Article, Biochemistry

How reliable are electrolyte and metabolite results measured by a blood gas analyzer in the ED?

a b s t r a c t

Introduction: Blood gas analysis is a frequently ordered test in emergency departments for many indications. It is a rapid technique that can analyze electrolyte and metabolites in addition to pH and blood gases. The aim of this study was to investigate the correlation of electrolyte and metabolite results measured by blood gas and core lab- oratory analyzers.

Methods: This was a prospective, single-center observational study conducted in a tertiary care center’s emergen- cy department. All adult patients requiring arterial/venous blood gas analysis and core laboratory tests together for any purpose were consecutively included in the study between April 2014 and July 2015. Patients younger than 16 years, having any intravenous infusion or blood transfusion prior to sampling, or who were pregnant were excluded.

Results: A total of 1094 patients’ (male = 547, female = 547) paired blood samples were analyzed. The mean age was 58.10 +- 21.35 years, and there was no difference between arterial and venous sampling groups by age, pH, or sex (P = .93, .56, and .41, respectively). Correlation coefficients for hemoglobin, hematocrit, glucose, potassi- um, sodium, and Chloride levels measured by Blood gas analyzer and core laboratory analyzers were 0.922, 0.896, 0.964, 0.823, 0.854, and 0.791, respectively.

Conclusion: Blood gas analysis results were strongly correlated for hemoglobin, hematocrit, glucose, potassium, and Sodium levels but were only moderately correlated for chloride levels. These parameters as measured by a blood gas analyzer seem reliable in critical decision making but must be validated by core laboratory results.

Introduction

Laboratory results are of great importance in the diagnosis and man- agement of patients in the emergency department (ED). Core laboratory services can increase the length of emergency stay; furthermore, de- layed results can reduce the quality of patient care, especially for criti- cally ill patients [1,2]. Reliable point-of-care systems are currently in

? Prior publication: This article has been presented as an oral presentation in the “4th Eurasian Congress on Emergency Medicine, 2014, Antalya, Turkey.”

?? Conflict of interest: We declared that we have no commercial, financial, and other re-

lationships in any way related to the subject of this article that might create any potential conflict of interest.

* Corresponding author. Department of Emergency Medicine, Etimesgut Military Hos-

pital, Etimesgut Asker Hastanesi, Etimesgut, Ankara 06797, Turkey. Tel.: +90 312 2491011.

E-mail addresses: eminuysal79@mynet.com (E. Uysal), yahya_acar@yahoo.com (Y.A. Acar), ahmetkutur@mynet.com (A. Kutur), cevikerdem@yahoo.com (E. Cevik), dr_necatisalman@yahoo.com (N. Salman), dr.onurtezel@gmail.com (O. Tezel).

¹ Tel.: +90 212 440 4000.

² Tel.: +90 432 222 3329.

³ Tel.: +90 312 2491011.

clinical use, but these systems are not widely used in lower-income countries because of their higher costs [3-5].

Blood gas analyzers are used in almost all EDs, and blood gas analysis (BGA) is a frequent test ordered for many indications. Blood gas ana- lyzers can measure parameters such as hemoglobin, hematocrit, glu- cose, sodium, potassium, chloride, and calcium in addition to pH and blood gases. Blood gas analysis has the advantage of being faster com- pared with core laboratory analyses. It has been suggested as a point- of-care test in pulmonology clinics, but debate continues regarding its use in EDs [6,7].

This study was designed to investigate whether hemoglobin, hemat- ocrit, glucose, sodium, potassium, and chloride levels measured by a blood gas analyzer and a core laboratory analyzer are correlated in emergency patients.

Methods

This was a prospective observational study conducted in a tertiary center’s ED, which has an annual admission rate of 396000. The study

http://dx.doi.org/10.1016/j.ajem.2015.11.025

Table 1

Descriptive statistics of the study

Parameter		n (%)	Mean +- SD	CI	P
Age	Arterial	219 (20)	58.22 +- 21.73	-3.32 to 3.02	.93
	Venous	875 (80)	58.07 +- 21.27
pH	Arterial	219 (20)	7.39 +- 0.13	-0.02 to 0.01	.56
	Venous	875 (80)	7.39 +- 0.08
Sex (male/female)	Arterial	219 (20)	115/104		.41
	Venous	875 (80)	432/443

Abbreviation: CI, confidence interval.

protocol was approved by local ethical committee, and all procedures were performed in compliance with the Helsinki Declaration.

