Whole blood glucose: what are we actually measuring?

Reliability of Handheld Blood Glucose Monitors in Neonates: Trustworthy Arterial Readings but Capillary Results Warrant Caution for Hypoglycemia

BACKGROUND

Accurate glucose monitoring is vitally important in neonatal intensive care units (NICUs) and clinicians use blood glucose monitors (BGM), such as the Inform II, for bedside glucose monitoring. Studies on BGM use in neonates have demonstrated good reliability; however, most studies only included healthy-term neonates. Therefore, the applicability of results to the preterm and/or ill neonate is limited.

OBJECTIVES

In preterm and ill neonates, quantify differences in glucose concentrations between (1) capillary glucose (measured by BGM) and arterial glucose (measured by YSI 2300 Stat Plus) and (2) between aliquots from the same arterial blood sample, one measured by BGM versus one by YSI.

DESIGN/METHODS

Forty neonates were included in the study. Using Inform II, we measured glucose concentrations on blood samples simultaneously collected from capillary circulation via heel puncture and from arterial circulation via an umbilical catheter. Plasma was then separated from the remainder of the arterial whole blood sample and a YSI 2300 Stat Plus measured plasma glucose concentration.

RESULTS

The dominant majority of arterial BGM results met the Clinical and Laboratory Standard Institute (CLSI) and Food and Drug Administration (FDA) tolerance criteria. Greater discrepancy was observed with capillary BGM values with an average of 27.5% of results falling outside tolerance criteria.

CONCLUSIONS

Blood glucose monitor testing provided reliable results from arterial blood. However, users should interpret hypoglycemic results obtained from capillary blood with caution.

Instrument-free single-step direct estimation of the plasma glucose level from one drop of blood using smartphone-interfaced analytics on a paper strip

We demonstrated an instrument-free miniaturized adaptation of the laboratory gold standard methodology for the direct estimation of plasma glucose from a drop of whole blood using a low-cost single-user-step paper-strip sensor interfaced with a smartphone. Unlike a majority of the existing glucose meters that use whole blood-based indirect sensing technologies, our direct adaptation of the gold-standard laboratory benchmark could eliminate the possibilities of cross interference with other analytes present in the whole blood by facilitating an in situ plasma separation, capillary flow and colorimetric reaction occurring concomitantly, without incurring additional device complexity or embodiment. The test reagents were dispensed in lyophilized form, and the resulting paper strips were found to be stable over three months stored in a normal freezer, rendering easy adaptability commensurate with the constrained supply chains in extreme resource-poor settings. Quantitative results could be arrived at via a completely-automated mobile-app-based image analytics interface developed using dynamic machine learning, obviating manual interpretation. The tests were demonstrated to be of high efficacy, even when executed by minimally trained frontline personnel having no special skill of drawing precise volume of blood, on deployment at under-resourced community centres having no in-built or accessible healthcare infrastructure. Clinical validation using 220 numbers of human blood samples in a double-blinded manner evidenced sensitivity and specificity of 98.11% and 96.7%, respectively, as compared to the results obtained from a laboratory-benchmarked biochemistry analyser, establishing its efficacy for public health and community disease management in resource-limited settings without any quality compromise of the test outcome.

Point-of-Care Glucose and Ketone Monitoring

Early and rapid identification of hypo- and hyperglycemia as well as ketosis is essential for the practicing veterinarian as these conditions can be life threatening and require emergent treatment. Point-of-care testing for both glucose and ketone is available for clinical use and it is important for the veterinarian to understand the limitations and potential sources of error with these tests. This article discusses the devices used to monitor blood glucose including portable blood glucose meters, point-of-care blood gas analyzers and continuous glucose monitoring systems. Ketone monitoring options discussed include the nitroprusside reagent test strips and the 3-β-hydroxybutyrate ketone meter.

Patient acuity exacerbates discrepancy between whole blood and plasma methods through error in molality to molarity conversion: "Mind the gap!"

OBJECTIVE

A mathematical constant factor is proposed to convert measured whole blood glucose molality to plasma-equivalent molarity. The objective of this study was to determine the distributions of conversion factors for groups of patients with different acuity and to assess the gap or error in plasma-equivalent glucose reporting that would occur when a mathematical constant conversion factor is used in patients.

METHODS

Distributions of hematocrit, red blood cell water and plasma water were determined in patients from the community, hospital and adult intensive care unit. Volume displacement conversion factor distributions and glucose error were determined for each group.

