Clinical laboratory in emergency medicine
The Feasibility and Accuracy of Point-of-Care Lactate Measurement in Emergency Department Patients with Suspected Infection

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Abstract

Background: Prior studies show that lactate is a useful prognostic marker in sepsis. Objective: To study the feasibility and accuracy of a point-of-care (POC) analyzer capable of performing bedside serum lactate measurements; and to determine if other measurements (pH, base excess) are predictive of mortality. Methods: Design: prospective cohort study of adult (age 18 years or older) Emergency Department (ED) patients with suspected infection during the study period of May 2006 through March 2007. Setting: A 55,000-annual-visit urban tertiary care ED. Intervention: A point-of-care device (i-STAT, Abbott Point of Care Inc., Princeton, NJ) was deployed using a standardized training and quality assurance process. Using POC testing, we measured serum lactate, pH, and base excess, as well as concomitant lactate measurement via a central laboratory. Statistics: Area under the curve (AUC) for receiver operator characteristic curve, Bland-Altman statistics along with a correlation coefficient, and relative risk with 95% confidence intervals reported. Results: There were 699 patients enrolled, of whom 34 (4.9%) died. The AUCs for mortality prediction were: POC lactate 0.72, laboratory lactate 0.70, pH measurement 0.60, and base excess 0.60. Bland-Altman showed that POC lactate was, on average, 0.32 (95% confidence interval −0.35–0.98) lower than laboratory lactate, with agreement kappa = 0.97. Conclusions: A point-of-care testing device provides a reliable and feasible way to measure serum lactate at the bedside. The pH and base excess were less helpful.

Introduction

There are an estimated 571,000 cases of severe sepsis that present to United States emergency departments (EDs) each year, with an unacceptably high mortality rate between 20% and 50% (1, 2, 3). Early identification of the “at-risk” patient represents a challenge to the ED physician, as the presentation of sepsis is often subtle and difficult to assess. Although there is no universally accepted gold-standard screening test, the measurement of serum lactate level is useful for the identification of ED patients at increased risk of mortality from sepsis (3, 4, 5). The Surviving Sepsis Campaign, an international, multidisciplinary consensus effort, endorses obtaining a serum lactate as one of its core sepsis bundles (6). Additionally, the use of similar markers of hypoperfusion, namely low pH and base excess, have been proposed but not previously well studied. The identification process of patients at increased risk of adverse outcome is important, as septic patients benefit from early and aggressive resuscitation protocols (1).

In order for a blood lactate level to provide utility for clinical decision-making, an accurate result must be readily available in a timely fashion. A major problem in obtaining accurate blood lactate levels relates to sample handling before analysis. Once the blood sample is drawn, lactate levels continue to rise in the sample as the result of red blood cell metabolism. If the sample can be analyzed immediately, the effect is minimal. But, if the sample needs to be transported to a central laboratory or requires centrifugation before analysis, the delay results in falsely elevated lactate levels. From a practical standpoint, one may divide the currently available lactate methods into three groups: 1) standard enzymatic spectrophotometric methods, performed with blood collected in special preservative tubes, requiring centrifugation; 2) electrode-based amperometric methods, performed on anticoagulated whole blood but which may require transport to a laboratory (see Methods); 3) electrode-based amperometric methods, performed on whole blood at the bedside (see Methods) For the first method, delays are unavoidable, thus the recommendation that samples be collected in tubes that minimize red cell metabolism (e.g., so-called “gray-top” tubes containing NaF). Due to the need for transportation, centrifugation, and typical instrument analysis times, turnaround times are often 2–3 h (or longer). For the electrode-based methods, centrifugation is not required and analysis time is minimal (< 5 min). For the second method, though, actual turnaround time may be prolonged significantly by transportation delay; during those delays, lactate levels will rise, potentially significantly. As a result, there are criticisms of this methodology, but it was, in fact, the method used in many of the studies establishing blood lactate as a valuable ED risk stratification tool for patients with infection (3, 4, 5). With a turnaround time of typically 30 min or less, this is within a time frame that is useful for clinical decision-making. The third method, point-of-care lactate, offers the advantage of rapidly available results at the bedside with reduced time-to-assay that would potentially reduce time for in vitro metabolism; however, its feasibility and reliability is relatively unproven in this setting.

Because conventional measurement in a central laboratory is not available in all institutions, or may be associated with significant delays, a rapid and accurate point-of-care (POC) test could contribute to improved and timely care of the septic patient. Accordingly, we undertook this study to determine if a POC device would be reliable in the ED for identification of patients at risk for adverse outcomes in sepsis. The objective of this investigation was to study the feasibility and accuracy of a POC analyzer capable of performing bedside serum lactate measurements in patients with suspected infection, and to determine if other POC acid-base measurements (pH, base excess) hold similar predictive ability.

Section snippets

Study Design and Selection of Participants

This was a prospective cohort study of a convenience sample of adult (age 18 years or older) ED patients with suspected infection during the study period of May 1, 2006–March 15, 2007 who had a POC lactate measurement obtained with a mandatory confirmatory lactate measurement performed by the hospital's clinical laboratory. The central laboratory determinations of lactate were done using whole blood on a Siemens/Bayer RapidLab 1265 Analyzer (Siemens Healthcare Diagnostics, Deerfield, IL), which

Characteristics of Study Subjects

There were 699 patients enrolled, of whom 34 (4.9%) died. The population had a mean age of 60.4 years (95% confidence interval [CI] 58.9–61.2) (Table 1). Bland-Altman statistics showed that POC lactate was accurate for clinical decision-making compared to the laboratory lactate. There was an average bias for POC lactate of 0.32 (SD 0.45) lower than laboratory lactate, with the limits of agreement ranging from –1.1 to 0.50 (the range over which 95% of the differences between the POC and

Discussion

The findings from this study support the use of POC lactate as one clinically useful methodology to measure a blood lactate level to risk-stratify patients with suspected sepsis. One may surmise that POC is feasible by virtue of conducting 699 tests in the ED, and accurate based on the Bland-Altman results, yielding a mean difference of 0.32 with reasonable limits of agreement, which may be interpreted as clinically acceptable. Although we did not assess the impact of POC testing on time to

Conclusion

A POC testing device provides a practical, feasible, and reliable way to measure blood lactate at the bedside that is predictive of death in sepsis. The venous pH and base excess were less helpful. Future experimental studies that directly test whether the use of POC lactate levels improve outcomes in patients with sepsis compared to routine testing would be informative.

Acknowledgments

This study was funded by Abbott Point of Care Inc. Nathan I. Shapiro, md, mph, is funded in part by grants from the National Institutes of Health/National Institute of Heart, Lung, and Blood (1R01HL091757-01A1) and National Institute of General Medical Sciences (1P50GM076659-01). Dr. Trzeciak is supported by a grant from the National Institutes of Health/National Institute of General Medical Sciences (K23GM083211).

Stephen Trzeciak, md, mph, receives research support from Novo Nordisk, Eli

References (9)

  • N.I. Shapiro et al.

    Serum lactate as a predictor of mortality in emergency department patients with infection

    Ann Emerg Med

    (2005)
  • E. Rivers et al.

    Early goal-directed therapy in the treatment of severe sepsis and septic shock

    N Engl J Med

    (2001)
  • N. Shapiro et al.

    The association of sepsis syndrome and organ dysfunction with mortality in emergency department patients with suspected infection

    Ann Emerg Med

    (2006)
  • S. Trzeciak et al.

    Serum lactate as a predictor of mortality in patients with infection

    Intensive Care Med

    (2007)
There are more references available in the full text version of this article.

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