Comparison of different methods for measurement of electrolytes in patients admitted to the intensive care unit

Objectives:

To investigate whether electrolyte levels measured by using blood gas analyzers (ABG) and auto-analyzers (AA) are equivalent and can be used interchangeably.

Methods:

This observational prospective study was conducted in 100 patients admitted to the Intensive Care Unit, Adnan Menderes University School of Medicine, Aydin, Turkey, between March and August 2014. Samples for both AA and ABG analyzers were collected simultaneously from invasive arterial catheters of patients. The electrolyte levels were measured by using 2 methods.

Results:

The mean sodium level measured by ABG was 136.1±6.3 mmol/L and 137.8±5.4 mmol/L for AA (p=0.001). The Pearson’s correlation coefficient was 0.561 (p<0.001). The Bland-Altman 95% limits of agreement were -9.4 to 12.6 mmol/L. The mean potassium levels measured by ABG was 3.4±0.7 mmol/L and AA was 3.8±0.7 mmol/L (p=0.001). The Bland-Altman comparison limits were -0.58 to 1.24 and the associated Pearson’s correlation coefficient was 0.812 (p<0.001).

Conclusion:

The results of the 2 analyzing methods, in terms of sodium, were not equivalent and could not be used interchangeably. However, according to the statistical analyses results, by including, but not blindly trusting these findings, urgent and vital decisions could be made by the potassium levels obtained from the BGA, but a simultaneous follow-up sample had to be sent to the central laboratory for confirmation.Electrolytes are very important for the continuation of the physiological functions of the human body. They play vital roles in: regulation of the cell membrane potential, steady process of neurohormonal pathways, energy transformation and the fluid, and acid-base balance in the body. Signs and symptoms of electrolyte disorders may be nonspecific in an intensive care unit (ICU) patient.1-3 The therapies directed for maintaining vital organ functions affect the electrolyte balance. Consequently, electrolyte disorders are more common in critically ill patients than non-critically ill patients.1 The incidence of electrolyte disorders is nearly 25% in ICU patients.2 In recent studies,4-6 it is shown that in ICU patients, serum sodium and potassium levels are significant predictors of mortality. Therefore, prompt and complete correction of electrolyte disorders in ICU patients is vitally important. Under these circumstances, the importance of obtaining the results of serum electrolyte levels at the earliest is obvious. In routine application, serum electrolytes are measured by the indirect ion-sensing (ISE) method using auto-analyzers (AA) located in the central laboratories of hospitals. In this analyzing method, the processing time is longer because of a delay in the transportation of the samples to the central laboratory on account of several reasons.7 Hence, point-of-care (POC) testing methods, such as, arterial blood gas (ABG) analyzers have been increasingly used in the daily assessments of ICU patients. Blood gas analyzers; use the direct ISE method with short processing time that provides time and rapidity to the physician in the patient’s treatment decisions.7,8 The United States Clinical Laboratory Improvement Amendments (US CLIA) accepts a 0.5 mmol/l difference in the measured potassium levels and a 4 mmol/l difference in the measured sodium levels, in the gold standard measure of the standard calibration solution.9 In some recent studies, the data revealed the difference in the electrolyte levels between the ABG and AA results.10,11 Furthermore, there are also studies that suggest that there is no significant difference between these measuring methods.12,13 Physicians want to trust the veracity of the ABG results of electrolytes such as sodium and potassium because, by this method, the delay in reaching the results is surpassed, and risks arising from this delay may be reduced. However, the results of the above-mentioned studies are confusing and still a diagnostic challenge for physicians. On account of the hesitation by the physicians, we decided to investigate whether the sodium and potassium levels measured by using ABG and AA were equivalent. We conducted a prospective study comparing the electrolyte level results measured in the arterial blood samples by 2 different methods. We tried to notice all limitations of previous similar studies and designed our study according to these points. Thereby, we aimed to improve the accuracy of our study results.     

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