RESULTS

With increasing patient acuity the median hematocrit decreased, median plasma water increased and variation of these parameters increased. In hospital patients, the molality to molarity conversion factor distribution interval was 1.04-1.16, rather than a constant 1.11. Assuming direct electrode glucose devices only have error attributed to analytical imprecision (coefficient of variation of 5%), it is predicted that only 2% of community patients will have glucose results that exceed 10% of the target values. In the same device, due to variance in hematocrit and plasma water affecting the factor for conversion of molality to molarity, it is predicted that 8.2% of adult intensive care unit patients would have glucose results that exceed 10% of the target value.

CONCLUSIONS

Changes in hematocrit and plasma water concentration are predicted to affect a gap or error between whole blood direct reading biosensors and central laboratory plasma methods. This error increases and becomes more variable as patient acuity increases.

Accuracy of bedside capillary blood glucose measurements in critically ill patients

OBJECTIVE

To compare the accuracy of fingerstick with laboratory venous plasma glucose measurements (laboratory glucose) in medical ICU patients and to determine the factors which interfere with the accuracy of fingerstick measurements.

PARTICIPANTS

The study included 80 consecutive patients aged 58+/-7 years, BMI 29.5+/-9.0, and APACHE II score 15+/-6 (277 simultaneous paired measurements).

MEASUREMENTS

This prospective observational study compared fingerstick measurements to simultaneously sampled laboratory glucose once a day in patients in our medical ICU (twice daily if on an insulin infusion). Data recorded included patient demographics, admission diagnoses, APACHE II score, BMI, daily hematocrit, arterial blood gasses, chemistry results, concomitant medications (including vasopressors and corticosteroids), and upper extremity edema. Accuracy was defined as the percentage of paired values not in accord (>15 mg dl(-1)/ 0.83 mmol(-1)l(-1) difference for laboratory values <75 mg dl(-1)/4.12 mmol(-1)l(-1) and >20% difference for laboratory values >or=75 mg/dl). Outliers (blood glucose difference >100 mg dl(-1)/5.56 mmol(-1) l(-1)) were excluded from the correlation and distribution analyses.

RESULTS

Mean fingerstick glucose was 129+/-45 mg/dl (7.2+/-2.5 mmol/l) and mean laboratory glucose 123+/-44 mg/dl (6.8+/-2.4 mmol/l). The correlation coefficient between the two values was 0.9110 (Clinical and Laboratory Standards Institute threshold 0.9751). The mean difference (bias) between the two methods was 8.6+/-18.6 mg/dl (0.48+/-1.0 mmol/l) and limits of agreement +45.8 and -28.6 mg/dl (+2.5 and -1.6 mmol/l). Fifty-three (19%) paired measurements in 22 patients were not in accord (CLSI threshold <or=5%). In 44 (83%) of these paired measurements fingerstick glucose was greater than laboratory glucose.

CONCLUSIONS

The findings suggest that capillary blood glucose as measured by fingerstick is inaccurate in critically ill ICU patients and does not meet the CLSI standard. It is unclear whether the sampling method, device used, or both contributed to this inaccuracy. The wide limits of agreement suggest that fingerstick measurements should be used with great caution in protocols of tight glycemic control.

Blood glucose measurements in the critically ill: more than just a blood draw

A crucial determinant for the success of intensive insulin therapy in critically ill patients is the frequent and accurate measurement of blood glucose values with immediate feedback of results. In general, therefore, this is achieved by point-of-care testing, raising the question of the best way of monitoring blood glucose. Corstjens and coworkers, in the previous issue of Critical Care, demonstrate that, in spite of good correlation to "conventional" laboratory glucose assessment, absolute glucose levels may differ systematically. This commentary reviews the problems of glucose measurements arising from matrix effects, interferences and the use of different assays.

Expression of bone matrix proteins during the osseus healing of topical conditioned implants: an experimental study

OBJECTIVES

Osseointegration of implants depends on time and local bone conditions regarding quality and quantity. This led to the bone classification by Lekholm et al. The aim of the present study was to follow the expression of bone matrix proteins during the phase of osseointegration after conditioning of the bone bed by means of immunohistochemistry.

