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CE Article

Differences in Glucose Values Obtained From Point-of-Care Glucose Meters and Laboratory Analysis in Critically Ill Patients

Corresponding author: Lisa Halvorsen, RN, PHD, Providence Portland Medical Center, 4805 NE Glisan St, Portland, OR 97213 (e-mail: Lisa.Halvorsen{at}providence.org).

	Abstract

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Background Blood for glucose analysis is often obtained interchangeably from indwelling catheters and fingersticks.

Objectives To determine the level of agreement between glucose values obtained by laboratory analysis and with a point-of-care device for blood from 2 different sources: fingerstick and a central venous catheter.

Methods A method-comparison design was used. Point-of-care values for blood from fingersticks and catheters were compared with laboratory values for blood from catheters in a convenience sample of 67 critically ill patients. The effects of hematocrit level and finger edema on differences in glucose values between the 2 methods were also evaluated. A t test was used to determine differences in glucose values obtained via the 2 methods. Differences and limits of agreement were also calculated.

Results Laboratory glucose values for blood from a catheter differed significantly from point-of-care values for blood from the catheter (t_1,66 = –9.18; P < .001) and from a fingerstick (t_1,66 = 6.53; P < .001). Glucose values for the 2 methods differed by 20 mg/dL or more for 1 of 6 patients (15%) for catheter samples and for 1 of 5 (21%) for fingerstick samples. Point-of-care glucose values for fingerstick and catheter samples did not differ (P = .98). Hematocrit level significantly explained the difference in glucose values between the 2 methods for both catheter (R² = 0.288; P < .001) and fingerstick (R² = 0.280; P = .02) samples.

Conclusions Use of a commonly used point-of-care device when precise glucose values are needed may lead to faulty treatment decisions.

Notice to CE enrollees:A closed-book, multiple-choice examination following this article tests your understanding of the following objectives: Describe the relationship of euglycemic glucose levels to outcomes for critically ill patients. Understand the importance of differences between point-of-care (POC) glucose testing and laboratory glucose testing used with critically ill patients. Recognize the relationship between hemoglobin levels and the accuracy of POC test values for blood glucose. To read this article and take the CE test online, visit www.ajcconline.org and click "CE Articles in This Issue." No CE test fee for AACN members.

The purpose of our study was to compare (1) glucose values obtained with a POC device and blood from a CVC or blood from a fingerstick with (2) glucose values obtained by laboratory analysis of blood from a CVC. A secondary purpose was to determine if hematocrit level and/or degree of finger edema was a significant contributor to differences between laboratory and POC glucose values. The methods used for POC glucose testing were purposely designed to more closely reflect how POC devices are used in clinical practice and thereby provide a better estimate of the performance of the devices in practice. Of particular concern was the common practice of using blood from fingerstick and CVC sources interchangeably. Because most POC glucose meters have an adjustment to correct glucose values from capillary blood to more closely approximate laboratory glucose values of venous blood, use of venous blood with a POC device designed for capillary blood could introduce additional measurement error.

POC glucose devices are calibrated for use with capillary fingerstick blood, but blood from central or arterial catheters is often used clinically.

 

	Materials and Methods

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
The study was conducted in an intensive care unit at Providence Portland Medical Center, Portland, Oregon, a 483-bed, not-for-profit community hospital. The study was approved by the medical center’s investigational review board.

	Sample

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Participants were a convenience sample of critically ill patients who required glucose monitoring for therapeutic care. Inclusion criteria included presence of a CVC (double- or triple-lumen CVC or a peripherally inserted central catheter). Power analysis was used to determine the sample size: multiple regression analysis, effect size of 20% (moderate), power of 80%, level of .05, 2 independent variables.¹¹

	Study Design

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
A method-comparison design was used to evaluate different methods (POC vs laboratory analysis) for determining blood glucose values. Each patient served as his or her own control. The primary dependent variables were differences between glucose values obtained with the standard reference method (laboratory analysis of a CVC blood sample) and values obtained with the test methods (POC analysis of fingerstick and CVC blood samples). Hematocrit level and degree of finger edema, factors that might explain any differences between the 2 methods of glucose analysis, were also evaluated.

	Instruments

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
For laboratory analysis of CVC blood samples, an Olympus AU640 glucose analyzer (Olympus America Inc, Melville, NY) was used according to the manufacturer’s guidelines. Manufacturer’s specifications for glucose analysis include a range of 10 to 800 mg/dL (to convert to millimoles per liter, multiply by 0.055), and precisions of 1.6% for a glucose mean of 59 mg/dL and 1.5% for a mean of 258 mg/dL.

