A practical method for dealing with missing Glasgow Coma Scale verbal component scores

The Glasgow Coma Scale (GCS) is used for the assessment of impaired consciousness; however, it is not always possible to test each component, most commonly the verbal component. This affects the derivation of the GCS sum score, which has a role in systems for predicting patient outcome. Imputation of missing scores does not add extra information, but it does allow use of tools for predicting outcome that require complete data. The authors devised a simple and practical tool to employ when verbal component data are missing. They then assessed the tool's utility by application to the GCS-Pupils plus age plus CT findings (GCS-PA CT) prognostic model.

The authors inspected data from the International Mission for Prognosis and Analysis of Clinical Trials in Traumatic Brain Injury (IMPACT) cohort to characterize the frequency of missing verbal scores. The authors identified a single verbal score to impute for each eye and motor combined sum (EM) score from distributions of verbal scores in a published database of 54,069 patients. The effectiveness of the imputed verbal score was assessed using a dataset containing information from the IMPACT and Corticosteroid Randomisation After Significant Head Injury (CRASH) databases. The authors compared the performance of the prognostic model using actual verbal scores with the performance using imputed verbal scores and assessed the information yield using Nagelkerke's R2 statistic.

Verbal data were most commonly missing in patients with no eye opening and with a motor score of 4 or less. The "simple" imputation model that was developed performed as well as a more complex model involving distinct combinations of eye and motor scores. The imputation model consisted of the following: EM scores 2-6, add 1; EM score 7, add 2; EM score 8 or 9, add 4; and EM score 10, add 5 to provide the GCS sum score. Modeling without information about the verbal score reduced the R2 from 32.1% to 31.4% and from 34.9% to 34.0% for predictions of death and favorable outcome at 6 months, respectively, compared with using full verbal score information.

This strategy is particularly valuable for imputation in clinical practice, enabling clinicians to make a rapid and reliable determination of the GCS sum score when the verbal component is not testable. This will support clinical communication and decisions based on estimates of injury severity as well as enable estimation of prognosis. The authors suggest that external validation of their imputation strategy and the performance of the GCS-PA charts should be undertaken in other clinical populations.