Researchers say this could lead to diagnostic tests and treatments. Global Center for Health Security

Long COVID has mystified the medical community for years, making it a tough condition to diagnose, let alone treat. However, a growing body of research has found more information on what may be behind long COVID, with the hope of eventually finding an effective treatment. Now, a new study has made an interesting discovery on what may cause long COVID symptoms: a change in the immune system that may be detected via a blood test.

That’s the major takeaway from a new study published in the journal Science. For the study, researchers followed 113 patients with COVID-19 and 39 healthy patients as controls. After six months, 40 of the COVID-19 patients developed symptoms of long COVID.

The researchers analyzed blood samples from those patients and found that they had a group of proteins that showed that a portion of the immune system called the complement system was ramped up well after the patients recovered from COVID-19.

The study analyzed 6,596 proteins across 268 blood samples, which were collected during patients’ acute phase and again six months later. Researchers found several differences in the blood of people with long COVID compared to the healthy patients, including an imbalance in proteins involved in blood clotting and inflammation. Researchers also found that those with long COVID had a group of proteins that showed that a portion of the immune system, called the complement system, was ramped up well after the patients recovered from COVID-19.