Scoring of anatomic injury after trauma: AIS 98 versus AIS 90 do the changes affect overall severity assessment?

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1 Injury, Int. J. Care Injured (2007) 38, Scoring of anatomic injury after trauma: AIS 98 versus AIS 90 do the changes affect overall severity assessment? Nils O. Skaga a, *, Torsten Eken b, Morten Hestnes a, J. Mary Jones c, Petter A. Steen d a Department of Anaesthesiology, Ulleval University Hospital, 0407 Oslo, Norway b Department of Anaesthesiology, Aker University Hospital, Trondheimsveien 235, 0514 Oslo, Norway c Mathematics Department, Keele University, Keele, Staffordshire ST5 5BG, UK d University of Oslo, University Division UUH, Department of Anaesthesiology, Ulleval University Hospital, 0407 Oslo, Norway Accepted 16 April 2006 KEYWORDS ISS; NISS; AIS; Anatomic injury; Probability of survival; Intraclass correlation coefficient (ICC) Summary Background: Although several changes were implemented in the 1998 update of the abbreviated injury scale (AIS 98) versus the previous AIS 90, both are still used worldwide for coding of anatomic injury in trauma. This could possibly invalidate comparisons between systems using different AIS versions. Our aim was to evaluate whether the use of different coding dictionaries affected estimation of Injury Severity Score (ISS), New Injury Severity Score (NISS) and probability of survival (Ps) according to TRISS in a hospital-based trauma registry. Materials and methods: In a prospective study including 1654 patients from Ulleval University Hospital, a Norwegian trauma referral centre, patients were coded according to both AIS 98 and AIS 90. Agreement between the classifications of ISS, NISS and Ps according to TRISS methodology was estimated using intraclass correlation coefficients (ICC) with 95% CI. Results: ISS changed for 378 of 1654 patients analysed (22.9%). One hundred and forty seven (8.9%) were coded differently due to different injury descriptions and 369 patients (22.3%) had a change in ISS value in one or more regions due to the different scoring algorithm for skin injuries introduced in AIS 98. This gave a minimal change in mean ISS (14.74 versus 14.54). An ICC value of (95% CI ) for ISS indicates excellent agreement between the scoring systems. There were no significant changes in NISS and Ps. * Corresponding author. Tel.: ; fax: addresses: noskaga@online.no, nosk@uus.no (N.O. Skaga) /$ see front matter # 2006 Elsevier Ltd. All rights reserved. doi: /j.injury

2 Scoring of anatomic injury after trauma 85 Conclusions: There was excellent agreement for the overall population between ISS, NISS and Ps values obtained using AIS 90 and AIS 98 for injury coding. Injury descriptions for hypothermia were re-introduced in the recently published AIS We support this change as coding differences due to hypothermia were encountered in 4.3% of patients in the present study. # 2006 Elsevier Ltd. All rights reserved. Introduction To improve quality of care for severely injured trauma patients, continuous analysis of performance data over time and versus other institutions or a recognised standard is essential. Standardised tools are required to enable such comparisons. Appropriate classification of injuries by type and severity is fundamental for quality assessment analyses. The worldwide recognised tool for anatomic injury scoring following trauma is the Abbreviated Injury Scale (AIS). The most common edition in use is AIS The coding dictionary is divided into nine anatomic body regions; in each region every anatomic injury is given a seven-digit injury description indicating body region, type and specification of anatomic structure, level and severity. AIS classifies individual injuries by body region on a six-point ordinal severity scale ranging from AIS 1 (minor) to AIS 6 (lethal) and is the foundation for Injury Severity Score (ISS), 8 a recognised tool for assessment of overall injury severity. The ISS is the sum of the squares of the highest AIS code in each of the three most severely injured ISS body regions. 5 The New Injury Severity Score (NISS) 16 is the sum of the squares of the three highest AIS codes disregarding body region localisation of the injuries. The probability of survival (Ps) of a trauma patient is most frequently estimated with the Trauma Score Injury Severity Score (TRISS) method, which since 1989 has been based on classification of overall anatomic severity assessment according to ISS, physiological derangement according to the Revised Trauma Score (RTS), age and mechanism of injury (blunt versus penetrating). 10,11,13 The 1998 AIS edition, AIS 98, 5 is implemented for instance in the Trauma Audit & Research Network (TARN) 1 in UK, in South Western Sydney Regional Trauma Registry 4 in Australia and in our own registry at Ulleval University Hospital (UUH) in Oslo, while National Trauma Data Bank (NTDB) 3 in the US still uses the 1990 version, AIS To our knowledge no existing registry has yet implemented the new revised and expanded AIS 2005 edition. 