For POC analysis of both fingerstick and CVC blood samples, a SureStepFlexx blood glucose meter (Johnson & Johnson LifeScan, Inc, Milpitas, California) was used. Manufacturer’s specifications on device performance indicated a correlation (r) of 0.995, with a precision of ±3.2% for glucose values in normal ranges.¹² Six POC glucose meters were available on the study unit for data collection. Although no effort was made to use the same meter for all patients, for each patient the same POC meter was used for the 2 POC tests.

For measurement of hematocrit levels in CVC blood samples, a Coulter Gen-S system (Beckman Coulter, Inc, Miami, Florida) was used according to standard manufacturer’s procedures. Manufacturer’s specifications for hematocrit analysis indicate a range of 0% to 75%, with precision of 3.0% for hematocrit values in the 0% to 75% range.

	Study Procedure

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
After training on the proper methods for obtaining samples of CVC and fingerstick blood and use of the POC glucose meter, 4 investigators obtained blood from the CVC and then from a fingerstick. Before blood was withdrawn from the CVC, all intravenous infusions through the catheter were stopped for 3 minutes. Blood was then withdrawn from the catheter (>10 times the catheter dead space) before the blood for glucose testing was obtained. Blood for laboratory glucose analysis was placed in a separator vacuum test tube and immediately sent to the laboratory for analysis. A drop of blood from the syringe used for the laboratory sample was then used for POC testing. The amount of peripheral edema in the digit used for fingerstick blood sampling was rated from 0 to 4+,¹³ and the patient’s most recent hematocrit level was transcribed from the medical record.

Glucose levels in CVC and fingerstick blood samples were measured with the POC glucose meter according to the manufacturer’s directions. A drop of blood was placed on the chemical reagent strip (SureStepPro Test Strips, Johnson & Johnson LifeScan, Inc), and the strip was placed into the glucose meter for analysis. Quality control testing of the glucose meters was performed daily according to the manufacturer’s directions. Both high- and low-quality samples were tested with the meters, and meters that did not pass the quality control test were replaced.

Blood from the CVC was sent to the laboratory for stat processing. Mean time from blood sampling to analysis in the laboratory was less than 1 hour for stat glucose measurements.

	Data Analysis

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Data were summarized by using descriptive statistics. A t test was used to determine if significant differences occurred between the reference glucose value (laboratory glucose) and the POC glucose values (CVC and fingerstick blood). Differences (bias) and limits of agreement (precision) between the POC glucose meter (CVC and fingerstick) and reference standard glucose values were calculated and graphed by using the Bland-Altman method.^14–18 Multiple regression analysis was used to determine if hematocrit level and/or finger edema significantly explained the difference in glucose values between the laboratory and POC methods. The level of significance for all statistical tests was P < .05.

POC glucose values from CVC and fingerstick blood were both different from lab values.

 

	Results

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
A total of 67 patients were studied during a 5-month period. Mean age of the patients was 58.4 years (SD, 13.7). Glucose values ranged from 62 to 218 mg/dL. Hematocrit values ranged from 22% to 46.2%; 21 patients (31%) had hematocrit values less than 30%. A total of 76% of the patients had peripheral edema scores of 0 or 1+; 24% had scores of 2+ or 3+.

Mean glucose values, bias (mean difference scores between POC and laboratory values), precision (standard deviation of the mean difference scores), and root-mean-square of the differences scores for the 67 patients are summarized in the Table. Among the 67 patients, 10 (15%) had differences of 20 mg/dL or greater between the CVC POC and laboratory glucose values, and 14 (21%) had differences of 20 mg/dL or greater between the fingerstick POC and laboratory glucose values.

View larger version (30K): [in this window] [in a new window]   Figure 1 Bland-Altman graphs of difference scores for point-of-care (POC) and laboratory (lab) glucose values in 67 critically ill patients. A, fingerstick POC and laboratory values. B, Central venous catheter (CVC) POC and laboratory values. C, fingerstick POC and CVC POC values.

View larger version (30K): [in this window] [in a new window]   Figure 2 Regression plots of difference between point-of-care (POC) and laboratory (lab) glucose values with hematocrit as the independent variable in 67 critically ill patients. A, fingerstick POC value – laboratory value. B, Central venous catheter (CVC) POC value – laboratory value.

The difference between finger-stick POC and lab glucose values was 20 mg/dL in 21% of cases.