METHODS

In the porcine frontal skull, implant beds of identical size were created. Before placement of the implants (Ankylos 4 x 3.5 mm), the implant beds were conditioned using bone condensation (cond), an osteoinductive collagen (Co) and platelet-rich plasma (PRP). These conditioning methods were compared with standard procedure. The animals were sacrificed after 2, 4 and 8 weeks. The specimens were then analyzed by light microcopy and immunohistochemistry for expression of bone morphogenic proteins (BMP)2, procollagen I and osteocalcin (OC).

RESULTS

Light microscopy revealed an initial effect of condensation and the bovine collagen at 2 weeks in comparison with the standard group. The PRP did not achieve a significant effect. At 8 weeks, the results of the standard, bone condensation and the bovine collagen group had aligned. The PRP group showed a significantly lower bone-implant contact (BIC) (P=0.003) compared with the standard group. BMP2 expression was significantly higher in all evaluated test groups at 4 and 8 weeks, as well as at 2 weeks in the condensation group. The procollagen I expression at 2 weeks was significantly increased for PRP and lower in the collagen and condensation group compared with standard procedure. Values for 4 and 8 weeks were slightly higher than in the standard group. No significant differences were obvious in the OC group at any time.

CONCLUSIONS

During the initial healing phase, an effect of the evaluated methods of topical bone conditioning can be demonstrated by differences in the expression of BMP2 and procollagen I. These findings had leveled at 8 weeks and were, in contrast, not detectable in the expression of OC or by means of light microscopy.

Evaluation of a glucose meter for determining the glycemic responses of foods

BACKGROUND

Glucose meters are convenient for measuring postprandial glycemic responses. However, their performance for this purpose has not been evaluated.

METHODS

Glucose responses of 7 potato meals were measured using the One Touch Ultra(R) (OTU) glucose meter and a reference method (Yellow Springs Instruments Glucose Analyzer, YSI) and the incremental areas under the curves (AUC) and glycemic index (GI) values compared.

RESULTS

Mean AUC(OTU) was greater than AUC(YSI) (192+/-8 vs. 175+/-7 mmolxmin/l, p=0.001), but GI(OTU) tended to be less than GI(YSI) (69+/-3 vs. 74+/-3, p=0.052). Bland-Altman difference plots showed wide 95% limits of agreement for AUC (-84 to 119 mmolxmin/l) and GI (-21 to 26) values of individual subjects, and for the mean GI values of the 7 foods (-11 to 21). Total and error variance of AUC and GI values were greater for OTU than YSI, and food means differed significantly for YSI (p<0.01) but not OTU (p=0.11).

CONCLUSION

AUC and GI values determined by OTU are more variable and do not agree well with those obtained by YSI. Thus, the OTU is not recommended for determining AUC or GI in normal subjects. This conclusion does not necessarily apply to other glucose meters whose performance should be evaluated.

Biosensors in clinical chemistry

Biosensors are analytical devices composed of a recognition element of biological origin and a physico-chemical transducer. The biological element is capable of sensing the presence, activity or concentration of a chemical analyte in solution. The sensing takes place either as a binding event or a biocatalytical event. These interactions produce a measurable change in a solution property, which the transducer converts into a quantifiable electrical signal. Present-day applications of biosensors to clinical chemistry are reviewed, including basic and applied research, commercial applications and fabrication techniques. Recognition elements include enzymes as biocatalytic recognition elements and immunoagents and DNA segments as affinity ligand recognition elements, coupled to electrochemical and optical modes of transduction. The future will include biosensors based on synthetic recognition elements to allow broad applicability to different classes of analytes and modes of transduction extending lower limits of sensitivity. Microfabrication will permit biosensors to be constructed as arrays and incorporated into lab-on-a-chip devices.

Problems of comparing blood glucose molality and molarity determined with an Omni, an EML 105 and an Ebio analyser

The comparability between glucose concentrations measured in various sample systems is still a matter of debate. Decision limits are usually determined in venous plasma and then converted to either blood or to the aqueous compartment (activity). The conversion factors recommended have not yet been generally accepted. In the present study, glucose concentrations were determined in blood and plasma with an Ebio analyser (molarity) and in the aqueous compartment with both an EML 105 and an Omni (molality). All analytical results were referred to the same aqueous standard solution. The Ebio results agreed with reference method values in control materials. Concentrations determined in the various sample systems from patients (molarity) correlated well with the molality values measured either with the EML or the Omni. However, the mean values of the EML were not consistent with those derived theoretically by considering the different water content. With the Omni, only molality values in whole blood appeared plausible, but not in plasma, although the two sample systems should provide identical molality values. The EML results were almost identical in whole blood and plasma. Theoretically, glucose molality would be the ideal diagnostic quantity. However, no diagnostic advantage of molality determined in whole blood with the Omni vs. molarity values could be detected in a group of 40 non-diabetic and 27 diabetic subjects.