Data collection

All adult patients admitted to the ED for any reason were assessed for eligibility between April 2014 and July 2015. The study period was prolonged because the ED underwent reconstruction for 6 months and did not accept patients between July 31, 2014, and January 31, 2015. Subjects were enrolled consecutively from the patients who required routine biochemical analyses and arterial or venous BGA at the same time. No additional sampling or intervention was performed for the study. No selection criteria were applied and patients were enrolled consecutively. Exclusion criteria were being younger than 16 years, hav- ing any intravenous infusion or blood transfusion prior to sampling, and being pregnant. After obtaining informed consent, laboratory results were recorded from the patients’ medical records.

For arterial and venous blood gas sampling, BD Vacutainer Preset (Becton, Dickinson and Company Inc, Franklin Lakes, NJ) syringes con- taining calcium-balanced heparin were used. Samples were analyzed with a RAPIDLab 1265 (Siemens, Berlin, Germany) device immediately after drawing. For Arterial puncture, the Radial artery was the vessel of choice. Alternatively, the femoral artery was used for sampling.

Venous blood samples were collected in standard containers through standard in-hospital procedures by experienced emergency staff and analyzed as standard procedure. Hematology parameters were analyzed with a CELL-DYN Ruby (Abbott, Abbott Park, IL) hema- tology device. Electrolytes and glucose were analyzed with a Cobas

6000 analyzer (Roche Diagnostics, Indianapolis, IN) device. All devices were located in the central laboratory and calibrated regularly accord- ing to manufacturer specifications by accredited companies. Core labo- ratory results were considered as the criterion standard in the assessment of blood gas results.

Once analyses were completed and reported, hemoglobin, hemato- crit, glucose, sodium, potassium, and chloride levels were recorded in the study medical records by principal investigators.

Statistical analyses

Descriptive statistics were stated as frequency, percentage (%), and mean +- SD. Differences between core laboratory and blood gas mea- surements were calculated for each parameter and means were com- pared with Student t test. Pearson correlation coefficients were calculated for each parameter; values higher than 0.8 were considered to be a strong correlation. To assess agreement, the Bland-Altman graphical method with 95% limits of agreement (LoA) was used. All sta- tistical tests were performed with the Predictive Analytics Software (PASW, version 18; SPSS Inc, Chicago, IL).

Results

A total of 1094 patients’ (male = 547, female = 547) paired blood samples were included in the study. The mean (SD) patient age was

58.10 (21.35) years (minimum, 16 years; maximum, 101 years), and there was no difference between arterial and venous groups (P =

.93). Descriptive statistics are presented in Table 1.

The artery BGA sample size was 219 (20.0%), whereas the venous BGA sample size was 875 (80.0%). Indications for BGA were metabolic (n = 408; 37.3%), respiratory (n = 365; 33.4%), toxicological (n =

132; 12.1%), trauma (n = 8; 0.7%), neurologic (n = 116; 10.6%), and

others (n = 65; 5.9%).

There was a strong correlation between BGA results and core labora- tory results for hemoglobin, hematocrit, glucose, sodium, and potassium measurements but only a moderate correlation for chloride measure- ments (Table 2).

Table 2

Correlation of the blood gas and core laboratory results

Parameter	n	Minimum difference	Maximum difference	Mean difference +- SD	P		95% CI	Pearson correlation coefficient
Hemoglobin (mg/dL)
– All	1092	-7.37	8.42	-0.027 +- 1.01		.37	-0.87 to 0.03	.922
– Arterial	218	-7.18	3.16	0.12 +- 0.85		.045	0.003 to 0.23	.934
– Venous	874	-7.37	8.42	-0.06 +- 1.04		.072	-0.13 to 0.006	.920
Hematocrit (%)
– All	1092	-24.59	30.46	-2.19 +- 3.34	b	.001	-2.39 to -1.99	.896
– Arterial	218	-24.59	10.94	-1.86 +- 3.05	b	.001	-2.27 to -1.46	.901
– Venous	874	-22.89	30.46	-2.27 +- 3.41	b	.001	-2.50 to -2.05	.896
Glucose (mg/dL)
– All	1094	-210	310.00	-6.05 +- 25.42	b.001		-7.56 to -4.54	.964
– Arterial	219	-52.00	310.00	0.40 +- 25.45	.815		-2.99 to 3.79	.947
– Venous	875	-210.00	182.00	-7.67 +- 25.17	b.001		-9.34 to -5.99	.969
Sodium (mmol/L)
– All	1094	-29.20	14.40	-1.63 +- 3.30	b.001		-1.82 to -1.44	.823
– Arterial	219	-13.50	6.80	-2.17 +- 3.09	b.001		-2.58 to -1.75	.894
– Venous	875	-29.20	14.40	-1.49 +- 3.34	b.001		-1.72 to -1.28	.796
Potassium (mmol/L)
– All	1094	-3.41	5.41	-0.46 +- 0.45	b.001		-0.49 to -0.43	.854
– Arterial	219	-1.84	1.77	-0.47 +- 0.34	b.001		-0.51 to -0.42	.931
– Venous	875	-3.41	5.41	-0.46 +- 0.47	b.001		-0.49 to -0.43	.829
Chloride (mmol/L)
– All	326	-6	15.50	4.74 +- 3.35	b.001		4.37 to 5.10	.791
– Arterial	49	-4.70	15.50	5.90 +- 3.80	b.001		4.81 to 6.99	.846
– Venous	277	-6.00	14.10	4.53 +- 3.23	b.001		4.15 to 4.91	.785

