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Importance: Estimated base rates of invalid performance on baseline testing (base rates of failure) for the management of sport-related concussion range from 6.1% to 40.0%, depending on the validity indicator used. The instability of this key measure represents a challenge in the clinical interpretation of test results that could undermine the utility of baseline testing. Objectives: To determine the prevalence of invalid performance on baseline testing and to assess whether the prevalence varies as a function of age and validity indicator. Design, Setting, and Participants: This retrospective, cross-sectional study included data collected between January 1, 2012, and December 31, 2016, from a clinical referral center in the Midwestern United States. Participants included 7897 consecutively tested, equivalently proportioned male and female athletes aged 10 to 21 years, who completed baseline neurocognitive testing for the purpose of concussion management. Interventions: Baseline assessment was conducted with the Immediate Postconcussion Assessment and Cognitive Testing (ImPACT), a computerized neurocognitive test designed for assessment of concussion. Main Outcomes and Measures: Base rates of failure on published ImPACT validity indicators were compared within and across age groups. Hypotheses were developed after data collection but prior to analyses. Results: Of the 7897 study participants, 4086 (51.7%) were male, mean (SD) age was 14.71 (1.78) years, 7820 (99.0%) were primarily English speaking, and the mean (SD) educational level was 8.79 (1.68) years. The base rate of failure ranged from 6.4% to 47.6% across individual indicators. Most of the sample (55.7%) failed at least 1 of 4 validity indicators. The base rate of failure varied considerably across age groups (117 of 140 [83.6%] for those aged 10 years to 14 of 48 [29.2%] for those aged 21 years), representing a risk ratio of 2.86 (95% CI, 2.60-3.16; P < .001). Conclusions and Relevance: The results for base rate of failure were surprisingly high overall and varied widely depending on the specific validity indicator and the age of the examinee. The strong age association, with 3 of 4 participants aged 10 to 12 years failing validity indicators, suggests that the clinical interpretation and utility of baseline testing in this age group is questionable. These findings underscore the need for close scrutiny of performance validity indicators on baseline testing across age groups.