Discrepancies Between Point of Care and Central Laboratory Sodium and Potassium Measurements in ICU: Analytical...

Blood electrolyte panels, especially sodium ion (Na + ) and potassium ion (K + ), are fundamental and routine tests. In critical care settings, these measurements are typically performed by central laboratories using indirect potentiometry (IP) or via point-of-care blood gas analyzers using direct potentiometry (DP). Discrepancies between the two methods exist, and clinicians often have limited knowledge regarding the strengths and weaknesses of each technique. We wanted to quantify these observations.

We compared prospectively Na + and K + measurements obtained from the same blood draw using both IP and DP (including analyses of correlation, agreement, and discrepancies). We also conducted a brief survey among physicians to assess their knowledge regarding IP and DP.

A tertiary hospital.

A total of 501 paired measurements were prospectively collected from an ICU population.

None.

We assessed the degree of agreement between the two methods. We also examined the impact of proteinemia and hemolysis on Na + and K + values, respectively. Only 31.1% of the 103 responding physicians were aware of the analytical bias in Na + measurements obtained by IP, and 45.6% considered K + measurements from DP to be as reliable or more reliable than those obtained by IP. The agreement between the two methods was moderate (Lin's concordance correlation coefficient: 0.90 for Na + and 0.93 for K + ). The 95% limit of agreement for Na + was particularly large (10.48 mmol/L). The divergence between the two methods, defined as a discrepancy in classification (within, below, or above the normal range), occurred in approximately 10% of cases, for both Na + and K + .

IP and DP were not interchangeable, each exhibiting distinct strengths and limitations. Enhancing physician awareness of the differences between these methods could improve the quality of care.