	Discussion

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
We found significant differences between POC glucose values and values obtained with the reference, laboratory method. Bias and precision of the POC device were large across the range of predominantly normal glucose values studied (62–218 mg/dL). POC glucose values differed from the laboratory glucose value by 20 mg/dL or greater in 1 of 6 patients (15%) for fingerstick samples and in 1 of 5 (21%) for CVC samples. In addition, hematocrit levels significantly explained differences in POC and laboratory glucose values, with greater overestimation of glucose by the POC device at lower hematocrit levels. No significant differences were found between the glucose differences of the 2 POC values and the laboratory values for glucose determinations ([CVC POC – laboratory] vs [fingerstick POC – laboratory]).

	POC vs Laboratory Glucose Values

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Our findings of significant differences between POC glucose values and the laboratory glucose values are similar to findings of previous studies.^{4–8,10,19–27} In most of those studies, however, POC glucose testing was done with fingerstick blood; blood was obtained from an existing catheter device (arterial catheters) and appropriate statistical analysis was done in only 2 studies.^7,8 Our study extends the findings of significant differences between POC and laboratory glucose analysis to CVC blood.

We simultaneously obtained fingerstick and CVC blood for POC testing to determine if either source could be used interchangeably in clinical POC testing. Using fingerstick and CVC blood samples interchangeably is a common practice today; clinicians assume that the results from these 2 sources are equivalent. However, the results may not be equivalent because the POC meter is calibrated to account for differences between capillary blood and venous blood used in laboratory analyses. In only 1 other study⁸ did researchers seek to determine if the 2 POC blood sources (catheter, fingerstick) yielded equivalent blood glucose levels. Much as we did for CVC blood, Lacara et al⁸ found no difference between POC glucose values obtained with arterial catheter or fingerstick blood. These results support the equivalence of using either a fingerstick or a catheter blood source if a POC device is used for glucose testing. However, although the fingerstick and catheter POC glucose values were equivalent to each other, we found significant differences between the POC and laboratory glucose values.

Although most studies did not report the number of patients with large differences between POC and laboratory glucose values, we found that 15% of CVC and 21% of fingerstick blood samples in our patients had differences of 20 mg/dL or greater between the POC and the laboratory values. These findings, coupled with the significant difference between POC and laboratory methods for glucose analysis and the large bias and precision of the POC device, underscore the need for caution when using POC values to guide insulin dosing with narrowly defined insulin administration protocols.

We attempted to use methods in our study that mimic actual clinical practice procedures, in which multiple POC devices are used by multiple caregivers. Error associated with the use of multiple POC devices by multiple caregivers most likely is greater than in studies in which a single POC device is used and 1 person does all the POC glucose testing. In previous studies, the number of POC devices and the number of persons doing the testing were limited—a good research method for improving interrater reliability, but an approach that is unlikely to reflect the results obtained in actual clinical use. We made no effort to use the same POC glucose meter for all patients, because a single device is not normally used in actual clinical practice. Our goal was to determine the difference and limits of agreement between POC and laboratory glucose values that exist in actual clinical practice situations.

	Effect of Hematocrit and Peripheral Edema

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
In previous studies on POC glucose devices, researchers examined several factors that may adversely affect the performance of the device when patients have physiological derangements. Confounding factors identified in earlier studies include pH, shocklike states, hypoxemia, hematocrit level, vasopressor use, and peripheral edema.^{5,7,9,10,19,20,22,28–34} Hoping to use the results of our study to affect the way POC testing is done in our unit, we selected 2 of these variables to evaluate, because POC testing is often done in patients with some or all of the confounding factors previously studied.

Multiple regression analysis indicated that differences between POC and laboratory glucose values were greater in patients with lower hematocrit levels. The negative relationship between hematocrit and differences between POC and laboratory glucose values for both venous and capillary blood in our study was similar to findings in previous studies^{5,19,20,28,30–32,34} in which the impact of hematocrit level on the performance of POC devices was examined. The manufacturer’s guidelines for the POC devices we used include cautionary statements about use of the devices with blood from patients with abnormally high and low hematocrit levels.¹² Our results support that caution for critically ill patients with low hematocrit levels. However, our sample did not include any patients whose hematocrit levels were elevated.

Finger edema was selected for study because clinicians in our unit have anecdotally reported situations in which large glucose differences occurred in patients with excessive finger edema compared with patients who have little or no finger edema. Similar to Kanji et al⁷ and Lacara et al,⁸ we found that finger edema did not help explain differences between POC and laboratory glucose values (P = .54). Several reasons may account for the lack of difference with finger edema: the limited number of high scores for edema in our study (<25% of patients had scores of 3+ or 4+), poor interrater reliability in scoring of edema, and a lack of effect of edema on POC and laboratory differences.

