Lay summary – GFAP is a protein that can be released into the blood after serious brain injuries, such as traumatic brain injury, stroke, or loss of oxygen to the brain. This study tested whether a quick, portable blood test could reliably detect raised GFAP levels compared with a highly accurate laboratory test. In samples from people with traumatic brain injury, the portable test was good at identifying raised GFAP levels and gave results that closely matched the laboratory test. This suggests it could help support earlier assessment of people with acute brain injuries, including before they reach hospital.