CHAPTER 6 TRAUMA SCORING

Trauma was the first medical specialty to regionalize health care delivery to specialized centers and to systematically measure health care outcomes. The first trauma scores were designed for a specific purpose: to standardize injury descriptions and rank injury severity to effectively triage injured patients to the appropriate trauma center.¹ Since then trauma scores have evolved to serve two new purposes: to allow risk adjustment for comparisons of outcomes for research and quality performance, and to predict the probability of survival.¹^–³ An additional purpose that trauma scores have only started to address is predicting functional impairment or disability.⁴ Currently, trauma scores play a major role in quality improvement processes and patient safety by identifying unexpected deaths for peer review audit.^2,^5,⁶ While existing scoring systems are reasonably predictive of survival, they are inadequate for measuring quality performance.⁷^–¹⁰

Most trauma scoring is based on anatomical injury descriptors or physiological derangements. Current scoring systems have been modeled to address one principle outcome—mortality—while little attention has been paid to other quality performance outcomes such as functional impairment and quality of life issues.^11,¹² Only recently have efforts been made to incorporate into scoring systems the impacts of demographics, comorbidities, and mechanism of injury. Unlike other medical scoring systems involving more uniform populations of patients and conditions (i.e., ischemic heart disease), it has proven extremely difficult to design a satisfactory scoring system in the heterogeneous trauma population. For example, there are a handful of ICD-9-CM descriptors that fully describe ischemic heart disease versus approximately 2000 descriptors for traumatic injuries. Patients with ischemic heart disease tend toward a uniform set of comorbidities and demographics, whereas trauma patients span the entire spectrum. Thus, scoring of traumatic injuries in a way that reduces the variables to a single numeric score, results in loss of detail, and generates similar or identical numeric scores for patients whose conditions are not comparable.

In the past, scoring systems were derived by consensus and did not undergo statistical modeling before release. During the last decade, much of trauma research has focused on the development, comparison, and validation of trauma scoring systems. In the future, to properly serve the new purpose of quality improvement, an ideal trauma scoring system must factor in the following:

Severity of injury

Physiologic derangements

Patient demographics

Mechanism of injury

Comorbidities

Only by accounting for all significant variables can trauma scoring systems support accurate risk stratification for outcomes research and benchmarking performance.

ANATOMIC SCORING SYSTEMS

Anatomic scoring systems require a lexicon to describe the anatomy and severity of the large number of potential injuries that result from trauma. Traditionally, this was provided by the Abbreviated Injury Scale (AIS), but more recently descriptors from the ICD-9-CM (Clinical Modification of the 9th revision of International Classification of Disease) diagnosis codes have been used. The Injury Severity Scale (ISS), Anatomic Profile (AP), and New Injury Severity Scale (NISS) are based on AIS rubrics, whereas the ICD-9 Injury Severity Scale (ICISS) is based on ICD-9-CM injury codes. Despite an ever-increasing number of injury descriptors in both the AIS and ICD-9, there are still a number of injuries that are difficult to classify accurately. The soon-to-be-released ICD-10-CM has even a larger number of injury descriptors. A further limitation of anatomic scoring systems is the difficulty in identifying all of a patient’s significant injuries, particularly in patients who die at the scene or early in their hospitalization and do not undergo autopsy.¹³

Abbreviated Injury Scale

In 1971, the American Medical Association Committee on Medical Aspects of Automotive Safety, later to become the Association for the Advancement of Automotive Medicine (AAAM), published the AIS, the first widely recognized anatomic injury scale.¹⁴ The AIS rated the severity of tissue damage secondary to motor vehicle crashes, and provided standardized terminology to describe injuries. The AIS divides the body into nine regions: head, face, neck, thorax, spine, abdomen/pelvis, upper extremities, lower extremities, and unspecified. For each region a consensus-derived scale was developed for grading injuries from 1 (minor) to 6 (virtually unsurvivable). The AIS is not an interval scale; the increase in mortality from 4 to 5 is much higher than from 2 to 3. The first published AIS described 73 blunt injuries for five body regions. Since then the AIS has been updated six times, most recently in 2005 (AIS 2005), and now includes descriptors for more than 1300 injuries covering blunt, penetrating, and pediatric injuries.¹⁵ For the first time, AIS 2005 addressed the prediction of functional impairment or disability in its classification.¹⁶ The AIS remains the foundation of most anatomic trauma scoring systems used by trauma registries as well as the National Highway Traffic Safety Administration (NHTSA) and other injury research and education organizations. The Organ Injury Scale (OIS) is a similar injury scaling system developed by the American Association for the Surgery of Trauma (AAST).¹⁷ The OIS provides a common terminology and severity score to allow comparisons of equivalent injuries for clinical research. Unlike the AIS, the OIS is not used as part of any trauma scoring system.

