Labcorp data presented at IDWeek 2026 show measles PCR testing nearly quadrupled and positivity rose sharply from April 2025 through March 2026, as clinicians shift toward guideline-recommended molecular testing amid ongoing US outbreaks.
Measles PCR testing volume rose 284.5% and PCR positivity climbed 71.6% from April 2025 through March 2026 compared with the prior year, according to a retrospective analysis of Labcorp commercial laboratory data.¹ The findings were presented at
The analysis tracks a sustained rise in US
The investigators analyzed measles IgG (CLIA), IgM (IFA/EIA), and RNA PCR testing on swab and urine specimens, stratified by age, sex, geography, and ordering specialty.¹ The laboratory performed 1,405,327 IgG tests during the study period, a 32.4% increase in volume.¹ Overall IgG positivity reached 88.8%, up 1.7%.¹
Adults aged 30 to 39 years accounted for the largest share of IgG testing (n = 362,930; 26.6%), while adults aged 65 years and older had the highest positivity at 96.1%.¹ IgG immunity was highest in the Northeast and Southeast.¹ Children younger than five years had the lowest protection rates, a finding Charles Walworth, MD, associate vice president of medical affairs and education at Labcorp, described as expected.
Of 10,308 IgM tests, 3.2% were positive.¹ PCR testing totaled 7,098 tests, with 9.4% of samples positive.¹ Patients aged 5 to 19 years had the highest positivity for both IgM (11.7%) and PCR (18.4%), while children younger than five years accounted for 43.4% of PCR testing (n = 2,920).¹
IgM testing continued to outpace PCR in volume, although the PCR-to-IgM ratio exceeded 1.0 for the first time in January 2026.¹ Walworth linked the shift to the South Carolina outbreak and noted the ratio later declined. Primary care clinicians ordered most tests, and females accounted for a greater share of IgG and IgM testing, with no meaningful difference in positivity by sex.¹
Walworth suggested some clinicians may order IgM alongside IgG to assess protection and forgo further testing after a positive IgM result. He noted IgM assays can produce false-positive results from cross-reactivity with rheumatoid factor or human herpesvirus 6, or after recent measles vaccination. Timing also matters, since IgM may not appear until several days after rash onset, while PCR detects viral RNA sooner.
“You can actually have a false negative also if tested too early because it’s an antibody response,” said Walworth. “If you’re testing too early, you may miss the infection because the antibody has not yet developed.”
The investigators reported PCR orders have risen relative to IgM since February 2025, moving closer to guideline recommendations.¹ Walworth said PCR use also increases during outbreaks and in outbreak areas, and state-level heat maps showed shifts in IgM and PCR positivity in Utah, South Carolina, and Pennsylvania.
“The pediatricians seem to be best at utilizing PCR testing,” said Walworth.
For patients presenting with fever, cough, coryza, or sore throat before rash onset, Walworth advised weighing exposure history, travel, and vaccination status and acting quickly. He recommended oropharyngeal or nasopharyngeal swabs as preferred specimens, with urine as a secondary option, alongside IgM serology.
Updated Labcorp data through September 30, 2026, showed IgM volume down 18.2% vs 2025 but up 73% vs 2024, according to Walworth. PCR volume rose 5.1% vs 2025 and 510% vs 2024.
