Investigators have uncovered markers in the blood that identify people with long COVID, a breakthrough finding that means it may soon be possible to develop a diagnostic test for the condition. The discovery, which was published on Monday in the scientific journal Nature, may help doctors understand the cause of the condition’s devastating symptoms, including memory issues and chronic fatigue. It also offers scientific evidence that the illness is real, according to David Putrino, a leading researcher in the study. The experiment involved blood sample analysis of 268 people, including those who have never contracted the disease, those who have had COVID but recovered, and those who have experienced symptoms of long COVID for at least four months. Putrino told NBC News that long COVID patients had “irregular” immune system responses and displayed notably lower levels of the hormone cortisol, which keeps people alert. He explained that this could be the reason why long COVID patients often suffer from ongoing fatigue. According to data from the Centers for Disease Control and Prevention, 1 in 13 U.S. adults may have long COVID.