Capillary lactate concentration on admission of normotensive trauma patients: a prospective study

Increase in lactate concentration is a common indicator of severity in critically ill patients [1‐3]. After severe trauma, elevated serum lactate concentration was also associated with short-term outcomes. In preclinical studies, arterial lactate concentration was a strong predictor of blood loss after blunt or penetrating trauma [4‐6]. In the clinical setting, initial lactate measurement was associated with organ failure and mortality in 129 trauma patients [7]. Blood lactate concentration was also an independent variable associated with mortality in 586 trauma patients [8]. Taken together, these studies highlight the role of arterial lactate concentration to screen high-risk patients for transfusion in the trauma bay.

Other classic vital parameters like heart rate or systolic arterial blood pressure (SBP) are common markers to predict critical bleeding in severe trauma patients [9]. Therefore, the additional information brought by serial lactate measurements may be obscured by classic physiological parameters in shocked patients. Interestingly, the yield of lactate concentration in a selected population of trauma patients with SBP between 90 and 110 mmHg allowed for early identification of patients requiring significant transfusion [10]. In patients with normal vital signs, elevated arterial lactate concentration was also found to be associated with occult major trauma [11]. These results were further corroborated in normotensive elderly blunt trauma patients [12]. Therefore, the additional benefit of arterial lactate concentration to detect patients at risk for transfusion may be superior in patients with normal vital signs compared to patients with shock on admission. However, measuring arterial lactate requires an automatic blood gas test and arterial blood sampling, which could be seen as invasive and expensive in patients with normal vital signs. To overcome these limitations, handheld point-of-care (POC) fingertip lactate measurement was implemented in emergency departments with adequate accuracy to determine moderate increase in lactate levels [13]. Using this device in the pre-hospital setting, capillary lactate concentration higher than 3.5 mmol/l was associated with in-hospital mortality in a study of 124 severe trauma patients requiring urgent ambulance dispatching [14].

The aim of the present study is to assess whether capillary lactate on admission can predict transfusion in trauma patients with a SBP higher than 90 mmHg. We also test the ability of the pre-hospital shock index defined as the ratio between heart rate and SBP to predict a significant transfusion. We assume that capillary lactate higher than 3.5 mmol/l can predict significant transfusion within the first 48 h post-trauma, with a higher accuracy than the pre-hospital shock index.

We included 120 consecutive patients within the study period. Characteristics of the trauma population are summarized in Table 1. The typical patient was a young, male adult admitted for blunt trauma. Trauma severity in our cohort was moderate (median ISS = 19) and only two patients out of 120 (2 %) did not survive. Median concentrations of capillary and serum lactate are presented in Table 1. Thirty-two (27 %) patients required emergency treatment for haemostasis (embolization or damage control surgery). Seventy-five (63 %) patients were directed to the ICU after their admission into the trauma bay. Thirty (25 %) patients received at least one unit of RBC within the first 48 h. Only four (3 %) patients received a transfusion of at least six RBC units. The low incidence of the primary outcome did not allow us to explore the diagnostic performance of capillary lactate to predict transfusion of at least six RBC. Nevertheless, twelve (10 %) patients had at least four RBC units within the first 48 h (secondary outcome).

The AUC-ROC of capillary lactate on admission to predict significant transfusion was 0.68 [95 % CI 0.58 – 0.78] (see Fig. 1 and Additional file 1). Maximizing the sensitivity, we found a threshold at 3.5 mmol/l. Sensitivity at this cut-off was 100 % (95 % CI 74–100 %) and specificity was 53 % (95 % CI 43–62 %). The AUC-ROC of the shock index was similar: 0.68 [95 % CI 0.51 – 0.85] (see Fig. 1 and Additional file 1). The AUC-ROC of capillary lactate on admission to predict any transfusion was only 0.59 [95 % CI 0.46 – 0.72] whereas the AUC-ROC of serum lactate was 0.77 [95 % CI 0.62 – 0.91].

Agreement between capillary lactate and serum blood lactate was performed in 60 patients. Bland & Altman analysis is presented in Fig. 2. Average bias between the two methods was 2.4 mmol/l with a standard deviation of 3.0 mmol/l. The capillary lactate minus blood lactate difference was positively correlated with the capillary and blood lactate average (capillary – blood lactate = -1.6 + 1.26 × (capillary + blood lactate)/2, p < 0.001), indicating larger discrepancy between the two measurements in patients with abnormal lactate concentration. We used this linear bias to assess the 95 % prediction limits for the blood lactate given the value by the capillary method (Fig. 2).

