Emergency Department Triage Scales and Their Components: A Systematic Review of the Scientific Evidence

Triage is a central task in an emergency department (ED). In this context, triage is viewed as the rating of patients' clinical urgency [1]. Rating is necessary to identify the order in which patients should be given care in an ED when demand is high. Triage is not needed if there is no queue for care. Triage scales aim to optimize the waiting time of patients according to the severity of their medical condition, in order to treat as fast as necessary the most intense symptom(s) and to reduce the negative impact on the prognosis of a prolonged delay before treatment. ED triage is a relatively modern phenomenon, introduced in the 1950s in the United States [2]. Triage is a complex decision-making process, and several triage scales have been designed as decision-support systems [3] to guide the triage nurse to a correct decision. Triage decisions may be based on both the patients' vital signs (respiratory rate, oxygen saturation in blood, heart rate, blood pressure, level of consciousness, and body temperature) and their chief complaints. Internationally, no consensus has been reached on the functions that should be measured. Apart from emergency care, triage may be used in other clinical activities, e.g. deciding on a certain investigation [4] or treatment [5].

Since the early 1990s, several countries have developed and introduced ED triage [6‐10]. Development of triage scales in some countries has been influenced largely by the seminal work of FitzGerald [11], resulting in most of the triage scales developed in the 1990s and 2000s being designed as 5-level scales. Of these, the Australian Triage Scale (ATS), Canadian Emergency Department Triage and Acuity Scale (CTAS), Manchester Triage Scale (MTS), and Emergency Severity Index (ESI) have had the greatest influence on modern ED triage [12‐15]. Other scales have not disseminated as widely around the globe, e.g. the Soterion Rapid Triage Scale (SRTS) from the United States and the 4-level Taiwan Triage System (TTS) [6, 7, 9, 16, 17]. Some countries, e.g. Australia, have a national mandatory triage scale while many European countries lack such standards [7, 9].

Patients may have a life-threatening condition, but show normal vital signs. Hence, in triaging the patient it is important to consider information given by patients or accompanying persons regarding the patient's chief complaints or medical history, which can provide essential information about serious diseases. The chief complaints describe the incident or symptoms that caused the patient to seek care.

In 2005, a joint task force of the American College of Emergency Physicians and the Emergency Nurses Association published a review of the literature on ED triage scales. Based on expert consensus and available evidence, the task force supported adoption of a reliable 5-level triage scale, stating that either the CTAS or the ESI are good choices for ED triage [18]. In 2002, a national survey conducted in Sweden identified the use of 37 different triage scales across the country. Further, some 30 EDs did not use any type of triage scale [19].

This systematic review aims to investigate the scientific evidence underlying published ED triage scales.

The following questions are addressed:

A systematic search of the international literature published from 1966 through March 31, 2009 explored the British Nursing Index, Business Source Premier, CINAHL, Cochrane Library, EMBASE, and PubMed. Inclusion was limited to studies of adult patients (≥15 years) visiting EDs for somatic reasons. Another criterion for inclusion was that the study design must contain a control, i.e. randomized controlled trials (RCT), observational studies with a control group based on previously collected data, and before-after studies. Descriptive studies without a control group and retrospective studies were excluded.

Figures 1 and 2 illustrate the results of the primary search.

Our systematic review shows that when adjudicated by standard criteria for study quality and scientific evidence, the triage scales used in EDs are supported, at best, by limited evidence. Often, the evidence is weaker, not above insufficient by the GRADE criteria. The ability of the individual vital signs included in the different scales to predict outcome has seldom, or never, been studied in the ED setting. The scientific evidence for assessing interrater agreement (reproducibility) was limited for one triage scale (Brillman) whereas it was insufficient or lacking for all other scales. Two of the scales (CTAS and ATS) offered limited scientific evidence, and the scientific evidence for one scale (METTS) was insufficient to assess the risk of early death or hospitalization in patients assigned to the two lowest triage levels in 5-level scales; the studies showed the risk of death to be low, but a need for inpatient care was not excluded (about 5% hospital admission rate on average). Studies on validity of the triage scales across all levels, i.e. their ability to distinguish the urgency in patients assigned the five different levels, were generally of low quality. Consequently, evidence was insufficient to assess the validity of the scales.

