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Capillary and Venous Lactate Agreement: a pilot prospective observational study
  1. Deepankar Datta1,
  2. Julia Grahamslaw1,
  3. Alasdair James Gray1,
  4. Catriona Graham2
    1. 1Emergency Medicine Research Group Edinburgh, Department of Emergency Medicine, Royal Infirmary of Edinburgh, Edinburgh, UK
    2. 2Wellcome Trust Clinical Research Facility, Western General Hospital, Edinburgh, UK
    1. Correspondence to Dr Deepankar Datta, Emergency Medicine Research Group Edinburgh, Department of Emergency Medicine, Royal Infirmary of Edinburgh, Little France Crescent, Edinburgh EH16 4SA, UK; deepankardatta{at}nhs.net

    Abstract

    Background Blood lactate is a marker of patient illness severity. Capillary lactate (CAP-LACT) measurement can potentially improve patient screening; however, it has poor evidence of clinical utility.

    Aim We aimed to investigate agreement between CAP-LACT and peripheral venous lactate (PV-LACT).

    Methods We performed a prospective observational pilot study of 99 patients requiring lactate measurement. Paired CAP-LACT and PV-LACT was recorded. Agreement was determined by Bland-Altman analysis.

    Results Bias was 0.2 mmol/L, with 95% limits of agreement from −1.9 to 2.3.

    Conclusions CAP-LACT has poor agreement with PV-LACT. Further research is needed to improve its potential clinical utility.

    • equipment evaluation
    • assessment
    • clinical care

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    Introduction

    Lactate is a biochemical marker used to identify unwell patients with potential poor clinical outcomes.1 To improve patient screening we require a method of measurement that is acceptable to the patient, causes minimal discomfort and minimises staff workload.2 This may improve access to screening and monitoring of unwell patients versus current techniques. A potential solution is capillary lactate (CAP-LACT) measurement.

    Capillary sampling is routinely used in glucose measurement.3 CAP-LACT meters have been used for athletes; however, most meters are not medically licenced, with poor evidence into clinical utility.4 ,5 The StatStrip Lactate (Nova Biomedical, Waltham, Massachusetts, USA) is a recently developed licenced CAP-LACT meter. CAP-LACT has potential utility in environments lacking easy access to blood gas or formal laboratory analysers.

    This pilot study aims to investigate agreement between CAP-LACT and peripheral venous lactate (PV-LACT), to demonstrate the utility of CAP-LACT as a substitute, and provide data for future study design.

    Methods

    Study design and setting

    We performed a prospective observational cohort pilot study of patients presenting to a single UK NHS ED between October and December 2014. Patients' management remained at the discretion of the treating clinician. The study complied with the STROBE guidelines.6

    Population

    Eligible patients were age 16 or over, within 4 hours of ED presentation, and judged to require lactate measurement as part of their clinical management. Exclusion criteria included detention under Mental Health Act, prisoners, pregnancy and incapacitated patients. Patients were recruited by written informed consent.

    Measurements

    Paired blood samples (peripheral venous and capillary) were taken for lactate analysis within 20 min of each other, along with patient enrolment demographics. PV-LACT was analysed in the reference standard Gem Premier 4000 blood gas analyser (Instrumentation Laboratories, Bedford, Massachusetts, USA).7 The comparison was against PV-LACT as the accepted ED standard of care.7 CAP-LACT was measured using the StatStrip Lactate analyser. CAP-LACT measurements were only taken by research nurses trained in a manufacturer-developed skin-cleaning protocol. Lactate measurements were recorded in mmol/L to 1 decimal place.

    Data analysis

    A pilot sample size of 100 was judged adequate to allow formal calculations for a definitive study. Agreement was determined by Bland-Altman analysis, with 95% limits of agreement (LoA) and their respective CIs.8 The SD of the bias allowed calculation of a definitive study sample size, by evaluating optimal LoA 95% CIs to improve upon the pilot study findings.9 Analysis was performed using the statistics software R.10

    Results

    Ninety-nine patients were recruited (table 1, figure 1 and see online supplementary table S1). The mean difference (bias: CAP-LACT minus PV-LACT) was 0.2 (95% CI 0.0 to 0.5), with LoA from −1.9 (95% CI −2.2 to −1.5) to 2.3 (95% CI 2.0 to 2.7) (figure 2). Only five samples had a PV-LACT greater than 4 mmol/L. We calculated a sample size of 500 for a definitive study, to estimate LoA 95% CI to ±0.2 mmol/L.9

    Table 1

    Patient demographics

    Figure 1

    STROBE diagram. CVLA, Capillary and Venous Lactate Agreement.

    Figure 2

    Bland-Altman plot of lactate agreement (CAP-LACT minus PV-LACT). CAP-LACT, capillary lactate; PV-LACT, peripheral venous lactate.

    Supplementary table

    Supplementary recruitment information

    Discussion

    The results demonstrate poor agreement between CAP-LACT and PV-LACT (figure 1). The wide LoA indicates poor precision: CAP-LACT approximates PV-LACT to ±2 mmol/L, a large discrepancy making CAP-LACT measurement impractical with commonly accepted thresholds of 2 and 4 mmol/L.1 Four measurements with a large bias (figure 1) contributed to poor precision: the reasons for these large biases are unclear. One possible explanation, which requires further evaluation, is inadequate skin cleaning. The overall low systematic bias, however, suggests potential clinical utility.

    The need for proper skin cleaning, to avoid measurement of sweat lactate, is a limitation of the study. The practicality of a 30 s skin-cleaning protocol in a busy ED needs to be investigated. Furthermore, this protocol was only tested on relatively well patients (table 1), a limitation from the design as a pilot study.

    Conclusions

    The poor precision suggests that CAP-LACT cannot currently be recommended for routine use. Further studies to practically optimise skin cleaning, and evaluate high lactate data points, may improve potential clinical utility.

    Acknowledgments

    The study team would like to acknowledge Linda McGough, Dinesh Kumar and Germano Ferrari from Nova Biomedical for their support of this project. We would also like to acknowledge the Emergency Medicine Research Group Edinburgh (EMERGE) team, with particular note to research manager Judy Coyle, with their assistance in this study. We would like to thank the staff of the Department of Emergency Medicine, Royal Infirmary of Edinburgh (http://www.edinburghemergencymedicine.com; Twitter @EdinburghEM) for their assistance in completing this study. We would also like to thank Dr Angela Oglesby, chairperson of the Medic One Charitable Trust, for her support in disseminating the results of this study.

    References

    Footnotes

    • Twitter Follow Deepankar Datta at @deepankardatta or @emerge_research

    • Collaborators Judy Coyle, Rachel O'Brien, Polly Black, Kirsty Simpson, Amy Dunn, Jen Simpson, Miranda Odam.

    • Contributors All authors contributed to the planning of the study. DD and JG obtained research ethical, Caldicott and governance approval. JG supervised patient recruitment, data collection and data entry. CG analysed the data. All authors contributed to the final write-up and review of the manuscript. DD as corresponding author will be responsible for the overall content as guarantor.

    • Funding The Capillary and Venous Lactate Agreement study had no direct funding. Nova Biomedical supported the study with equipment and supplies, however, had no role in the design and conduct of the study.

    • Competing interests As noted in manuscript and ICMJE, Nova Biomedical supplied equipment and supplies for this study, but the company was not involved in the design and conduct of the study.

    • Patient consent Obtained.

    • Ethics approval Institutional Review Board ethical approval was granted by the South East Scotland Research Ethics Service (REC no. 14/SS/1028).

    • Provenance and peer review Not commissioned; externally peer reviewed.

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