(NEW YORK) — While there is no specific test to determine if someone is experiencing long COVID, a new study published in Nature used blood tests to find new insight into what biological markers are associated with this collection of mysterious conditions reported by millions of Americans.

Researchers used machine learning to help analyze immune markers and hormone levels in 273 adult participants at Mount Sinai and Yale University and compared those with and without long COVID symptoms at least one year after having COVID-19.

Long COVID, defined in this study as persistent symptoms more than six weeks after infection, was associated with lower levels of a hormone called cortisol and had some distinct differences in certain immune cells and inflammatory markers circulating in the blood.

These levels are identified through blood tests, but this is not a blood test that specifically tests for long COVID.

“[These results] suggests some potential mechanisms leading to long COVID that might be amenable to treatment. It also may help in identifying patients with long COVID,” Dr. Alison Morris, division chief of pulmonary, allergy, critical care and sleep medicine at the University of Pittsburgh School of Medicine, told ABC News.

“I think one of the most important findings of the paper is that it validates the symptoms people have by finding biological differences between them and healthy controls,” Morris said.

“It is a truly remarkable study,” Dr. Shari Barnett Brosnahan, M.D., M.Sc., a COVID-19 researcher who was not involved in the study and assistant professor of the division of pulmonary, critical care and sleep medicine at New York University Langone Health System, told ABC News.

Brosnahan does note that “it’s a limited analysis that they did, and I think that there’s still more work to be done.”

This study was done on relatively few people, so researchers say more studies are needed to better understand the significance of these results more broadly. Still, they say this study helps scientists get one step closer to knowing more about long COVID. If there are biological markers that are specific to long COVID, it could help confirm a diagnosis or help target treatments.

This research joins a large movement to gain more understanding of long COVID conditions and efforts to provide services to those affected, including efforts by the Biden administration which recently announced a new Office of Long Covid Research.

Long COVID is a term used to characterize signs, symptoms and conditions that persist for at least four weeks after getting infected with COVID-19 that may last months to years, according to the working definition developed by the U.S. Department of Health and Human Services.

These symptoms can range in severity and impact multiple organ systems in the body. Common complaints include fatigue, brain fog, sleep problems, difficulty breathing or shortness of breath, dizziness upon standing, and stomach issues, according to the CDC.

In July 2021, long COVID was recognized as a condition that could qualify as a disability under the Americans with Disabilities Act if it “substantially limits major life activities.” Research that may help doctors make a diagnosis could be crucial for those most severely impacted to receive these necessary services.

“I also hope that continuing to have these studies that show objective evidence help validate people and understanding [of] their long COVID,” Brosnahan said. “And help us as a medical community validate the disease of long COVID a little bit more.”

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