As none of the studies reported on mortality rates adjusted for differences in age and gender between the triage levels, we could not evaluate the validity of the triage scales across all triage levels as regards the risk of early death. To estimate the safety of the scales, we studied early death among patients assigned to the lowest triage levels (green and blue/4-5). Two triage scales (ATS and CTAS) offered limited scientific evidence for assessing safety. In both scales, the patients assigned to the two lowest triage levels had a very low risk of dying within 24 hours after triage. Hence, in this respect, the scales are safe to use. Scientific evidence for METTS, the newly developed Swedish triage scale, was found to be insufficient to assess safety. Since the study recorded the risk of dying during the in-hospital stay, mortality was higher than in the studies on ATS and CTAS.

In using the need of hospitalization as a measure of safety, the situation was found to be more complex. Again, none of the studies reported on hospital admission rates adjusted for age and gender, so we could not evaluate the validity of the triage scales across all triage levels. However, on average, about 5% (in some studies up to 17%) of patients in the lowest (4-5/green-blue) triage levels in ATS, ESI, and SRTS were reported to be admitted as inpatients. The variations were wide not only between different triage scales, but also between studies using the same scales. This indicates differences between the studies in (a) patient populations in the ED, (b) access to hospital beds, (c) hospital admission policies and traditions, and/or (d) inaccurate triage decisions (i.e. patients were rated as less urgent than their actual urgency).

No definitive conclusions could be drawn regarding which of the scales was the safest as measured by the need of hospitalization. Hence, we suggest that none of the scales be used in referral of patients in the lowest triage levels (4-5/green-blue), e.g. to primary care, without further medical examination in the ED.

New diagnostic tests typically need to meet rigid criteria before they can be accepted for widespread use. These criteria include documentation on precision. For non-laboratory tests, interrater agreement (reliability) is a key precision issue. Our review shows that most triage scales present insufficient scientific evidence for assessing interrater agreement. The study designs used to estimate interrater agreement have often been suboptimal. Most of the studies are based on fictitious cases rather than on authentic patients in real-life settings. The value of the studies as regards interrater agreement is also compromised by the fact that the mean age of patients assessed has either been low (as low as 30 years) or unreported. The generalizability to real-life ED patients must therefore be questioned.

All 5-level triage scales present insufficient evidence on interrater variability. The few studies that have been published (most of low quality) have reported widely divergent interrater agreement, with kappa values ranging from 0.2 (slight agreement) to 0.9 (almost perfect). Only a single study [32] presented limited scientific evidence. This was a 4-grade scale reporting a kappa value of 0.45, a value usually considered to be in the moderate agreement range [42]. It is evident that inter-observer agreement in triage scales must be documented in greater detail, and, if low, actions must be taken to reduce variability.

The literature shows variations in the vital signs and chief complaints applied in triage scales. It is unclear whether the selected vital signs are the best at distinguishing different risk groups. Further, evidence supporting the selected thresholds for continuous variables is deficient. The inclusion criteria for this systematic literature review place considerable emphasis on relevance. Triage scales are intended to be used in EDs irrespective of specific symptoms or disease. Hence, only studies of unselected patient populations in ED settings were included, greatly limiting the number of studies on the ability of individual vital signs to predict outcome. Our literature search revealed that many more studies had been performed in intensive care units, or soon after hospital admission.

Regarding specific vital signs, limited scientific evidence supports the use of oxygen saturation and consciousness level as predictors of mortality early after triage. However, scientific evidence was found to be insufficient as regards respiration and pulse, blood pressure, and body temperature. Hence, it remains unclear whether the selected vital signs are the best ones to use in distinguishing different risk groups. Moderate scientific evidence indicated age as a predictor of mortality early after triage, yet most triage scales do not take age into account.

MTS and eCTAS include the chief complaint leading to the ED visit, but we did not find any studies that analyzed which of the chief complaints are important predictors of mortality early after triage. It appears likely that in the construction of triage scales, much of the information was deduced from studies performed in settings other than EDs.

This systematic literature review reveals shortcomings in the scientific evidence on which presently available triage scales are based. Stronger scientific evidence is needed to determine which of the vital signs and chief complaints have the greatest prognostic value in triage. Interrater agreement (reliability), validity, and safety of triage scales need to be investigated further, and head-to-head comparisons are needed to determine whether any of the scales have advantages over others.

This review was confined to ED triage scales for adult ED patients with non-psychiatric illnesses or injuries. In the absence of an internationally agreed outcome measure for ED triage scale validity, the proxy variables hospital admission and mortality were used in the current study. These proxy variables have limitations with regards to ED triage scale validity as the variables may be affected by events occurring after the triage assessment. Further, comparison between ED triage scales need to be done with caution as there may be contextual differences influencing the result.