NEW DELHI: Blood biomarkers for long Covid, or specific blood substances indicating the disease, have been identified, and could be used to accurately distinguish patients with the disease, scientists report in a new research published in the journal Nature.

The long Covid patients also exhibited an increased antibody circulation and markedly lower levels of the hormone cortisol, released by body under stress, compared to those not infected by SARS-CoV-2, they said.

The antibodies circulating also included those that help the body fight non-Covid viruses, such as the Epstein-Barr virus, which is a human herpes-causing virus linked with many cancers, the scientists from Yale University and other US institutes found by analysing the patients' blood samples.

"The work is a decisive step forward in the development of valid and reliable blood testing protocols for long COVID," said principal investigator David Putrino, from the Icahn School of Medicine at Mount Sinai, New York, US.

Further, "the findings can inform more sensitive testing and personalised treatments for long COVID patients," said Putrino.

For the study, the researchers recruited 271 patients between January 2021 and June 2022 and grouped them as such - those with no prior SARS-CoV-2 infection; those fully recovered from a clinically confirmed case of COVID-19; those with active long COVID symptoms for at least four months after confirmed COVID-19 infection.

The researchers analysed the patients' blood samples and applied machine learning algorithms to determine the biomarkers most effective for identifying patients with long-term COVID-19.

The patients were also asked to respond to questionnaires enquiring about their symptoms, medical history, and health-related quality of life.

The researchers said that the algorithm was 96 per cent accurate in identifying people with and without long Covid and that it detected the condition based on the distinct features exhibited by the affected participants' blood - those of enhanced antibody circulation and low levels of cortisol production.

"These markers need to be validated in larger studies, but provide a first step in dissecting the disease pathogenesis of long COVID," said co-principal investigator Akiko Iwasaki, professor of immunobiology and of molecular, cellular & developmental biology at Yale School of Medicine.

"There is no 'silver bullet' for treating long COVID, because it is an illness that infiltrates complex systems such as the immune and hormonal regulation. Complex illnesses require complex treatment solutions and we need more rapid research to better understand long COVID and discover new and promising therapies," said Putrino.