Even though millions of people worldwide are believed to have long COVID and hundreds of studies have been published, there’s still no clear, agreed-upon definition of what the condition actually is.
That lack of clarity is creating big problems for doctors, researchers, and patients.
A new study from UCLA, published on August 12 in JAMA Network Open, shows just how much definitions vary—and how that changes the numbers.
The research found that depending on how long COVID is defined, the percentage of people considered to have it can range widely, even in the same group of patients.
“The findings highlight the need for a standard definition for long COVID,” said study lead Lauren Wisk, assistant professor of medicine at the David Geffen School of Medicine at UCLA.
“Up to one third of the variation in published studies may be because they use different definitions.”
The UCLA team looked at data from 4,700 U.S. patients in the INSPIRE study.
They applied five different definitions of long COVID used in earlier research from the U.S., U.K., Netherlands, Sweden, and Puerto Rico.
Each definition set its own rules—for example, how long symptoms must last (anywhere from four weeks to six months) and how many possible symptoms to include (ranging from nine to 44).
When the researchers applied these definitions to the same group of patients, the estimated prevalence of long COVID ranged from 15% to 42%. In the original studies that created those definitions, the reported rates had been anywhere from 2.6% to 61.9%.
The team also tested how well each definition matched patients’ own reports of whether they thought they had long COVID.
None of the definitions performed perfectly. Some missed people who considered themselves to have the condition, while others included people who didn’t believe they had it.
Senior author Dr. Joann Elmore compared the problem to using different measuring sticks. “Without a clinically usable and standardized research definition of long COVID, it’s like every study is using a different measuring stick,” she said. “That makes it hard to compare results, develop treatments, or track progress.”
This isn’t just a scientific headache—it affects real patients. Without a shared definition, some people with genuine long COVID symptoms might be overlooked, while others might be wrongly told they have it. That could mean delays in treatment, misdirected care, and slower progress in finding effective therapies.
The study’s authors say that until there’s an objective test, like a blood marker, researchers and doctors will need to carefully choose the definition that best fits their needs—and be open about the trade-offs between stricter and looser criteria.
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Source: UCLA.
