Performance of cassette-based blood gas analyzers to monitor blood glucose and lactate levels in a surgical intensive care setting

The effect of cardiopulmonary bypass on blood thiamine concentration and its association with post-operative lactate concentration

Objective

Cardiothoracic surgery is a large field in Australia, and evidence suggests post-cardiopulmonary bypass (CPB) hyperlactataemia is associated with higher morbidity and mortality. Low thiamine levels are a potentially common yet treatable cause of hyperlactataemia and may occur in the setting of exposure to CPB non-biological material. We hypothesized that cardiopulmonary bypass would result in decreased whole-blood thiamine levels, which may therefore result in increased whole-blood lactate levels in the post-operative period.

Methods

Adult patients undergoing non-emergent CPB were recruited in a single centre, prospective, analytic observational study at Townsville University Hospital, Australia. The primary outcome was a comparison of pre- and post-CPB thiamine diphosphate level, secondarily aiming to assess any relationship between lactate and thiamine levels. Prospective pre- and post-CPB blood samples were taken and analysed at a central reference laboratory.

Results

Data was available for analysis on 78 patients. There was a statistically significant increase in thiamine diphosphate level from pre-CPB: 1.36 nmol/g Hb, standard deviation (SD) 0.31, 95% confidence intervals (CI) 1.29–1.43, to post-CPB: 1.77 nmol/g Hb, SD 0.53, 95% CI 1.43–1.88, p value < 0.001. There was a non-statistically significant (p > 0.05) trend in rising whole-blood lactate levels with increasing time. Analysis of lactate levels at varying time periods found a significant difference between baseline measurements and increased levels at 13–16 h (p < 0.05). There was no significant relationship observed between whole-blood thiamine levels and post-operative lactate levels.

Conclusion

Whole-blood thiamine levels were found to increase immediately post-CPB in those undergoing elective cardiac surgery. There was no correlation between whole-blood thiamine levels and post-operative arterial lactate levels.

Automation on an Open-Access Platform of Alzheimer's Disease Biomarker Immunoassays

The lack of (inter-)laboratory standardization has hampered the application of universal cutoff values for Alzheimer's disease (AD) cerebrospinal fluid (CSF) biomarkers and their transfer to general clinical practice. The automation of the AD biomarker immunoassays is suggested to generate more robust results than using manual testing. Open-access platforms will facilitate the integration of automation for novel biomarkers, allowing the introduction of the protein profiling concept. A feasibility study was performed on an automated open-access platform of the commercial immunoassays for the 42-amino-acid isoform of amyloid-β (Aβ_1-42), Aβ_1-40, and total tau in CSF. Automated Aβ_1-42, Aβ_1-40, and tau immunoassays were performed within predefined acceptance criteria for bias and imprecision. Similar accuracy was obtained for ready-to-use calibrators as for reconstituted lyophilized kit calibrators. When compared with the addition of a standard curve in each test run, the use of a master calibrator curve, determined before and applied to each batch analysis as the standard curve, yielded an acceptable overall bias of -2.6% and -0.9% for Aβ_1-42 and Aβ_1-40, respectively, with an imprecision profile of 6.2% and 8.4%, respectively. Our findings show that transfer of commercial manual immunoassays to fully automated open-access platforms is feasible, as it performs according to universal acceptance criteria.

Analytical performance of three whole blood point-of-care lactate devices compared to plasma lactate comparison methods and a flow-injection mass spectrometry method

OBJECTIVES

Point of care (POC) whole blood lactate testing may facilitate rapid detection of sepsis. We evaluated three POC methods against both plasma lactate comparison methods and a flow-injection mass spectrometric (MS) method.

DESIGN AND METHODS

Nova StatStrip, Abbott i-STAT CG4+ and Radiometer ABL90 POC lactate methods were evaluated against the mean of Cobas Integra 400 and Vitros 350 plasma lactate. POC methods were also compared to a flow-injection mass spectrometric assay measuring lactate in ZnSO₄-precipitated whole blood extracts. Intra- and inter-assay precision was determined using quality control material. Method comparison included specimens from normal donors at rest, after exertion, and after spiking with lactic acid.