7 The use of different versions of the coding dictionary could possibly invalidate comparisons between systems. In AIS 98 5 numerous injury descriptions were changed, and a major change was introduced in the algorithm for calculating ISS for patients with skin injuries. While it is stated in the preface to the coding manual 5 that these changes should not affect overall severity assessment, no quantitative evaluation of the consequences for calculating ISS, NISS or probability of survival according to TRISS has to our knowledge been published. The aim of the present study was to analyse whether the changes in injury descriptions and/or ISS calculations introduced in AIS 98 5 caused significant changes in ISS, NISS, Ps or body region specific ISS in our trauma population. Materials and methods Population and data source Ulleval University Hospital is the major trauma hospital for 540,000 citizens in Oslo and the trauma referral centre for 2.5 million people, i.e. half of the Norwegian population. A systematic approach for care of the severely injured and a dedicated trauma team were introduced in 1984, and a custom built hospital based trauma registry was implemented in The selection of data fields, data definitions and coding guidelines were partly based on information encountered in NTDB, South Western Sydney Regional Trauma Registry and TARN. International Classification of Diseases, 10th revision (ICD-10) 2 was employed to define external causes of injury. We have implemented a rigorous protocol for data quality checks, performed by the trauma registrar, a registered nurse anaesthetist with previous trauma team experience. He is educated in injury coding (AIS 98) through a standard course of the Association for the Advancement of Automotive Medicine in the US and is under continuous supervision by the physician in charge of the registry. Approximately, 900 patients are included in our trauma registry annually, around 350 (40%) with ISS > 15. Ninety percent are exposed to blunt and 10% to penetrating injury, and the number of children <15 years is steadily increasing, at present amounting to approximately 100 per year (11%).

3 86 N.O. Skaga et al. Inclusion criteria All patients with ISS 10 admitted between August 1, 2000 and July 31, 2002 were included, whether they were admitted to Ulleval directly or via a local hospital within 24 h after injury. As we wished to include patients with only minor anatomic injury if they were severely physiologically deranged, we also included all patients admitted with trauma team activation and/or penetrating injuries towards the torso and/or proximal to elbow or knee, irrespective of ISS. We did not include patients with isolated fracture of a single extremity, as Ulleval receives approximately 700 patients per year with femoral neck fracture after low falls, many of whom had ISS 10 (9 for femoral fracture, 1 for a frequently encountered skin contusion). Transferrals more than 24 h after injury were included only if the trauma team was activated. The trauma team was activated if patients were obviously severely injured, were unstable (circulatory/respiratory instability or reduced level of consciousness), had been victims of high-energy trauma, and in other situations with a high index of concern. Patients classified as dead on arrival were also included. Our study was approved by the Regional Committee for Research Ethics. Changes in coding dictionary and guidelines The number of injury descriptions increased from 1312 in AIS 90 to 1339 in AIS 98. The changes comprised 51 new codes, 26 codes were changed and 24 removed (Table 1). Among the removed codes were injury descriptions for accidental hypothermia, inexpedient for coding of trauma in our country where hypothermia on admission is a serious concern. In addition to these changes in injury descriptions, changes were introduced in the algorithm for calculation of ISS in cases of skin injury. In AIS 90, skin injuries of minor severity (AIS 1 and AIS 2) were marked with an asterisk. Table 1 Modifications from AIS 90 to AIS 98 N % Injury descriptions AIS Injury descriptions AIS New Removed Changed Six-digit injury identifier AIS severity score Reduced Increased The guidelines for ISS calculation were (AIS 90 coding manual, 6 page 11): If the asterisked injury is the only injury in a body region, locate it under the body region in which it occurs, but assign it to ISS body region, external. If the asterisked injury accompanies an injury to a deeper structure, code the asterisked injury under the body region in which it occurs. The injury to the deeper structure in that body region will take precedence over the external injury for ISS. In AIS 98, all asterisks were removed from the text, and the algorithm was changed as follows (AIS 98 