Comparison of point-of-care-testing glucose meters with standard laboratory measurement of the 50g-glucose-challenge test (GCT) during pregnancy

OBJECTIVES

Although glucose meters are well-established instruments for self-monitoring blood glucose levels, diagnostic and screening procedures should be performed using standard laboratory methods. In addition to standard laboratory methods, HemoCue is authorized for screening and diagnostic purposes in Germany. The rapid development of other glucose meters makes it necessary to re-evaluate this recommendation. Our objective was to test the usefulness of glucose meters in screening pregnant patients for gestational diabetes.

METHODS

The 50-g glucose challenge test was administered to one hundred and ninety-three pregnant patients whose blood glucose levels were then simultaneously measured with five portable meters and the HemoCue. The results were compared to our standard method (Hexokinase). A cut-off of 7.8 mmol/L was used and sensitivity, specificity, accuracy, the Youden index, and the Kappa index were calculated. The tests were performed by well-trained personnel (C.D. and U.S.).

RESULTS

1212 measurements were performed on 193 patients. All glucose meters showed a very good correlation (r > 0.90). None of the measurements showed an extreme deviation necessitating the error grid analysis. The GlucoTouch (5.93% +/- 7.4) and the HemoCue (-9.04% +/- 5.9) showed a mean deviation greater than 5%. None of the meters had a mean deviation greater than 10%. The accuracy fluctuated between 0.85 and 0.94. The Kappa index was between 66 to 85. In our clinical trial, the Accu-Chek, Glucotouch, OneTouch, and Precision demonstrated greater accuracy and a higher Kappa index than the HemoCue.

CONCLUSIONS

Our data showed good concordance in statistical and clinical parameters for most of the six glucose meters. The HemoCue, recommended as a standard method in several countries, did not show better concordance than most of the tested glucose meters. When used by well-trained personnel, the accuracy of the Accu-Chek, Glucotouch, One-Touch, and Precision was acceptable for use in gestational diabetes screening.

Comparison of several point-of-care testing (POCT) glucometers with an established laboratory procedure for the diagnosis of type 2 diabetes using the discordance rate. A new statistical approach

The applicability of point-of-care testing (POCT) glucometers for monitoring blood glucose concentrations has been demonstrated. However, their use in diagnosing type 2 diabetes is still debated. Therefore, a new statistical procedure for estimating discordance rates (DRs) was applied in comparing a well-established laboratory method (Ebio) with another laboratory method (Cobas Integra 700) and with several POCT glucometers (Accu-Chek, Accutrend, Elite, HemoCue, Omni) in detecting glucose intolerance states. All procedures led to parallel glucose concentration patterns in capillary blood, venous plasma, and venous blood during oral glucose tolerance tests. However, the mean concentrations differed more or less. The Ebio and Integra results agreed within a maximal deviation of 3%. In blood samples, the HemoCue and Accutrend results were closest to the laboratory procedures (Ebio and Integra) and the highest differences were obtained with the Elite. Comparing whole blood values with those obtained in the aqueous blood compartment (Omni), even greater differences were observed. When all procedures were referred to the same glucose standard, the Ebio, Integra, Accutrend, and Omni results remained almost unchanged, whereas the Elite "moved" toward the Ebio results, and the Accu-Chek results toward the Omni results. Thus, traceability to an aqueous standard was observed with the Ebio, Integra, Accutrend, and Elite in all three sample systems. The Accu-Chek was only traceable in the presence of albumin, and HemoCue was not traceable at all. The clinical relevance of the differences observed between Ebio and POCT glucometers was tested by comparing the relative number of discordant classifications. The highest DRs were observed in the fasting state. They were higher in capillary blood than in the other sample systems. The DRs were found higher with POCT glucometers than with the other established laboratory procedure (Integra). Thus, at least in the fasting state, all POCT glucometers were less reliable than the established laboratory procedures and above the chosen criteria of clinical acceptability (DR < or = 5%). After transforming all glucometer results with a regression function (bias correction), the DRs were less than 5% if compared with the Ebio procedure in all sample systems. In conclusion, the WHO recommendation not to use POCT glucometers for diagnosing type 2 diabetes must be supported. However, after proper recalibration, the tested systems were acceptable. Therefore, manufacturers should reconsider their calibration procedure. Those POCT procedures should be preferred that can be referred to aqueous glucose solutions.

