Abbreviated Injury Scale: not a reliable basis for summation of injury severity in trauma facilities?

Injury severity is most frequently classified using the Abbreviated Injury Scale (AIS) as a basis for the Injury Severity Score (ISS) and the New Injury Severity Score (NISS), which are used for assessment of overall injury severity in the multiply injured patient and in outcome prediction. European trauma registries recommended the AIS 2008 edition, but the levels of inter-rater agreement and reliability of ISS and NISS, associated with its use, have not been reported.

Nineteen Norwegian AIS-certified trauma registry coders were invited to score 50 real, anonymised patient medical records using AIS 2008. Rater agreements for ISS and NISS were analysed using Bland-Altman plots with 95% limits of agreement (LoA). A clinically acceptable LoA range was set at ± 9 units. Reliability was analysed using a two-way mixed model intraclass correlation coefficient (ICC) statistics with corresponding 95% confidence intervals (CI) and hierarchical agglomerative clustering.

Ten coders submitted their coding results. Of their AIS codes, 2189 (61.5%) agreed with a reference standard, 1187 (31.1%) real injuries were missed, and 392 non-existing injuries were recorded. All LoAs were wider than the predefined, clinically acceptable limit of ± 9, for both ISS and NISS. The joint ICC (range) between each rater and the reference standard was 0.51 (0.29,0.86) for ISS and 0.51 (0.27,0.78) for NISS. The joint ICC (range) for inter-rater reliability was 0.49 (0.19,0.85) for ISS and 0.49 (0.16,0.82) for NISS. Univariate linear regression analyses indicated a significant relationship between the number of correctly AIS-coded injuries and total number of cases coded during the rater's career, but no significant relationship between the rater-against-reference ISS and NISS ICC values and total number of cases coded during the rater's career.

Based on AIS 2008, ISS and NISS were not reliable for summarising anatomic injury severity in this study. This result indicates a limitation in their use as benchmarking tools for trauma system performance.