It has been suggested that a ratio score of part B to part A of the Trail Making Test (TMT) may have clinical utility as a measure of cognitive efficiency and/or dissimulation. We investigated the clinical utility of the TMT B/A ratio score in the evaluation of patients with traumatic brain injury. Part B of the TMT demonstrated a statistically significant difference between groups with mild (n = 59) versus moderate-severe (n = 41) injuries. However, the B/A ratio score did not demonstrate sensitivity to injury severity and also failed to discriminate reliably individuals who had independent psychometric indicators of possible dissimulation from those who did not. We conclude that in individuals with traumatic brain injury, the B/A ratio score does not appear to enhance the clinical utility of the TMT and that interpretation of performance on this test should instead involve comparing subtest completion times to established normative data.