Figure 2. Receiver operating characteristic (ROC) curve of King-Devick (K-D) test for distinguishing concussed from nonconcussed athletes.

ROC curve areas represent the probability that a test can distinguish concussed from nonconcussed athletes, and range from 0.5 (probability no better than chance) to 1.0 (perfect ability to distinguish). Area under the curve = 0.91. The optimal cutoff point of 2 seconds worsening predicts concussion (sensitivity 90%, specificity 91%).