RESULTS

Intra- and inter-assay coefficient of variation was <5% for i-STAT and ABL90; but ranged from 3.1-8.2% on two StatStrip meters. Mean (±SD) bias between POC and plasma lactate ranged from -0.2±0.9 (i-STAT and ABL90) to -0.4±1.2 (StatStrip) mmol/L. At concentrations >6mmol/L, all POC methods showed proportional negative bias compared to plasma methods; but this bias was not observed when compared to the MS method. Despite proportional negative bias, all POC methods demonstrated acceptable concordance (94-100%) with plasma lactate within the reference interval (<2.3mmol/L) and >4mmol/L, commonly used clinical cut-offs for detection of sepsis.

CONCLUSIONS

POC lactate methods demonstrate acceptable concordance with plasma lactate across commonly used clinical cut-offs for detection of sepsis. Due to systematic negative bias at higher lactate concentrations, POC and plasma lactate should not be used interchangeably to monitor patients with elevated lactate concentrations.

Point-of-care diagnostics for niche applications

Point-of-care or point-of-use diagnostics are analytical devices that provide clinically relevant information without the need for a core clinical laboratory. In this review we define point-of-care diagnostics as portable versions of assays performed in a traditional clinical chemistry laboratory. This review discusses five areas relevant to human and animal health where increased attention could produce significant impact: veterinary medicine, space travel, sports medicine, emergency medicine, and operating room efficiency. For each of these areas, clinical need, available commercial products, and ongoing research into new devices are highlighted.

Modeling of effect of glucose sensor errors on insulin dosage and glucose bolus computed by LOGIC-Insulin

BACKGROUND

Effective and safe glycemic control in critically ill patients requires accurate glucose sensors and adequate insulin dosage calculators. The LOGIC-Insulin calculator for glycemic control has recently been validated in the LOGIC-1 randomized controlled trial. In this study, we aimed to determine the allowable error for intermittent and continuous glucose sensors, on the basis of the LOGIC-Insulin calculator.

METHODS

A gaussian simulation model with a varying bias (0%-20%) and CV (-20% to +20%) simulated blood glucose values from the LOGIC-1 study (n = 149 patients) in 10 Monte Carlo steps. A clinical error grid system was developed to compare the simulated LOGIC-Insulin-directed intervention with the nominal intervention (0% bias, 0% CV). The severity of error measuring the clinical effect of the simulated LOGIC-Insulin intervention was graded as type B, C, and D errors. Type D errors were classified as acutely life-threatening (0% probability preferred).

RESULTS

The probability of all types of errors was lower for continuous sensors compared with intermittent sensors. The maximum total error (TE), defined as the first TE introducing a type B/C/D error, was similar for both sensor types. To avoid type D errors, TEs <15.7% for intermittent sensors and <17.8% for continuous sensors were required. Mean absolute relative difference thresholds for type C errors were 7.1% for intermittent and 11.0% for continuous sensors.

CONCLUSIONS

Continuous sensors had a lower probability for clinical errors than intermittent sensors at the same accuracy level. These simulations demonstrated the suitability of the LOGIC-Insulin control system for use with continuous, as well as intermittent, sensors.

Analytic evaluation of a new glucose meter system in 15 different critical care settings

BACKGROUND

Maintaining appropriate glycemic control in critically ill patients reduces morbidity and mortality. The use of point-of-care (POC) glucose devices is necessary to obtain rapid results at the patient's bedside. However, the devices should be thoroughly tested in the intended population before implementation. The use of POC glucose meters in critically ill patients has been questioned both in the literature and by regulatory agencies. The aim of this study was to determine if the ACCU-CHEK® Inform II system (Roche Diagnostics) POC glucose meter demonstrated the desired accuracy and precision, as defined by Clinical and Laboratory Standards Institute guideline POCT12-A3, in a large number of critically ill patients from multiple intensive care settings at two academic medical centers.

METHODS

A total of 1200 whole blood meter results from 600 patients were compared with central laboratory plasma values. Whole blood aliquots from venous samples were used to obtain duplicate meter results with the remaining sample being processed to obtain plasma for central laboratory testing within 5 min of meter testing.

RESULTS

A total of 1185 (98.8%) of the new meter's glucose values were within ± 12.5% (± 12 mg/dl for values ≥ 100 mg/dl) of the comparative laboratory glucose values, and 1198 (99.8%) were within ± 20% (± 20 mg/dl for values <100 mg/dl).

CONCLUSIONS

Considering the large number of patients from numerous critical care units examined, the new glucose meter system appears to have sufficient analytic accuracy for use in critically ill patients.