118 research about the same. The arterial blood gas (ABG) machines and AA differ in several aspects as listed in Table 1 [4]. The United States Clinical Laboratory Improvement Amendment (US CLIA) 2006 [5] accepts a difference of 0.5 mmol/l in measured potassium, and 4 mmol/l in measured sodium, from the gold standard measure of standard calibration solutions. Previous studies [6] that measured the accuracy of electrolyte values obtained by ABG machines concluded that the results from two different measurement technolo- gies differed significantly for plasma sodium and chloride concentrations. These differences significantly affected the calculated anion gap and strong ion difference values. Others have also found statistically significant differ- ences in measures of pH, potassium, and hematocrit between ABG machines and leading bench-top analyzers [6, 7]. Experts have cited theoretical reasons for these differ- ences based on the chemical reactions employed in the AA and the auto-analyzers [8]. 1. 2. Dilution with heparin raises the volume of the sample, thereby lowering the value of the measured electrolytes on the ABG. High volume of heparin itself binds the electrolytes, thereby lowering the value of measured electrolytes on the ABG. Our study had a hypothesis that there is no difference between the electrolyte measurements by the AA and ABG machines. Materials and methods The study was approved by the ethics committee of Apollo Hospitals, Hyderabad, India. The study was designed to be Table 1 Differences between the blood gas analyzer and auto-analyzer Blood gas analyzer Auto-analyzer Analyzes whole blood Arterial sample used conventionally Uses direct ion selective electrode Uses heparin-diluted sample Processing time is short No effect of protein levels in blood Analyzes serum Venous sample used conventionally Uses indirect ion-selective electrode technology Serum sample diluted with fixed volume diluent Processing time is long Affected by protein levels in blood Int J Emerg Med (2009) 2:117–120 Table 2 Statistical analysis of sodium samples Sample Blood gas analyzer Auto-analyzer Mean mmol/l 131.2 136.4 SD 7.3 6.5 an observational study of a consecutive cohort of patients admitted to the ICU. Paired blood samples from 200 consecutive adult patients admitted to the Medical ICU of Apollo Hospitals were included in the study. The study period started in August 2005 and ended in May 2006. Blood samples were collected simultaneously for ABG and AA for electrolyte analysis. All samples were analyzed on the ABL555 blood gas analyzer and the Dade Dimension RxL Max, both located in the central laboratory. Both of these are considered as benchmark top models in their respective technologies. Quality control was ensured by having the blood samples collected by trained staff of a single ICU unit in the hospital and analyzed in the two analyzers located in the central laboratory under similar environmental conditions, unlike in the previous studies where the analyzers were in different environments [7]. Both analyzers were calibrated [10] according to the Joint Commission on Accreditation of Hospitals (JCAHO) requirements for hospital certification prior to each measurement cycle. Time of collection of samples was documented on both sample groups for reference. Operators drawing and handling blood samples did so with only latex gloves without using any hand disinfectants just prior to handling blood [11]. The 200 paired samples included a venous blood sample for auto-analyzer electrolyte measurement and an arterial blood sample for blood gas analysis. Data were collected, and the values of sodium (Na+) and potassium (K+) from these measurements were compared using the paired t-test. Results The analysis of sodium values (Table 2) showed that the mean sodium measured on the ABG was 131.28 mmol/L (SD 7.33), and the mean sodium measured by the AA was 136.45 mmol/L (SD 6.50). The maximum difference in sodium value was 30 mmol/l, and the minimum difference was 0mmol/l. The mean difference was 5.96 mmol/l with SD 5.09 (p<0.001, significant difference detected, hence hypothesis rejected). The correlation coefficient (r) was 0.68. The adjusted r2 was within a 95% confidence interval from