coding manual, 5 page xix): If a skin injury occurs in combination with an underlying injury, both are coded under the specific body region. The skin injury is assigned to external body region for ISS. Exceptions to this rule are open fractures and penetrating injuries where the skin injury is implicit in the code of the deeper injury and should not be coded separately. This change in algorithm for coding of skin injuries in AIS 98 is a simplification with respect to injury coding. Injury coding AIS 98 5 is the edition of the scoring dictionary that is implemented in our trauma registry. We recoded 528 patients according to the AIS 90 6 edition of the coding dictionary in a copy of our trauma database. Identification of all injury descriptions that had been changed in the revision process from AIS 90 to AIS 98 was achieved by thorough examination of both coding dictionaries (Table 1). Patients affected by the changes were identified and recoded by the physician in charge of the registry. The next step was identification of all patients in the study database with skin injuries of minor severity (AIS 1 and AIS 2). We identified six hundred and twenty nine patients (38.0% of the total study population) of whom 369 (22.3%) had an injury to a deeper structure in the same ISS body region in which the skin injury occurred, giving precedence over external injury for ISS calculation. In these 369 patients, skin injury should not be assigned to external region for ISS calculation according to AIS 90 coding guidelines. The last focus for recoding was patients with accidental hypothermia (<35 8C) on admission (71, 4.3%), who were assigned an AIS 90 injury description that did not have an AIS 98 counterpart. Finally, ISS, NISS and Ps were calculated for all patients in both editions of the study database.

4 Scoring of anatomic injury after trauma 87 Ps was calculated according to the TRISS convention 9 using the revised regression coefficients based on AIS 90 published by Champion et al. 12 To our knowledge, new regression coefficients for calculation of Ps according to TRISS have not been computed based on scoring according to AIS 98 for use in the original TRISS model. Hence, the same coefficients were used for calculation of Ps in both editions of the study database. We have chosen to present statistical differences between the databases as AIS 98 data minus AIS 90 data. Statistical analysis ISS, NISS and Ps were analysed as continuous variables. Explorative data analyses revealed a clear trend of increasing mortality rates with increasing ISS and NISS. The magnitude of the differences (AIS 98 AIS 90) in ISS, NISS, Ps and body region specific ISS (i.e. the squared value of the highest AIS score in the affected ISS body regions, defined by convention; head or neck, face, chest, abdomen or pelvic contents, extremities or pelvic girdle, external) were estimated using the mean and 95% confidence interval (CI). Measure of agreement between the two editions of the coding dictionary was analyzed by intraclass correlation coefficients (ICC) with 95% CI. ICC is defined as a ratio of the variance of interest over the sum of the variance of interest plus error. 18 It gives an overall measure of agreement for numerical data, and was obtained using a one-way analysis of variance. Interpretation has been performed according to Shrout 17 ; 0 indicating agreement no better than chance, 1 indicating perfect agreement and a value above 0.8 substantial agreement. Descriptive statistics for the main parameters are also presented. Data analysis was undertaken using Statistical Package for the Social Sciences, release 11.0 (SPSS, Inc., Chicago, IL). Results Of the 1681 patients included in the trauma registry during the study period, 27 were excluded due to lack of anatomic injury suitable for AIS scoring. This included patients with hypothermia as single injury, drowning, strangulation or syncope, all registered in our trauma database due to trauma team activation. Of the remaining 1654 patients constituting the study population, 147 (8.9%) were coded differently using AIS 98 versus AIS 90 due to different injury descriptions. Seventy-one (4.3%) of those patients were hypothermic. 369 patients (22.3%) had a change in ISS value in one or more regions due to the change in the scoring algorithm for skin injuries. Overall ISS score changed in 378 patients (22.9%). Descriptive statistics for ISS, NISS and ISS body region external are presented in Table 2. The number of patients that underwent coding changes was very small (<10) in all ISS body regions except external, and hence statistical analyses have not been performed for these subgroups (head or neck, face, chest, abdomen or pelvic contents, extremities or pelvic girdle). For ISS body region external Table 2 Difference between AIS 98 and AIS 90 for individual patients Change in ISS value ISS region External Total ISS Total NISS N % N % N % Number of patients Patients with changes