	Clinical Implications

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Discrepancies between POC and laboratory glucose values were statistically significant. The magnitude of those differences could be considered clinically significant if the glucose values were used to direct treatment decisions requiring precise glucose measurements. Of particular concern is the frequent use of POC glucose values for intravenous insulin titration in glucose management protocols with narrowly defined glucose ranges for different insulin treatment levels. The large bias and precision in our study, in which we used a common POC glucose device and methods similar to those of usual practice, could easily result in erroneous insulin dosages based on POC glucose values. The large bias and precision data in our study may be greater in patients with abnormally low hematocrit values, a common physiological derangement in critically ill patients.

POC fingerstick and CVC blood glucose values are equivalent.

 

Use of POC glucose analysis to decrease the time to treatment decisions, the rationale often given for why POC rather than laboratory testing for glucose is best, must be balanced by how precise the glucose value must be for correct decisions about a patient’s management. The greater the need for precision, the greater is the importance of using laboratory analysis.

Most POC glucose devices were never intended to be used for treatment situations in which precise measurement of glucose is required; the devices were designed for monitoring trends in glucose levels.

Another reason clinicians give for using POC glucose testing rather than laboratory analysis is cost. A recent study,³⁵ however, casts doubt on any cost savings associated with POC rather than laboratory glucose analysis. Of additional concern is the large amount of time that critical care nurses estimate is spent on POC testing activities (approximately 6 h/d per patient) in a time when appropriate use of nursing staff is crucial.³⁶

These differences in glucose values may be problematic when values are used for titration of intravenous insulin with narrowly defined glucose ranges.

 

Although critical care nurses can easily learn to competently perform laboratory tests, consideration should be given to how best to use their nursing expertise, particularly when the accuracy of the POC glucose devices is less than optimal.

When use of POC glucose analysis is appropriate, the use of CVC or fingerstick sources of blood for POC testing will provide equivalent results. For patients who require frequent glucose testing, obtaining blood samples from a catheter rather than fingerstick would be less painful. This pain consideration maybe overshadowed if blood acquisition from the CVC increases infection risk and/or excessive blood wasting due to the lack of closed blood withdrawal systems with reinfusion capability.

	Study Limitations

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Although we attempted to use study procedures similar to those used in clinical practice, we only had 4 investigators performing POC testing after special training before data collection began. In most critical care units, POC testing is done by almost every nurse or unlicensed nursing personnel providing patient care, a situation that is likely to increase the imprecision of the values obtained with the POC device. In future studies, researchers should examine the performance of POC glucose devices when the devices are used by all of the unit clinicians, not just a select few who have had additional training beyond that required for yearly laboratory certification and competency validation. Although research methods that optimally control variables that may influence device performance are important in early testing of any medical technology, later testing also should be done in conditions that more closely mimic actual clinical practice to better elucidate the likely performance characteristics of the device during usual use.

Because all of the patients in our sample were being treated according to an aggressive glucose management protocol that included frequent glucose measurement and insulin adjustment, we did not observe glucose values in the extremes for either hypoglycemia or hyperglycemia. Likely, performance of the POC device would have had even greater differences and limits of agreement if more of our patients had tested values in the extreme ranges for abnormal glucose values. In future studies in which researchers attempt to use procedures similar to use of POC devices in clinical practice, the performance of the devices in patients with hypoglycemia and/or hyperglycemia should be examined.

	Conclusions

Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 
Although POC glucose analysis is often used to guide management decisions in critical care, we found significant differences between glucose values obtained with a POC device and by the laboratory. The magnitude of these differences is such that use of a POC device in situations in which knowledge of precise glucose values is needed (eg, tight glucose management protocols) may lead to faulty treatment decisions. Our findings that POC values differed from the laboratory values by 20 mg/dL or more for 1 of 5 fingerstick samples and 1 of 6 CVC samples emphasizes this concern. The accuracy of POC devices is further eroded when patients have low hematocrit levels. Clinicians should use caution when interpreting POC glucose values in these situations. We did find that POC glucose values obtained from CVC and fingerstick blood are equivalent.

	ACKNOWLEDGMENTS

 
Special thanks to Marianne Chulay, RN, PHD, FAAN, for assistance with study design, data analysis, and manuscript preparation.

FINANCIAL DISCLOSURES
None reported.

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Top Abstract Materials and Methods Sample Study Design Instruments Study Procedure Data Analysis Results Discussion POC vs Laboratory Glucose... Effect of Hematocrit and... Clinical Implications Study Limitations Conclusions References

 

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