Iran Journal of Nursing

Introduction
Diabetes is the most common disease of the endocrine system. Each year, an average of 500,000 people in Iran are diagnosed with diabetes. This condition increases patients’ vulnerability to various cardiovascular diseases, as diabetic people often experience significant fluctuations in their blood glucose levels. Failure to maintain blood glucose balance after heart surgery can lead to severe complications, such as hemodynamic disorders and cardiac arrhythmias. Additionally, errors in the sampling process or the use of inappropriate methods can result in incorrect interpretations and the selection of improper treatments. Therefore, accurate measurement and effective management of blood glucose level are crucial in reducing complications in diabetic patients, where intensive care nurses play a key role. Given the conflicting results regarding the accuracy of blood glucose measurement using capillary glucometers, this study aims to determine the difference in the accuracy of capillary and arterial blood glucose values in patients undergoing heart surgery.

Methods 
This cross-sectional study was conducted on patients with diabetes undergoing heart surgery and hospitalized in the intensive care unit of a specialized cardiology hospital in northern Iran. The inclusion criteria were age over 18 years, the diagnosis of diabetes, and undergoing heart surgery. Using a sequential sampling method, 73 patients were selected. 
An 11-item form was used to collect demographic and clinical information from the patients, such as age, sex, body mass index (BMI), duration of diabetes, insulin use before surgery, insulin dose, history of blood pressure, smoking habits, mean arterial pressure, cardiac output, and type of heart surgery. Three types of blood glucose samples were taken from the patients; an arterial line sample sent to the laboratory, an arterial line sample monitored by a glucometer, and a capillary sample monitored by a glucometer. 
To ensure measurement accuracy, glucometer results from arterial and capillary samples were compared with laboratory (arterial) results. The reliability of glucometer devices was assessed by simultaneously measuring blood glucose samples from 10 diabetic patients using two glucometer devices from different brands. The average error and average absolute error of blood glucose measurement methods were calculated to record the error rate. 
Statistical analysis was conducted using ANOVA (to compare blood glucose levels among the three methods), Bonferroni test (for pairwise comparisons), McNemar’s test (to compare the frequency of correct estimations between two methods), linear regression analysis  (for accurate laboratory estimation based on the arterial and capillary glucometer values), and Bland-Altman plots (to visualize the error rate and importance). The significance level for all tests was set at P<0.05.

Results 
Most of the participants were male (64.38%), overweight (42.47%), and non-smokers (63.01%). Their mean age was 60.34±8.26 years, with an mean BMI of 26.37±3.92 kg/m². More than half of the participants (57.53%) had a history of high blood pressure, and 58.9% had a cardiac output greater than 40%. The mean duration of diabetes was 8.21±4.15 years, and none of the patients had received insulin before the surgery.
The mean laboratory (arterial) glucose level was 213.88±69.11 mg/dL. The mean arterial glucometer value was 215.47±71.16 mg/dL, and the mean capillary glucometer value was 225.22±71.22 mg/dL. The mean absolute error between the laboratory glucose level and the arterial glucometer value was 11.12±13.16 mg/dL. The mean absolute error between the laboratory glucose level and the capillary glucometer value was 18.96±17.43 mg/dL, while the mean absolute error between the arterial and capillary glucometer values was 15.93±78.17 mg/dL.
Statistical analysis revealed significant differences (P<0.001) in the mean absolute errors among the three methods, indicating a greater difference between the capillary glucometer value and laboratory glucose level, compared to the arterial glucometer value. The arterial glucometer value achieved an accuracy rate of 94.5%, while the capillary glucometer value showed 89% accuracy relative to the laboratory level. McNemar’s test results indicated no statistically significant difference in accuracy between the capillary (89.04%) and arterial (94.52%) glucometer values (P=0.344). 
Both capillary and arterial blood glucose measurements using a glucometer tended to overestimate blood glucose levels compared to the laboratory test, where the capillary blood glucose measurement by a glucometer showed a greater tendency for higher estimates. The correlation coefficient between the arterial glucometer value and the laboratory level was 0.985, while the coefficient between the capillary glucometer value and laboratory level was 0.972. The agreement coefficient between the arterial and capillary glucometer values was 0.976. By considering an expected agreement coefficient value of 0.8, these results demonstrated statistically significant agreement among all three methods.
Based on the linear regression analysis results, for every one mg/dL increase in the arterial glucometer value, the laboratory blood glucose level increases by 0.94 mg/dL. Also, for every one mg/dL increase in the capillary glucometer value, the laboratory blood glucose level increases by 0.91 mg/dL.

Conclusion
Both capillary and arterial blood glucose measurements using a glucometer are effective for assessing blood glucose levels in patients with diabetes undergoing heart surgery. However, the arterial glucometer value shows greater accuracy and reliability, making it the preferred choice for blood glucose level measurement. This preference is especially important in clinical settings where accurate glucose monitoring is essential for effective management and decision-making. The higher accuracy of the arterial glucometer value may be due to its ability to more accurately reflect the true physiological blood glucose level, minimizing potential discrepancies and providing more reliable data to healthcare professionals. Therefore, while the capillary blood glucose measurement by a glucometer offers results more conveniently and is suitable for daily blood glucose monitoring, the arterial blood glucose measurement by a glucometer is recommended for situations requiring high precision. This conclusion highlights the importance of choosing the appropriate method of blood glucose measurement in a glucometer based on the specific clinical context, and indicates the need for accurate blood glucose assessment.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the ethics committee of the Guilan University of Medical Sciences (Code: IR.GUMS.REC.1398.087). Full details about the study objectives were provided to the participants, and written informed consent was obtained from them. 

Funding
This study was funded by Guilan University of Medical Sciences.

Authors' contributions
Conceptualization, research, and data collection: Soolmaz Mousavi; writing and editing: Tara Alizadeh and Pooyan Ghorbani Vajargah; data analysis: Ehsan Kazemnejad Leili; supervision: Nazila Javadi Pashaki and Zahra Majd Teymouri.

Conflict of interest
The authors declare no conflict of interest.

Acknowledgments
The authors would like to thank Guilan University of Medical Sciences for their financial support and all participants for their cooperation.


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