5 88 N.O. Skaga et al. Table 3 Variable Group differences between AIS 98 and AIS 90 for ISS region External, total ISS, total NISS and Ps Mean score AIS 90 Mean score AIS 98 Mean difference AIS 98 AIS 90 (with 95% CI) Intraclass correlation coefficient (ICC) (with 95% CI) ISS value in ISS region External for ( ) ( ) patients with injuries in that region (n = 867) ISS value in ISS region External ( ) ( ) for total study population (n = 1654) Total ISS ( ) ( ) Total NISS ( ) ( ) Ps ( ) ( ) the mean difference in ISS score for the total study population was 0.19 (95% CI ) with an increase for 348 patients (21% of total study population), a decrease for 21 patients (1.2%) and unchanged for 498 patients (30.1%). Mean ISS score difference from AIS 90 to AIS 98 for the 867 patients with ISS region external injuries was 0.37, 95% CI , cf. Table 2. The intraclass correlation coefficient (ICC) for the ISS scores in ISS region external (Table 3) suggests substantial agreement between the AIS 90 and AIS 98 scoring systems. Total ISS changed in 378 patients. This was mainly due to the changes in ISS region external (Table 2). Thus, the mean differences for the total study population were identical for overall ISS and ISS region external (both 0.19, 95% CI ; Table 3). The ICC value for ISS of (95% CI ) indicates excellent agreement between the two editions of the scoring system (Table 3). NISS changed for only 41 patients (2.5%), with no significant difference in mean NISS (Table 3). The ICC value of (Table 3) indicates excellent agreement. Neither was there a significant difference for the mean probability of survival, with ICC (Table 3). Discussion For 1654 trauma patients admitted to Ulleval University Hospital during a 2-year period in there were no important differences in mean Injury Severity Score, New Injury Severity Score or probability of survival with the 1990 versus the 1998 edition of the Abbreviated Injury Scale. This is important, as it potentially enables comparison of survival for registries which use AIS 98, like Ulleval, the Trauma Audit & Research Network (TARN) 1 in the UK and the South Western Sydney Regional Trauma Registry 4 in Australia, with data from registries which still employ AIS 90, e.g. the US National Trauma Data Bank. The intraclass correlation coefficients calculated also indicate excellent agreement between the two editions of the coding dictionary, as all ICC values were above 0.99 except for ISS region external where it was This was to be expected, as the most dramatic changes in the AIS coding system from the 1990 to 1998 edition were the removal of hypothermia as an injury description and the changes for the coding of skin injuries in the external region. These changes might therefore be important for studies and registries with focus on hypothermia, burn and other skin injuries. The removal of injury descriptions for accidental hypothermia is potentially problematic for evaluating systems or hospitals with a large percentage of hypothermic patients. If the occurrence of accidental hypothermia is very different in study and reference populations, this might markedly affect the results using AIS 98. Although the overall effect was small in our material, 26 of the 71 patients in our study with significant accidental hypothermia on admission (<35 8C) had lower ISS with AIS 98 than AIS 90. Hypothermia is a risk factor after injury, and used as a prognostication tool for an individual patient, this becomes particularly important. Six of the patients with accidental hypothermia can be identified in Table 2, region ISS external, displaying a reduction in ISS of between 8 and 16 with AIS 98. Their corresponding increase in Ps was between 5% and 27%. We therefore strongly support the reimplementation of descriptions for accidental hypothermia in the recently published 2005 edition of the coding dictionary. However, the actual consequences of this change should be evaluated for registries currently using AIS 98. Due to the changes in coding of ISS external, total ISS changed in 42.6% of those with an injury in the external region corresponding to 22.3% of the total patient population. One consequence of the AIS 98

