Early appreciation of major trauma enables emergency medical service (EMS) providers to match the available resources to each victim’s needs. Triage is the process of classifying patients according to injury severity and determining the priority for further treatment [1, 2]. Field triage has become increasingly important, as regionalised trauma care with dedicated trauma teams has been shown to improve patient outcome [3–5]. Nevertheless, some mistriage is unavoidable, as field triage is performed close to the time of injury, with limited diagnostic resources in a multifarious pre-hospital environment. If major trauma victims are undertriaged and therefore denied access to high-resource resuscitation, avoidable negative outcomes may ensue [1, 6]. Conversely, overtriage may cause minor trauma victims to be unnecessarily transferred to dedicated trauma care facilities, thereby consuming scarce financial and human resources. Overtriage thus decreases the available resources for other patients with greater needs [7, 8].

The rates of over- and undertriage are considered to be trauma system quality indicators [2]. Although these data are debated, the American College of Surgeons - Committee on Trauma (ACS-COT) states that an undertriage rate of 5–10% is unavoidable, and most systems are associated with an overtriage rate of 30–50% [2, 9].

The definition of major trauma provides the reference standard against which the over- and undertriage rates are calculated [10]. There is a 40-year tradition of grading the severity of individual injuries using the Abbreviated Injury Scale (AIS), and based on this scale, the Injury Severity Score (ISS) can be calculated as the sum of the squares of the highest AIS code in each of the three most severely injured ISS body regions [11, 12]. The US Major Trauma Outcome Study (MTOS) found that an ISS >15 was associated with a mortality risk of at least 10% and was related to a distinct increase in mortality [13]. Following this study, many subsequent triage studies dichotomised study populations into “major trauma” patients, who were defined as having an ISS >15, and “minor and moderate trauma” patients (ISS ≤15), and they presented two-by-two tables describing the diagnostic accuracy of triage algorithms [10, 14]. Several limitations of the ISS have been highlighted [15, 16], providing a basis for the New Injury Severity Score (NISS) [17]. The NISS is a simple modification of the ISS and is calculated from the three most severe injuries regardless of body region. The NISS has been considered to be more predictive of survival, especially in patients suffering from multiple head injuries or penetrating trauma [17–20]. Although the ISS is still the dominant scale in papers published on triage precision, an NISS >15 is recommended as an inclusion criterion in the Utstein template for uniform reporting of data following major trauma [21].

Mortality and morbidity are the principal outcomes after trauma, and their relevance remains undisputed [22, 23]. To define major trauma, several studies have therefore combined anatomic injury scales, such as the ISS or NISS, with variables associated with mortality, morbidity, type of injury, or resource consumption [24–26]. The rationale is an understanding of major trauma as more complex than anatomic injury alone. The compound definitions in these studies often include process-mapping variables, making the definitions more system-specific and thereby reducing the external validity and reproducibility.

Butcher et al. reported, in their review on the definitions of “polytrauma”, that there was no consensus on the term [27]. This lack of consensus was corroborated by a recent systematic review of pre-hospital prognostic trauma models [28], in which the authors also questioned the external validity of published studies on triage precision and emphasised the challenges inherent in the comparison of triage systems.

To compare data sets, assess external validity and facilitate multicentre trials, the impact of different definitions of major trauma on quality assessments should be clarified. The aim of the present study was to investigate how various definitions of major trauma influence the calculation of under- and overtriage in a trauma cohort.

The definition of major trauma is commonly based on anatomic injury alone, and both ISS >15 and NISS >15 are recommended cut-off values. Our study revealed no significant differences in the perceived under- and overtriage rates between NISS >15 or ISS >15 as the definitions for major trauma. This finding suggests that the outcomes of triage precision calculations may be comparable between trauma systems, regardless of the use of NISS >15 or ISS >15 as definitions. In contrast, the NISS will be equal to or greater than the ISS for any given patient, depending on the injuries sustained. Accordingly, utilising NISS >15 instead of ISS >15 will result in an increased number of included patients in most trauma populations (cf. Table 4 and Figure 1). In the present population, we found a 24% relative increase in the number of patients who were defined as having sustained a major trauma when NISS >15 was applied, compared to ISS >15 (from 470 to 585; see Table 4). This increase might be interpreted as improved sensitivity without loss of specificity, implying that NISS >15 is superior to ISS >15 as a definition of major trauma [18, 29]. However, this difference in sensitivity caused by the use of a different injury scale obviously makes the results less comparable. It has therefore been argued that a compound definition of major trauma is necessary [25]. Factors other than anatomic injury influence outcome, and the inclusion of the mechanism of injury and/or outcome variables, such as mortality, in the definition of major trauma seems relevant. The Utstein template recommends 30-day mortality, Glasgow Outcome Scale (GOS), discharge destination, and hospital length of stay (LOS) to be reported as outcome measurements after trauma [21]. In an attempt to capture the complexity of trauma, several studies have included such outcomes in their definitions of major trauma [6, 25, 30–32]. In our cohort, expanding the purely anatomic definition of major trauma by including proximal penetrating injury, 30-day mortality, ICU LOS greater than 2 days and transferred intubated out of the hospital at ≤2 days did not significantly influence the perceived triage precision.

Defining major trauma only in terms of the mechanism of injury, death within 30 days and resource consumption, without including anatomic injury scaling, drastically reduced the number of cases defined as major trauma. The proportions of perceived major trauma patients in the study population were reduced from 34.0% and 42.3% with ISS >15 and NISS >15, respectively, to 9.5% in the “extended” group and 22.6% in the “extensive” group (Table 4), thereby putting into serious doubt the usefulness of these definitions for triage precision calculations.

The definition of major trauma provides a reference standard when calculating the precision of trauma triage. However, the definitions are inconsistent in the current literature. In our cohort, although the proportion of patients who were defined as having sustained major trauma increased when NISS-based definitions were substituted for ISS-based definitions, the outcomes of the triage precision calculations did not differ significantly between the two scales. Additionally, adding the mechanism of injury and outcome variables did not significantly influence the triage precision calculations. Based on our findings we recommend that triage precision calculations should include anatomical injury scaling according to NISS. To further enhance comparability of trauma triage calculations, researchers should establish a consensus on a uniform definition of major trauma.