Analytical performance of glucometers used for routine glucose self-monitoring of diabetic patients

BACKGROUND

Glucometry is an essential part of diabetes treatment, but so far, no standard quality control procedure verifying blood glucose meter results is available. In this study, we evaluated the analytical performance of eight glucose meters: GX and Esprit (Bayer Diagn.), MediSense Card Sensor, ExacTech (MediSense) with strips Selfcare (Cambridge Diagn), One Touch Basic, One Touch II, One Touch Profile (Lifescan) and Glucotrend (Boehringer Mannheim/Roche).

METHODS

The evaluation included within-run imprecision, linearity, comparison with the laboratory method and calculation of differences between individual glucometers.

RESULTS

Within-run imprecision ranged from 1.5% to 4.5%, linearity assessed as the correlation between measured and calculated glucose concentrations yielded r(2) values from 0.97 to 0.981. Analytical bias of glucose concentration values obtained by the glucometry amounted from 0.14% to 16.9% of values measured by the laboratory method. Bias higher than 5% was found for One Touch Basic, II and Profile meters (however, glucose concentrations in plasma obtained by the laboratory method One Touch meters showed analytical bias from 3.0% to 8.8%). The regression analysis yielded slope values from 0.77 to 1.09 and r(2) values from 0.86 to 0.98. The best correlations with the laboratory method were found for One Touch Basic, II Profile, Glucotrend and Esprit meters. The calculated differences between the individual glucose meters can constitute 0.02-1.49 mmol/l (0.96-26.9%) at glucose concentration 5.55 mmol/l, and 0.16-4.16 mmol/l (0.96-24.96%) at glucose concentration 16.67 mmol/l. Error grid analyses have shown that Glucometers One Touch Basic and One Touch Profile yielded all results in zone A (acceptable). The remaining glucometers yielded 1-7% of results in zones B (insignificant errors), C or D (lack of detection and treatment).

CONCLUSIONS

All studied glucometers had both small deviation from laboratory reference values (<10%) and high concurrence with results obtained by the laboratory method.

The evaluation of analytical performance of the Precision G point-of-care glucometer

In this study analytical and functional performance of the Precision G "point-of-care" glucometer (MediSense Inc.) was evaluated. Studies were carried-out using capillary blood collected for routine monitoring of glycemia in diabetic patients. Each glucose test measurement with the glucometer was paralleled by the laboratory measurement of glucose on the same blood sample, using the GOD/PAP method. Mean accuracy error in the glucose concentration range of 1.1-33.3 mmol/l calculated for the glucometer vs. the laboratory method amounted to only 0.2%. However, for glucose concentrations below 4.4 mmol/l the mean accuracy error was 3.9%, and for the concentrations above 10.0 mmol/l it was 4.6%. Within-run CV for three concentration levels was 2.76%, 2.89%, and 4.22%, respectively. Linearity of the meter response in samples with glucose concentration ranging from 1.7 mmol/l to 16.7 mmol/l, expressed as the correlation coefficient r, yielded r=0.996 and linear regression equation [y1 = 0.996 y2 - 0.005], where y1 is the measured glucose concentration and y2 is the target glucose concentration calculated in diluted samples. Correlation studies on a set of 114 blood samples collected from patients and assayed by glucometer and by the laboratory method yielded a relationship expressed by the equation: y = 0.84x + 1.13 where y is glucometer read-out and x is glucose concentration obtained by the laboratory method. Passing-Bablok test showed a significant agreement between the glucometer measurements and the reference laboratory results in the studied glucose concentration range. The error grid analysis of series of the paired patient's samples showed that 95% of results were in the clinically acceptable zone A and 1% of results in zone D.

Accuracy of self-monitoring of blood glucose: impact on diabetes management decisions during pregnancy

PURPOSE

This study tested the hypothesis that the accuracy of self-monitoring of blood glucose (SMBG) values of patients with diabetes during pregnancy deviates substantially from reference values.