Difference between core laboratory and blood gas measurements was calculated for each parameter and tested for zero difference. Abbreviation: CI, confidence interval

Fig. 1. Scatter plots for the six parameters studied.

Scatter plots showed linear positive correlation, with r² values of 0.849 for hemoglobin, 0.803 for hematocrit, 0.930 for glucose, 0.678 for sodium, 0.729 for potassium, and 0.317 for chloride (Fig. 1).

According to the Bland-Altman comparison of 2 different analyzers, the 95% LoAs were as follows: -2.25 to 1.71 for hemoglobin, -8.75 to

-2.19 for hematocrit, -55.87 to 43.77 for glucose, -6.63 to 3.37 for

sodium, – 1.34 to 0.42 for potassium, and 1.74 to 7.74 for chloride

(Fig. 2).

Discussion

This study showed that BGA results and core Laboratory test results have a strong correlation for 5 parameters and moderate correlation for 1 parameter in adult emergency patients.

Laboratory results are of great importance in the clinical decision- making process. Fast and reliable laboratory results are essential for emergency physicians. Point-of-care systems have demonstrated good reliability and also have the advantages of being fast and easy to use [4,5]. However, these systems increase costs and are therefore not wide- ly used in lower-income countries [3]. Although there has been no con- sensus on using BGA as a Point-of-care test in the ED, our results suggest that BGA can be used for the first-line assessment of critical patients until core laboratory results are completed.

In their prospective single-center cohort study, Zhang et al [8] report- ed that arterial blood gas-measured hemoglobin, sodium, and Potassium levels were generally lower than laboratory-measured results. According to their study, the arterial blood gas-measured levels of these 3

parameters were reliable and did not exceed United States of America Clinical Laboratory Improvement Amendment (USCLIA)-determined ac- ceptable bias [8,9]. Our results showed similar correlations for arterial blood gas samples, and the BGA results were also within USCLIA limits.

Uyanik et al [10] and Leino and Kurvinen [11] found a poor correla- tion between arterial BGA and core laboratory results for glucose, sodi- um, calcium, and chloride; they attributed the differences to the different ion-selective electrode (ISE) measurement techniques (direct ISE with the blood gas analyzer and indirect ISE with the core analyzer). Measurement technique difference between blood gas analyzer and core laboratory systems was also valid in our study and that potentially affected the chloride results, resulting in the moderate correlation. How- ever, our results showed strong correlations for the other 5 parameters. Jain et al [12] also studied the reliability of arterial BGA results for so- dium and potassium levels. They found that ABG results were lower than auto-analyzer results and reported a correlation coefficient of

0.68 for sodium and 0.72 for potassium. They also reported that the re-

sults of arterial BGA were in the USCLIA range for both parameters [9,12]. In their retrospective study, Budak et al [13] reported similar re- sults for critical care patients. Our arterial subgroup results were consis- tent with the results for hemoglobin, sodium, and potassium levels reported in these 2 studies.

Our results showed that glucose was the most reliable among the 6 parameters. This is especially important because hyperglycemic and hy- poglycemic emergencies are frequent, and treatment is based on labora- tory results [14,15]. We did not find any study in the current literature on the reliability of BGA in hyperglycemic and hypoglycemic conditions,

Fig. 1. (continued).

but Uyanik et al [10] and Leino and Kurvinen [11] reported results con- sistent with those of our study [10,11].

In addition, our results showed that emergency physicians ordered venous BGA 4 times more frequently than arterial BGA; this could be a result of recent studies comparing arterial and venous BGA results [16-18]. However, we did not find any clinical study directly studying the reliability of venous BG results in the current literature, and our re- sults suggested that venous samples are as reliable as arterial ones.

Limitations

Subjects were selected consecutively and randomization was not ap- plied for this prospective study.

Conclusions

There was a good correlation between BGA and core laboratory results for hemoglobin, hematocrit, glucose, sodium, and potassium levels. Blood gas analysis might have the potential to help emergen- cy physicians in critical decision making, especially for hyperglyce- mic and hypoglycemic states, but they must be validated by core laboratory results. We also concluded that reliable, inexpensive, and rapid point-of-care measurement technologies could be desir- able for emergency physicians.

References

AJEM