Injury Severity Scale

The AIS failed to account for the cumulative effect of injury in different body regions, so in 1974 Baker proposed the ISS, an algorithm based on the AIS designed to improve the ability of the AIS to predict mortality.^1,¹⁸ The ISS divides injuries into six body regions compared to nine in the AIS. The ISS is calculated by taking the sum of the squares of the highest AIS from each of the three most severely injured body regions to achieve a score that ranges from 3 (least) to 75 (most) injured. By definition, an unsurvivable injury with an AIS of 6 is automatically given an ISS of 75. An ISS of 1–8 is considered minor, 9–15 moderate, 16–24 severe, and 25 and higher very severe. The ISS reduces the great variability of injury patterns to a much smaller range of values that can be used in outcomes research. Although the ISS score correlates well with mortality, the relationship is not linear and ISS methodology was not designed to predict disability or other outcomes.¹⁹ The ISS is integral to most trauma registries, and is the basis for the anatomic component of TRISS (Trauma Injury and Severity Score) discussed later.

A significant limitation of the AIS and ISS is the cost, time, and training involved in capturing the data and calculating the scores (hand coding), particularly in hospitals that do not use a trauma registry.²⁰ Determination of the AIS requires abstraction of the injuries from the medical record and appropriate training of the trauma registrar or coder, and is dependent on the methodology used to assign the AIS codes and the version of AIS or algorithm used by the registry software to calculate the ISS. There can be significant differences in the calculation of the AIS and ISS due to registry software or personnel. These factors limit the ability to compare outcomes with data derived from varying institutional practices.²¹ Commercial computerized applications (ICDMAP) are available that convert ICD-9-CM discharge diagnosis codes into AIS scores (ICD/AIS), which in turn can be used to calculate the ISS score.²² The level to which injuries can currently be mapped by ICD-9-CM is crude compared to the AIS, as the detail of the injury descriptors is inadequate. AIS and ICDMAP are proprietary software, and this limits their availability. Despite these limitations, there is good correlation between AIS and ICD/AIS.^6,²² The most recent iteration of AIS (AIS 2005) was considered in developing the injury portion of the upcoming ICD-10-CM; thus, mapping between ICD-10-CM and AIS 2005 is likely to be even better once software becomes available.¹⁶

The ISS is statistically problematic because it is based on the sum of squares of triplets. As a result, it is nonlinear and nonmonotonic, which means that mortality does not necessarily increase with successive values of ISS. This characteristic is frequently not accounted for in outcomes research.¹⁹ Of the 75 potential values, only 44 are represented by ISS scores and 11 of these scores are generated by pairs of triplets. Eight of these triplet pairs have mortality rates that are statistically different.²³ The reason that this difference exists is the variable maximal AIS scores within pairs of triplets. For example, an ISS score of 25 is generated both by the triplets 5,0,0 and 4,3,0. Intuitively, one would expect the ISS score based on the triplet containing the near lethal 5 AIS score to have a higher mortality. This was confirmed by Russell and colleagues,²⁴ who retrospectively calculated a mortality rate of 20.6% associated with the triplet 5,0,0 compared with 0% for the triplet 4,3,0. Thus, a trauma center with a higher percentage of the lethal triplets among its patients will have worse outcomes than expected if assessment is based on the ISS alone. Even for a single-value ISS triplet, one would expect significant variability in mortality rates depending on the body region affected. For example, the mortality rate for the same AIS of an isolated injury of the head would likely be more lethal than an isolated injury to the extremity. This was shown to be true when the mortality rate for an AIS of 4 of an isolated head injury was compared with an extremity injury and found to be 17.2% versus 0%.¹⁹ And finally, at the highest ISS values of 75, there are unexpected survivors due to AIS 6 patients who do not die. The statistical problems of the ISS could potentially be improved by representing the numerical data as a categorical rather than a continuous variable in regression models; however, this does not correct the underlying problem with its methodology.