METHODS

The patients' glucose values were measured on 6 different SMBG meters; reference values were from the HemoCue B Glucose Analyzer. Over a 5-year period, 1973 comparisons between SMBG values and reference values were recorded during clinic visits and used for this study. Data were analyzed for percent of values that varied more than +/- 10.5% and +/- 15.5% from the reference value. Out-of-range data at each variance level were analyzed to determine the impact on medical management if decisions were based solely on SMBG values.

RESULTS

One third of SMBG readings deviated significantly, which could adversely affect treatment for half of these patients if diabetes management was based on SMBG values. At the 10.5% deviation level, 34% of SMBG meter readings were out of range; 54% of these would have implied erroneous treatment. At the 15.5% deviation level, 18% were out of range; 63% of these would have implied erroneous management.

CONCLUSIONS

The accuracy of home meters should be verified at regular intervals, and SMBG values should not be the sole criterion for diabetes management during pregnancy.

IFCC recommendation on reporting results for blood glucose

In human beings, glucose is distributed like water between erythrocytes and plasma. The molality of glucose (amount of glucose per unit water mass) is the same throughout the sample. Different water concentrations in calibrator, plasma, and erythrocyte fluid can explain some differences that are dependent on sample type, methods requiring sample dilution, and direct reading biosensors detecting molality. Different devices for the measurement of glucose detect and report fundamentally different analytical quantities. The differences exceed the maximum allowable error of glucose determinations for diagnosing and monitoring diabetes mellitus, and they complicate the treatment. The goal of the International Federation of Clinical Chemistry, Scientific Division, Working Group on Selective Electrodes (IFCC-SD WGSE) is to reach a global consensus on reporting results. The document recommends harmonizing to the concentration of glucose in plasma (with the unit mmol/l), irrespective of sample type or technology. A constant factor of 1.11 will convert measured concentration in whole blood to the equivalent concentration in plasma.

Dynamic model of amperometric biosensors. Characterisation of glucose biosensor output

An integrated model for the characterisation of the output signal course of oxidase-bound amperometric biosensors is presented and evaluated in the case of glucose biosensors. This model integrates two earlier proposed models, the model of oxygen transducer-based biosensors, allowing the prediction of steady state parameters from the transient response and the dynamic signal lag model, characterising the electrochemical diffusion-limited sensors. The integrated model allows the characterisation of the whole biosensor signal output, originating from the output curve itself with errors less than 3% and no need to determine the system's geometrical, diffusion and partition parameters.

Evaluation of a real-time blood glucose monitor in children with diabetic ketoacidosis

Use of a real-time bedside glucose monitor was analyzed during the course of management of diabetic ketoacidosis (DKA) in children. Simultaneous determinations of blood glucose were obtained, using three methods: bedside glucose meter (One Touch II), laboratory glucose analyzer (YSI 2300 STAT), and a real-time bedside glucose monitor (VIA 1-01G Blood Chemistry monitor). Study patients included seventeen patients < 18 years of age admitted to a Pediatric Intensive Care Unit, with blood samples obtained during treatment of DKA by continuous insulin infusion. Four patients did not complete the study. Three experienced temporary technical problems with the monitor, and four required repeat IV placement. Duration of monitor use ranged between 6 and 47 h (mean 24 +/- 4 h). Blood glucose values ranged between 2.6 and 22.5 mmol/l. Overall correlation of blood glucose values were as follows: 0.965, 0.965, 0.973, VIA 1-01G vs. One Touch II, VIA 1-01G vs. YSI 2300 STAT, and One Touch II vs. YSI 2300 STAT, respectively (all P-values < 0.0001). This real-time bedside glucose monitor is accurate at glucose values < 13.8 mmol/l, and reliable for rapid, repetitive analyses. Results indicate that blood glucose values obtained using this real-time monitor are comparable to those using standard methods of measurement, and that this device is clinically applicable for use in management of children with DKA.

