A comparative study on reliability of point of care sodium and potassium estimation in a pediatric intensive care unit.

OBJECTIVE: To compare sodium and potassium levels in children as done with Blood Gas Analyzer (BGA) at point of care testing in pediatric ICU vs. that done in laboratory electrolyte analyzer.

METHODS: This prospective method comparison study was done from February to April 2012 in Pediatric ICU of tertiary care hospital at Delhi. Sixty consecutive patients were tested during the period. Paired blood samples for venous blood gas to be tested on BGA and serum electrolytes to be tested on auto-analyzers (AA) were taken as per standard technique. Data was collected and 59 paired samples were analyzed for sodium and potassium levels. They were analyzed according to CLSI document EP15-A2 using ACB method comparison software.

RESULTS: Mean sodium measured on the BGA was 132.8 ± 12.2 mmol/L where as measured by AA was 141.5 ± 11.1 mmol/L. The mean difference between the two was -8.76 mmol/L (p < 0.001). The difference was statistically significant in all three subgroups of hypernatremia, isonatremia and hyponatremia (p < 0.001). Potassium level in BGA was 3.53 ± 0.81 mmol/L and AA was 4.28 ± 1.05 mmol/L. The mean difference between the BGA and AA was -0.75 mmol/L (p < 0.0001). The difference was statistically significant in patients with normokalemia and hyperkalemia (p < 0.0001). The difference was non significant in patients with hypokalemia (p = 0.051).

CONCLUSIONS: Blood gas analyzers underestimates Na + and K + values if sampling is done using liquid sodium heparin and if all other potential pre-analytical errors of testing are taken care of. The Bland Altman's analysis in the present study showed a significant systematic bias and very wide limits of agreement for both sodium and potassium, which is not clinically acceptable.