6 Scoring of anatomic injury after trauma 89 revision, probably not foreseen, is that patients with an isolated open fracture may actually end up with a lower ISS than with a similar closed fracture with minimal skin contusion: In AIS 98, a patient with an open fracture of the femoral shaft is assigned an AIS severity code of 3 and consequently ISS 9, as the skin injury is part of the code for the deeper structure (coding rule 8, page xvi, AIS 98 5 ). However, a patient with a closed fracture and smaller skin contusion receives ISS 10; 9 for the femoral shaft fracture and 1 for skin contusion/laceration (ISS body region external). For burn injuries, it is important to recognise that a new code for minor third degree burn injuries (<100 cm 2, face <25 cm 2 ) was added in AIS 98, reducing the region specific ISS from four to one for this injury. The mean difference in Ps between AIS 90 and AIS 98 in the study population was virtually zero (Table 3) due to ISS being the only element that had changed in the Ps calculation and, in fact, its change was minimal between the two coding schemes. Osler et al. 16 have launched NISS as an improved way to calculate overall anatomic injury based on AIS coding. Their study indicated improved survival prediction over ISS. Recently, Lavoie et al. 14 have confirmed and extended this finding. They conclude that NISS has its particular strength versus ISS in populations with high injury severity, such as patients with head and neck injuries. This study is of specific interest for populations like ours, with a large portion of patients with head injuries. Others have been skeptic of Osler s conclusion; Tay et al. 19 and Meredith et al. 15 reported similar predictive values and misclassification rates for ISS and NISS. However, in the latter study duplicate coding (e.g., bilateral femoral fracture), which is one of the surmised strengths of NISS, 15 was not allowed. In our present study, the change in AIS protocol affected NISS in only 41 patients (2.5%), with virtually identical mean values and ICC of This robustness to changes in AIS protocol might be a further argument for implementing NISS in survival prediction models. Conclusion We found excellent agreement between the two different editions of the AIS coding dictionaries, AIS 90 and AIS 98. There were no differences in mean NISS or probability of survival (Ps) estimates. The differences in mean total ISS and external body region specific ISS between the two editions are so minimal that we do not consider them to be of clinical relevance when comparing trauma systems that use different AIS versions. A possible exception might be systems with an exceptionally high incidence of accidental hypothermia or skin/burn injuries. We strongly support the re-implementation of injury descriptions for accidental hypothermia in the recently published AIS Acknowledgements We thank The Norwegian Medical Association s Fund for Quality Improvement and the Research Council at Ulleval University Hospital, Oslo, Norway for financial support. References 1. The Trauma Audit & Research Network. The First Decade; The Trauma Audit & Research Network. Available at: Accessed February 2, External causes of morbidity and mortality. International Classification of Diseases, 10th revision (ICD-10). Geneva: World Health Organisation; American College of Surgeons. National Trauma Data Bank, Data Submission File Format 2.00; July Available at: Accessed January 19, South Western Sydney Regional Trauma Registry Report ; December 19, Available at: Accessed February 2, Association for the Advancement of Automotive Medicine. The Abbreviated Injury Scale 1990 revision-update 98Des Plains, IL USA: Association for the Advancement of Automotive Medicine. 6. Association for the Advancement of Automotive Medicine. The Abbreviated Injury Scale-1990 revisiondes Plains. IL 60018, USA: Association for the Advancement of Automotive Medicine. 7. Association for the Advancement of Automotive Medicine. The Abbreviated Injury Scale 2005Des Plains. IL 60018, USA: Association for the Advancement of Automotive Medicine; Baker SP, O Neill B, Haddon Jr W, Long WB. The injury severity score: a method for describing patients with multiple injuries and evaluating emergency care. J Trauma 1974;14: Boyd CR, Tolson MA, Copes WS. Evaluating trauma care: the TRISS method. Trauma score and the injury severity score. J Trauma 1987;27: Champion HR. Trauma scoring. Scand J Surg 2002;91: Champion HR, Copes WS, Sacco WJ, et al. The major trauma outcome study: establishing national norms for trauma care. J Trauma 1990;30: Champion HR, Sacco WJ, Copes WS. Injury severity scoring again. J Trauma 1995;38: Champion HR, Sacco WJ, Copes WS, et al. A revision of the trauma score. J Trauma 1989;29: Lavoie A, Moore L, LeSage N, et al. The new injury severity score: a more accurate predictor of in-hospital

7 90 N.O. Skaga et al. mortality than the injury severity score. J Trauma 2004;56: Meredith JW, Evans G, Kilgo PD, et al. A comparison of the abilities of nine scoring algorithms in predicting mortality. J Trauma 2002;53: discussion Osler T, Baker SP, Long W. A modification of the injury severity score that both improves accuracy and simplifies scoring. J Trauma 1997;43:922 5 [discussion 25-6]. 17. Shrout PE. Measurement reliability and agreement in psychiatry. Stat Methods Med Res 1998;7: Shrout PE, Fleiss JL. Intraclass correlations: uses in assessing rater reliability. Psychol Bull 1979;86: Tay SY, Sloan EP, Zun L, Zaret P. Comparison of the new injury severity score and the injury severity score. J Trauma 2004;56:162 4.