Close to 1 in 5 past diagnoses of multisystem inflammatory syndrome in children would now not technically qualify as cases under an updated definition from the CDC, according to a study published Jan. 11 in Pediatrics.
Increased cases of MIS-C following COVID-19 infections in early 2020 led the CDC to update its definition at that time and begin tracking it more closely, requiring clinicians to report cases in patients under 21.
Since then, a revised definition from the agency drafted in late 2022, which took effect on Jan. 1, 2023, changed what counts as an MIS-C infection. Now, after examining how classification differs from the 2020 definition compared to the 2023 revision, researchers revealed that a little over 17% of previously diagnosed MIS-C cases would no longer meet the requirements of the new definition.
Part of what prompted the 2023 revision was a need to further distinguish between MIS-C and Kawasaki disease (KD), another pediatric hyperinflammatory syndrome, which MIS-C is sometimes misclassified as.
"Development of an MIS-C case definition for national surveillance is challenging because of a lack of a confirmatory diagnostic laboratory test or pathognomonic clinical features that distinguish MIS-C from COVID-19 or other pediatric hyperinflammatory syndromes such as Kawasaki disease," the CDC wrote of its updated definition.
Researches explained that with the new definition in their study they "identified likely improvements in accuracy of MIS-C cases identified for surveillance and in the ability to differentiate between KD and MIS-C," they wrote, but also noted that "although the 2023 case definition seems better able to discern MIS-C from KD, there continues to be overlap."