Anatomic Profile and New Injury Severity Score

Another problem with the ISS is that it underestimates mortality resulting from multiple injuries to a single body region or organ because only the single most severe injury in each region is considered.^25,²⁶ The AP and NISS were designed specifically to address this limitation of the ISS. The AP score is a modification of the AIS and ISS that uses only four regions: brain and spinal cord, thorax and neck, all other serious injuries, and all other nonserious injuries. The AP score is calculated by taking the square root of the sum of the squares of all of the AIS scores within each region to give a summation score for each region, which is then used to calculate the ISS.²⁷ The AP performs better than the ISS in single-system injury.²⁷ The modified AP (mAP) only considers AIS values greater than 3, and coefficients derived from logistic regression analysis are then used to calculate the Anatomic Profile Score (APS) to predict survival.²⁷ The AP has found limited use as the anatomic component of ASCOT (A Severity Characterization of Trauma), detailed later in this chapter.²⁸ The NISS sums the squares of the three highest AIS score regardless of body region.²⁵ The NISS and APS predict mortality better than the ISS, especially in head injuries and higher injury-severity patients, but have not gained widespread use.^25,^26,²⁸^–³¹

The ideal number of injuries to include in trauma scoring is unknown. The ISS and NISS score up to three injuries per patient, while the AP includes all injuries in its score. Multiply injured patients are currently modeled as if the effects of their injuries are independent, not cumulative; some combinations of injuries are likely to be more lethal than predicted by individual models. However, including additional injuries in trauma scoring models has not improved performance. Indeed, it has been shown that regardless of scoring system, a patient’s worst injury predicts survival best.^6,³² Accounting for multiple injuries may be more important when outcomes such as morbidity, length of stay, and disability rather than mortality are being evaluated.³³

ICD-9 Injury Severity Score

The ICISS skirts all of the issues with the AIS and ISS by directly calculating the probability of survival (survival risk ratio [SRR]) from approximately 2000 individual trauma-related ICD-9-CM diagnoses.³³ The coefficients for the SRR are calculated from logistic regression from large databases. SRRs are only estimates of true survival and are database specific; however, they have been shown to be robust in terms of their application to other sets of injured patients from comparable populations.^33,³⁴ In general, SRRs are not calculated independently of other injuries, and thus are not true representations of individual injury risk; however, independent SRRs based on single-injury cases are available.³⁵

The original mortality tables for the ICISS SRRs are based on the non-trauma North Carolina Hospital Discharge Diagnosis (NCHDD) database.³⁶ The NCHDD is criticized for not being comparable to most populations of trauma patients with its overall low mortality, low numbers of trauma patients, and atypical injury patterns. Recalculated ICISS SRRs based on the National Trauma Data Bank (NTDB) and other databases have confirmed this, underscoring the need for adequate comparisons of SRRs from various sources.^7,^36,³⁷

The ICISS carries the advantage that ICD-9-CM codes are readily available from hospital discharge codes; thus, no additional costs are incurred or trained personnel needed for capturing the data. Furthermore ICD-9-CM is universally available, and most medical personnel are familiar with ICD-based diagnosis coding in contrast to AIS coding. Another advantage of ICD-9 scoring is that risk stratification can easily be expanded to include coded comorbidities.³⁵ ICISS does not include physiologic data; however, it predicts mortality, costs, and length of stay as well as or better than risk adjustment models like TRISS and ASCOT that do.³⁵^–⁴⁰

In ICD-9-CM, there are a limited number of rubrics for orthopedic, vascular, and solid organ injury descriptors, and severity of injury is not accounted for. Therefore, coding the best diagnosis with sufficient detail of the various potential injuries is problematic in ICD-9-CM. There has been an effort to correct these discrepancies in the ICD-10-CM, whose draft version is now available. ICD-10 is already in use in the United States for coding fatal injuries, but the clinical modification has not been finalized and approved yet. The number of injury descriptors in the ICD-10-CM is large and allows precise location of injuries, in particular of interest to researchers in transportation safety. A disadvantage that results when large numbers of descriptors are available is that the number of cases on which to base each SRR will be small, thus diminishing the accuracy of the SRRs. ICISS is rapidly becoming the trauma score of choice for mortality prediction and quality improvement processes and this trend will likely continue as ICD-10-CM becomes available.³⁷