It is recognised that elevated blood lactate levels increase the risk of mortality, even in those patients who appear physiologically stable (Vincent et al, 2016). The National Early Warning Score (NEWS) was introduced in the United Kingdom (UK) as a risk management tool in the Emergency Department (ED). Based on easy to establish physiological data, ED staff are better able to appropriately triage patients. Blood lactate has become a vital measure of the body's physiological response to an insult or injury. The authors of this study were assessing whether adding a serum lactate reading (NEWS–L) to the NEWS assessment could improve the triage of patients in the ED.
Using the NEWS and NEWS–L tools, a retrospective cohort study was undertaken at an urban tertiary hospital in South Korea over two consecutive months. The primary outcome was based on patients' scores predicting the likelihood of mortality in 48 hours (primary outcome); whilst secondary outcomes included the need for critical care intervention (defined as Intensive Care Unit admission) and/or advanced airway management in the ED and/or cardiac/cardiovascular drug therapy in the ED; 2-day composite outcome (2-day mortality plus the need for critical care); 7-day mortality; and in-hospital mortality. The NEWS–L score was presented as NEWS + lactate, for example, a NEWS of 3 and a blood lactate of 2mmol/l would give a score of 5 (3+2).
In total, 4624 patients attended ED in the study period, with full data available for 71.6% (n=3309). The paper presents detailed tables of comparison results which are accessible to the reader, but beyond the scope of this review to discuss in depth.
The overall summary of the results indicated that the NEWS–L was a better predictor of all outcomes, and highlighted patients who were at greater risk and would need early intervention. Typically, these patients were elderly and presented to the ED via the ambulance service. A NEWS–L score of 7 indicated a patient was at high risk of death and more likely to require continuous monitoring and critical care. In particular, the NEWS–L shows excellent discriminative value in relation to predicting risk of 2-day mortality. However, it also demonstrated that patients with a low NEWS score but high lactate score (NEWS–L 4) were also at risk and would also require timely interventions.
One notable limitation the researchers acknowledge, is that in 28.4% of cases there were no lactate levels documented. As this was a retrospective study and no further data could be generated, the lower limit of lactate was substituted (0.3) in relation to these patients. The authors discuss the rationale for this but recognise that further work is required in this area to increase generalisability of the findings, ideally through the implementation of a prospective multi-centre study.
Research has shown that although a patient appears haemodynamically stable, the presence of elevated blood lactate increases the risk of mortality (Vincent et al, 2016). The NEWS–L tool could aid paramedics to better identify patients who may appear well, but who could require regular monitoring and treatment.
This raises the question of the practicality and validity of point-of-care (POC) testing of blood lactate in the pre-hospital setting. Capillary samples from the finger typically over-estimate serum lactate levels (Lightowler and Hoswell, 2016). This could lead to inappropriate triaging of patients and unnecessary and costly use of resources.
In published sport exercise studies where capillary blood lactate sampling sites have been compared (typically the finger, toe and ear lobe), the finger consistently shows a higher blood lactate reading than the ear lobe (Dassonville et al. 1998; Feliu et al 1999; Forsyth and Farrally, 2000). However, the statistical significance of these differences is not as consistent, although this could be due to variation in study design (exercise intensities and type of exercise, for example). It appears that no study has assessed ear lobe capillary lactate levels as a predictor of serum lactate. Due to its central location, the ear lobe may provide a more accurate site for blood lactate sampling; however, research into this area needs to be conducted to validate this.
Early identification and appropriate treatment of high blood lactate levels in patients is linked to better patient outcome. The use of NEWS–L may help clinicians identify those patients who would appear stable or low risk based on the NEWS tool, but, who would benefit from early intervention. Exploring the reliability and validity of capillary blood samples from the ear lobe could be a way forward when considering the possibilities of introducing NEWS–L into pre-hospital settings.