Proposal for standardizing direct-reading biosensors for blood glucose

Direct-reading glucose biosensors sense molality (glucose per unit water mass) in the sample. With aqueous calibration, a direct-reading glucose biosensor produces higher results in blood and plasma than methods measuring concentration, theoretically by the ratio of water concentrations in calibrator and sample. To confirm this, we measured glucose in 140 blood and 40 plasma samples with the direct-reading glucose sensor in the Chiron Model 860 Blood Gas and Critical Analyte System and with our routine method (ESAT 6660; Eppendorf). The Chiron instrument is calibrated with a 10 mmol/L (180 mg/dL) glucose calibrator (mass concentration of water = 0.99 kg/L). Assuming normal water concentrations of 0.84 and 0.93 kg/L in blood and plasma, respectively, we multiplied results from the Chiron sensor by 0.84/0.99 and 0.93/0.99 to obtain concentrations in blood and plasma. This conversion resulted in agreement of results with our routine method. An individual conversion based on hematocrit in each whole-blood sample was less satisfactory. To avoid confusion over variously measured and reported glucose results and reference values, we suggest standardization and reporting of whole-blood glucose results as equivalent plasma concentrations. This proposal may be conveniently achieved by using a commercially available reference material for glucose, NIST SRM 965.

Accuracy of devices used for self-monitoring of blood glucose

We measured the inaccuracy of 17 home blood glucose monitors (two visually read, eight colorimetric and seven amperometric). Using strips from a single batch, blood glucose measurements were performed by three medical laboratory technologists on at least 50 capillary blood specimens from patients attending two diabetes clinics. Additional capillary blood was deproteinized and assayed with hexokinase to give a whole blood glucose result. A dedicated glucose analyser was also studied to cross-validate the methodology. At a mean glucose concentration of about 9 mmol/L, monitor readings differed from the reference results by -5.1 to +19.5% with three systems failing to meet the American Diabetes Association guideline for total error of less than 15%. This problem would be alleviated by manufacturers adopting a common policy on calibration and on reporting as a plasma or whole blood value.

Accurate and precise isotope dilution mass spectrometry method for determining glucose in whole blood

An accurate and precise method to determine glucose concentration in whole blood is presented. The method, based on isotope dilution gas chromatography–mass spectrometry (ID GC-MS), was developed to be used as a Reference Method for determining glucose concentration in capillary or venous whole blood. Blood samples and standards are pipetted manually with “microcap” micropipettes, which makes it possible to collect samples even at the patient’s bedside. Glucose is quantified as its aldononitrile pentaacetate. [13C6]Glucose is used as an internal standard. Assay of Seronorm and Pathonorm L and H controls by ID GC-MS gave within-run CVs of 0.66%, 0.96%, and 0.92%, respectively. For whole blood with glucose concentrations in the low, normal, and high ranges, the within-run CVs were 1.27%, 0.91%, and 0.78%, respectively. The between-run CV for glucose calculated from 36 separate single analyses of Seronorm was 1.44%. In an accuracy assessment test of the HemoCue blood glucose analyzer, 140 capillary blood samples were measured in parallel after split-sampling. For all samples the HemoCue analyzer results had a mean bias of +2.0% compared with the ID GC-MS results.

Diagnosis of hypoglycaemia: effects of blood sample handling and evaluation of a glucose photometer in the low glucose range

Hypoglycaemia is a dangerous condition. Rapid and reliable blood glucose measurements are necessary for the initiation of treatment to reduce the risk of neurological sequelae. The aim of this study was to compare a bedside glucose photometer (HemoCue) with three methods of handling blood glucose measurements in a routine chemistry laboratory and to estimate the reliability of glucose measurements in the low glucose range during controlled hypoglycaemia. Nine children underwent an arginine-insulin tolerance test as part of a growth hormone deficiency investigation. Only blood samples below 4.0 mmol l-1 were included (n = 35). Significant (0.3-1.0 mmol l-1) differences in blood glucose measurements were found, depending on the handling of the blood sample. The differences seem primarily to be due to glycolysis which occurred in spite of the addition of the glycolysis inhibitor NaF to the blood samples. Immediate centrifugation and analysis of the supernatant or immediate analysis with the HemoCue results in higher, and presumably more correct, values than routine procedures and permits a more accurate diagnosis of hypoglycaemia.

Use of an enzyme thermistor for semi-continuous blood glucose measurements

A method to monitor glucose in whole blood is presented. The aim of the project was to develop a prototype for a bedside monitor system for semi-continuous monitoring of the blood glucose concentration, requiring only one calibration. This was made possible by using the special advantage of the thermal sensor technique in combination with the adjustment of flow. The glucose concentration was determined from the difference between the sensor response and an estimated background signal. Using standard addition technique, calibration factors for background and sensitivity were set and remained unchanged during the monitoring. The background signal was 45 +/- 8 mV (mean +/- S.D., n = 8) and the sensitivity was 28 +/- 1 mV/mmol (mean +/- S.D., n = 4). Recovery in whole blood was 90-98% (mean 94%, n = 12). With an injection interval of 3 min the precision with the sensor was < 3% over more than 100 blood samples. Response time was about 60 s. The calculated glucose values correlated, r = 0.98, with the values obtained with an YSI glucose analyser (Yellow Springs Instruments. Yellow Springs, OH, USA), over the range 2-20 mmol/l.

