Do you suspect you have long COVID, but aren’t sure?

The answer you get will largely hinge on whom you ask, a new study says.

The medical field still lacks a clear answer as to what constitutes long COVID, despite hundreds of published studies and millions of sufferers worldwide, researchers reported Aug. 12 in JAMA Network Open.

The definition of long COVID varies so widely that the percentage of people identified as having the ailment can differ dramatically, making it harder to properly diagnose and treat patients, researchers said.

“The findings highlight the need for a standard definition for long COVID,” lead researcher Lauren Wisk, an assistant professor of medicine at UCLA’s David Geffen School of Medicine, said in a news release.

A number of major organizations like the National Academies of Sciences, Engineering, and Medicine have advanced their own definitions for long COVID, but none has stuck and all feature some flaws, researchers said in background notes.

For example, the National Academies’ definition, released in 2024, is extremely broad and does not require lab confirmation that a person actually had an initial COVID-19 infection, researchers said.

For the study, researchers applied five published long COVID definitions from previous studies to a group of more than 4,500 COVID patients being tracked as part of an ongoing research project. The prior studies took place in the U.S., U.K., Netherlands, Sweden and Puerto Rico.

The five definitions differed by symptom duration, ranging from four weeks to six months, researchers said. The definition also varied by the number of potential symptoms, from nine to as many as 44.

The percentage of patients with long COVID varied from 15% to 42%, depending on which definition had been used, results showed.

These differences can lead doctors to miss legitimate long COVID cases while misdiagnosing others who actually don’t have the syndrome, said senior researcher Dr. Joann Elmore, a professor at David Geffen School of Medicine.

“Without a shared definition, we risk mislabeling patients and misguiding care,” she said in a news release. “This is more than an academic debate — it affects real people.”

These differences are also hampering medicine’s ability to figure out long COVID, Wisk said.

“If every study on long COVID uses a different definition for identifying who has it, the scientific conclusions become harder to compare across studies and may lead to delays in our understanding of it,” she said.

“In the absence of an objective measure, like a blood test, or a uniform standard for measuring long COVID, researchers and clinicians will need to decide which definition is best suited for their scientific question and be more transparent about the potential limitations of using a more versus less restrictive definition,” Wisk added.