Scientists have potentially identified a biomarker for chronic fatigue syndrome (CFS), sometimes called myalgic encephalomyelitis (ME), and used it to create a blood test for the disease. According to the Centers for Disease Control and Prevention, as many as 2.5 million people in the US have ME/CFS, which is characterized by overwhelming fatigue that’s not improved by rest. There are currently no simple tests doctors can use to diagnose ME/CFS; instead, their diagnoses are based on symptomatic criteria. Researchers led by Rahim Esfandyarpour of the University of California, Irvine, and Ronald W. Davis of Stanford University hypothesized that they could use cell stress responses as the basis for a possible diagnostic. They collected red blood cells from 20 healthy volunteers and 20 people with CFS and exposed the cells to salt-induced osmotic stress forces. Then, using a nanoelectronic needle array, they measured how the blood cells in a salty solution impeded an electrical current. There was a distinct difference in the cells’ electrical impedance. This suggests a diagnostic signature for the disease, although the authors are still working out the mechanism (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1901274116). Esfandyarpour and Davis’s team also applied a machine-learning algorithm to the results to improve the potential diagnostic. The researchers say the test they’ve developed could also be used to screen drugs for treating ME/CFS.