GFAP and brain injury: When examining group by time interaction, cadets in the concussion group had increased GFAP levels compared with those in the contact-control group at the acute postinjury point (mean difference in ln values, 0.34; 95% CI, 0.18-0.50; P < .001), the 24- to 48-hour postinjury point (mean difference in ln values, 0.22; 95% CI, 0.06-0.38; P = .007), and the asymptomatic postinjury point (mean difference in ln values, 0.21; 95% CI, 0.05-0.36; P = .01).