Reliability of the Hitachi sodium and potassium electrodes assessed

We compared the Hitachi 705 E sodium/potassium electrode system with the Radiometer flame photometer for measuring concentrations of sodium and potassium ions in serum and plasma. Our data show that the Hitachi results are slightly but significantly (less than 0.001) lower than the flame photometer results: Na+ 1.33 mmol/L for serum and 2.24 mmol/L for plasma; K+ 0.062 mmol/L for serum and 0.082 mmol/L for plasma. The magnitude of the bias varied considerably from day to day, ranging from -0.10 to 2.33 mmol/L for Na+ in serum and from 0.00 to 0.11 mmol/L for serum K+; for plasma specimens the corresponding ranges were -0.03 to 5.97 mmol/L and 0.00 to 0.17 mmol/L. These variations probably reflected difficulties in reproducing the calibration from day to day, because the daily means for a serum pool varied in parallel. We occasionally encountered serious problems with reproducibility.