.
The investigators retrospectively analyzed measles immunoglobulin G testing using chemiluminescent immunoassays, immunoglobulin M testing using immunofluorescence assays or enzyme immunoassays, and RNA PCR testing of swab and urine samples performed at a large U.S. commercial laboratory between April 2025 and March 2026. Testing volumes and positivity rates were compared with those from the preceding year, with results stratified by age, sex, geographic region and ordering specialty.
Measles IgG testing volume increased by 32.4% from the previous year, with 1,405,327 tests performed during the study period. Overall IgG positivity increased to 88.8%, up 1.7%. The highest testing volume was observed among individuals aged 30 to 39 years (n = 362,930; 26.6%), while those aged 65 years or older had the highest overall positivity rate (96.1%).
A total of 10,308 IgM tests were performed, representing a 41.9% increase from the previous year. Of these tests, 3.2% were positive, a 265.5% increase. Testing volume was highest among individuals aged 30 to 39 years (n = 2,260; 22.5%); the highest overall positivity rate was observed among those aged 5 to 19 years (11.7%).
Compared with the previous year, PCR testing volume increased by 284.5% to 7,098 tests, and the positivity rate increased by 71.6% to 9.4%. Testing volume was highest among children younger than 5 years (n = 2,920; 43.4%), while those aged 5 to 19 years again had the highest overall positivity rate (18.4%).
“Higher positivity rates with PCR testing compared to IgM continue, despite more IgM testing,” the authors wrote. Of note, PCR orders reportedly increased significantly relative to IgM orders beginning in February 2025, with the PCR-to-IgM order ratio exceeding 1.0 for the first time in January 2026 — a shift that the authors noted “align[s] closer to guideline recommendations, especially in pediatrics.”
Although females were found to account for a greater proportion of IgG and IgM testing, PCR testing was evenly distributed by sex, and positivity rates did not differ meaningfully across sexes. The highest levels of IgG immunity were observed in the Northeast and Southeast U.S., according to the authors, with primary care clinicians ordering most tests.
Dr. Gillim reported holding stocks and bonds in Labcorp; Martens reported holding stocks and bonds in Labcorp; and Dr. Walworth reported employment with Labcorp. Disclosure information was not provided for the remaining authors.