Microdialysis for metabolic monitoring in neonates after surgery

Microdialysis is a new method for continuous metabolic monitoring. We studied the possibility of using microdialysis in neonates treated in a paediatric intensive care unit after surgery. A microdialysis catheter was inserted in the abdominal subcutaneous adipose tissue in 14 neonates for a median of 93 h (range 24-106 h). In four neonates, two microdialysis catheters were used simultaneously. Samples were taken hourly for analysis of glucose, lactate and glycerol. Dialysate and blood concentrations were compared. Serum/whole blood glucose values (n = 68) were in the range 2.1-15.4 mM. The serum glucose levels showed good agreement with the dialysate concentrations of glucose, although these infants were subjected to various forms of stress, drugs and glucose infusions. The whole blood glucose levels were significantly lower than the dialysate levels. The microdialysis concentrations of glucose varied considerably. As almost identical dialysate glucose levels were found when two microdialysis catheters were used simultaneously, the variability probably reflects true changes in blood glucose levels. Our results indicate that microdialysis can be used in neonates.

Evaluation of the YSI 2300 glucose analyzer: algorithm-corrected results are accurate and specific

OBJECTIVES

To insure commutability (equivalence) between whole blood and plasma glucose values and to assess potential interferences in glucose oxidase methods.

DESIGN AND METHODS

We compared plasma glucose results by the Ektachem 700 analyzer to glucose results on whole blood by the Nova Stat Profile 6, and by the YSI 2300, both without hematocrit correction (YSIunc) and with two different hematocrit (Hct) corrections. The two correction methods were: (a) whole blood results multiplied by 1.11 (YSI1.11), and (b) whole blood results corrected for Hct with the programmed YSI algorithm (YSIcor). Several compounds were tested for interference.

RESULTS

88% of YSIcor results agreed with plasma results within plus/minus 5%, and 78% of Nova and 74% of YSI1.11 results were within this limit. YSIcor results were unaffected by Hct and were valid even when erythrocyte size was abnormal. At low Hct, Nova results were falsely decreased, while YSI1.11 results were falsely elevated. The Ektachem was most affected by ascorbic acid, and the Nova was especially affected by acetaminophen.

CONCLUSIONS

The YSI algorithm-corrected whole blood glucose results were commutable with plasma results and the YSI was unaffected by the compounds tested, including acetaminophen and acetylsalicylic acid.

Blood glucose reagent strip tests in the operating room: influence of hematocrit, partial pressure of oxygen, and blood glucose level--a comparison of the BM-test 1-44, BM-Accutest, and Satellite G reagent strip systems

OBJECTIVE

The objective of our study was to assess the influence of hematocrit (HCT), partial pressure of oxygen (PO2), and blood glucose level upon results obtained with three different blood glucose reagent strip tests used in conjunction with the appropriate meter: BM-Test 1-44, BM-Accutest, and Satellite G.

METHODS

Our study was designed as a consecutive sample study of patients undergoing coronary artery surgery. The setting was the hospital theater and intensive care unit. We conducted blood analysis for HCT, PO2 and blood glucose on 20 consecutive patients undergoing coronary artery surgery using three blood glucose reagent strip testing systems and a laboratory analysis of plasma glucose.

RESULTS

All three blood glucose reagent strip tests showed a significant bias when compared with plasma glucose: BM-Test 1-44, 0.89 mmol/L; BM-Accutest, -1.27 mmol/L; Satellite G, 0.75 mmol/L (p < 0.05). The error found when using the Satellite G system was worse than that of either of the other two systems. Results obtained with the BM-Accutest strips were unaffected by PO2 (p = 0.745). Blood glucose value and HCT both had an influence on the results of all three blood glucose strip systems.

CONCLUSIONS

Caution must be taken when using reagent strip systems in the operating room or intensive care setting because, of the three systems tested, all showed a significant bias, all were influenced by blood glucose level and HCT, and only the BM-Accutest reagent strips used with the Accutrend meter was unaffected by PO2.