Univariate analyses between the group of patients with capillary lactate ≥ 3.5 mmol/l (n = 62 patients) and the group of patients with capillary lactate < 3.5 mmol/l (n = 58 patients) is shown on Table 2. Patients in the abnormal capillary lactate group had lower capillary haemoglobin, higher prehospital heart rate and more emergency haemostatic treatment than the normal capillary lactate group. The allocation of transfusion in the two groups is presented in Fig. 3. All patients receiving at least four RBC units had capillary lactate higher than 3.5 mmol/L.

In a selected cohort of trauma normotensive patients, we found a poor diagnostic performance of capillary lactate to predict the transfusion of at least 4 RBC units. More concerning was the poor agreement between the capillary method and the serum blood lactate concentration. Although significant association was found between capillary lactate and transfusion requirements, the lack of accuracy of capillary lactate weakens the use of this POC device at the bedside to detect occult trauma in non-shocked trauma patients.

Capillary lactate higher than 3.5 mmol/l was statistically associated with transfusion requirements in our cohort. Indeed, this group of patients had more embolization procedures than patients with normal capillary lactate. Accordingly, transfusion needs were also higher in the capillary lactate ≥ 3.5 mmol/l group and all significantly transfused patients were part of this group. These findings were in line with prehospital evaluation of POC fingertip lactate device, where pre-hospital blood lactate levels were associated with in-hospital mortality [14]. In this study, capillary lactate was performed by nurses from the emergency medical service and provided more prognostic information than vital signs alone. In our study, we confirmed that capillary lactate could be achieved by the nurse in charge of the patient. However, we found statistical association but failed to demonstrate sufficient accuracy to predict transfusion needs. Indeed, the AUC-ROC was low with a lower limit of its 95 % confidence interval close to 0.5. This result clearly affected the diagnostic performance of capillary lactate at the early phase of severe trauma. In a larger retrospective cohort, blood lactate had larger AUC to predict significant transfusion at hospital admission (0.76) but no 95 % confidence interval was provided [10] and lactate was measured on blood samples. Interestingly, the same group found that prehospital shock index might facilitate the early identification of patients at risk for massive transfusion [20]. In our cohort of patients receiving less transfusion, we reported similar diagnostic performance for serum lactate (AUC ROC = 0.77 to predict any transfusion) but we did not report good diagnostic performance of prehospital shock index with an AUC-ROC equal to 0.68 [95 % CI 0.51 – 0.85]. Taken together, these findings illustrate the need for reliable information to detect occult severe trauma in normotensive patients. Unfortunately, in our study, neither capillary lactate nor shock index had sufficient diagnostic performance to be considered pertinent in determining patients’ severity at hospital admission.

The poor predictive value of capillary lactate might be related to the poor agreement between this method and the serum blood lactate measurement since average bias between the two methods was 2.4 mmol/l. Considering normal lactate range from 0 to 3.5 mmol/l, such bias limits the usefulness of POC device. One explanation for this result may be extra-capillary contamination by sweat composition [21]. Under intense physical exercise, lactate could be measured on the skin surface and may overestimate capillary lactate value. As severe trauma often occurs during sport-related accident in our region [22], this type of contamination may be expected in our patients. Other explanation may be interferences between capillary lactate assessment and interstitial oedema due to fluid overloading [23]. However, total infused volume was low in our study according to the moderate severity of the patients.

We acknowledge several limitations of our study. First, the incidence of the primary outcome was too low to explore capillary lactate in patients that received at least six RBC units. Nevertheless, the incidence of four RBC transfusions was relatively high in our cohort (10 %). Moreover, a transfusion of four RBC units may be considered significant from the clinical standpoint since this quantity represents a total volume of one litre. Second, the sample size of the cohort is relatively small and the limited number of events should be considered when interpreting our results. However the lack of agreement between the POC fingertip device and the reference method is valuable and questions the relevancy of capillary lactate measurement device in daily practice. Third, standardized capillary lactate measurement were not performed by one dedicated nurse. Nevertheless, this study was meant to be pragmatic and POC devices should be practical enough to be generalized in the trauma bay independently of the executing nurse.

Capillary lactate was associated with transfusion requirements in normotensive trauma patients. However, we did not find sufficient accuracy of this technique for transfusion prediction at the bedside. The poor agreement between this device and the standard method may hinder the usefulness of capillary lactate in daily clinical practice. Despite encouraging statistical associations, capillary lactate may not add relevant information about occult severe bleeding in non-shocked trauma patients.