Sepsis currently claims ~37,000 lives annually in England alone, costing the National Health Service £2 billion (1). Blood lactate has been identified as a clinically relevant biomarker during the diagnostic and treatment phases of septic shock (2), with the rate of lactate clearance able to distinguish between survivors and non-survivors (3). However, determining lactate clearance rates requires serial measurements over many hours, which may not always be practical. Therefore, the ability to continuously monitor blood lactate has important clinical value. Using exercise as a model to alter blood lactate concentration, we compare the accuracy of a novel intravascular microdialysis device providing real-time blood lactate concentrations to those obtained from serial venous catheter samples. Sixteen healthy human volunteers (age 28 ± 8 years; stature 176 ± 9 cm; mass 79.3 ± 14.0 kg) provided written informed consent and completed one 2.5-hour experimental visit. After measures of height and weight an 18-gauge Teflon catheter was inserted into an antecubital vein, and a Probe Scientific MicroEye® PME012 microdialysis device inserted into the catheter. The MicroEye® was perfused at 1mL/hour with 0.9% sodium chloride containing a small quantity of anticoagulant (Fondaparinux). The outlet of the MicroEye® was connected to the inlet of a ContinuMon® lactate flow cell, for continuous lactate monitoring, which was inserted into a wireless battery-operated ContinuMon® module worn on the participant’s wrist. The sensor signal was transmitted to a laptop running proprietary software. A second 18 g catheter was introduced into a peripheral vein in the contralateral forearm for reference blood sampling. After 30-minutes of supine rest participants completed five 3-minute cycling bouts, beginning at of 70 watts and increasing by 35 watts each stage. Samples were collected every 10 minutes at rest and at the end of each exercise stage. No signal filters or compensation algorithms have been applied to the ContinuMon® data and so results present the raw signal, albeit adjusted for the time delay (typically <2 minutes) and calibrated retrospectively. A total of 117 paired venous blood lactate and MicroEye® lactate samples were obtained. Mean Pearson co-efficient is 0.948, with 11 participants giving an r > 0.9, and 9 of those > 0.95. Bland-Altman analysis shows a mean bias between measures of -0.016 mM, with an upper and lower limit of agreement of -0.499 to +0.466 mM respectively. The combination of the MicroEye® microdialysis catheter and the ContinuMon® Continuous Lactate Monitoring system exhibited good performance in following blood lactate changes in the clinical range (0 – 4 mM), using an incremental exercise model without signal filters. Further studies to demonstrate performance in critical care settings are planned.