Blood Test to Diagnose Long COVID Could Be On Its Way Soon

One of the hardest things about diagnosing and treating long COVID has been the lack of lab tests to reliably identify this condition. But a new study suggests that it might one day be possible to diagnose long COVID with a blood test.

Researchers examined blood samples from 268 people, including some who’d never had COVID-19, some who had infections and recovered, and some who had long COVID (defined as symptoms that persist or develop at least four months after the initial infection).

Blood tests found clear differences in immune and hormone function in the people with long COVID, according to the study results, which were published in Nature.

“These findings are important — they can inform more sensitive testing for long COVID patients and personalized treatments for long COVID that have, until now, not had a proven scientific rationale,” said principal study investigator David Putrino, PhD, in a statement. Dr. Putrino is a professor and director of the Abilities Research Center at Icahn School of Medicine at Mount Sinai and the Cohen Center for Recovery From Complex Chronic Illnesses, both in New York City.

“This work is so exciting because it is one of the first to show us clear, measurable differences in blood biomarkers of people with long COVID compared with people who recovered fully from an acute infection and a group of people who have never been infected with SARS-CoV-2, the virus that causes COVID-19,” Putrino said. “This is a decisive step forward in the development of valid and reliable blood testing protocols for long COVID.”

Symptoms of Long COVID Vary Widely

Most people with COVID-19 get better within a few days or a few weeks of infection, according to the Centers for Disease Control and Prevention (CDC). People can be diagnosed with long COVID when they have symptoms four weeks or longer after their initial infections. Symptoms of long COVID can vary widely, and include post-exertional malaise (the worsening of symptoms after even minor physical or mental exertion), fatigue that interferes with daily life, “brain fog” (difficulty thinking or concentrating), and digestive issues.

Patients who have severe COVID infections or who aren’t vaccinated have a higher risk of developing long COVID, says the CDC.

About 6 percent of Americans have long COVID, according to a report published by the CDC in August 2023. The report also noted that more than one-quarter of those with long COVID experienced significant limitations in their daily activities.

Long COVID Patients Showed Differences in T-Cell Activity, Cortisol Levels

For the new study, patients completed a detailed set of questionnaires about their symptoms, medical history, and health-related quality of life. Then scientists took blood samples to look for distinct patterns that might be consistent with long COVID.

Patients with long COVID had unusual activity in certain immune system cells including disease-fighting T-cells, the analysis found. These patients also had significantly lower levels of the hormone cortisol, which can lead to weakness and fatigue.

In addition, lab tests showed active levels of Epstein-Barr and other herpes viruses. Most people get infected with Epstein-Barr, which causes mononucleosis infections, at some point in their lives and it remains in their body in an inactive state, according to the CDC. The study findings suggest that inactive Epstein-Barr may become reactivated in people with long COVID.

Blood Tests Could Be Available in a Year or 2

The study results are preliminary and must be confirmed with more research before blood tests for long COVID can become a routine tool for diagnosing the condition. But it’s possible that this type of diagnostic test might be widely available within a year or two, says Daniel Altmann, PhD, an infectious disease researcher at Imperial College London in England, who wasn’t involved in the new study.

Not only would blood tests help diagnose long COVID, it could also help speed development of new treatments, Dr. Altmann says. That’s because drug companies would have an easier time running clinical trials for potential therapies if they had a lab test that could clearly identify whether people had long COVID, a tool that currently isn’t available.

“The rewards for cracking it would be huge,” Altmann says.