Evaluating methods for estimating premorbid intellectual ability in closed head injury.

OBJECTIVES: The present study examines the utility of three measures of premorbid intellectual functioning in closed head injury, the National adult reading test (NART), the Cambridge contextual reading test (CCRT), and the spot the word test (STW).
METHODS: In the first experiment, a group of 25 patients with closed head injury were compared with 50 healthy controls and 20 orthopaedic trauma controls. In the second experiment, the strength of correlation between the premorbid measures and current intellectual level were assessed in 114 healthy adults.
RESULTS: The head injured group performed significantly more poorly than both control groups on measures of current intellectual ability. However, no significant differences emerged between the groups on any of the premorbid measures. In the large control sample, both the NART and the CCRT accounted for about 50% of the variance in current verbal intelligence. However, by contrast, the STW only accounted for 29% of the variability in verbal intelligence. Adding demographic variables to the prediction of current intellectual level increased the amount of variance explained to 60% for the NART, 62% for the CCRT, but only 41% for the STW.
CONCLUSION: The results provide supportive evidence for the use of the CCRT and NART in estimating premorbid intellectual functioning in patients who have sustained closed head injuries, but suggest caution when employing the STW.