Determination of potassium by dry reagent carrier technology: a multicentre evaluation

We describe the construction, the reaction principle and the performance of Reflotron K+, a new Reflotron test for the quantitative determination of potassium in serum and heparinized plasma. The reaction principle is based on the introduction of the potassium cation via valinomycin into a non-polar phase; the accompanying loss of protons from the non-polar phase is detected by the colour change of a pH indicator. The multicentre evaluation of the reagent carrier system showed in median CVs of < 0.9% (within-series in heparinized plasmas) and 1.3% (run-to-run in control sera). The recovery in control sera was +/- 4% for seven laboratories. In the method comparison with flame emission spectrometry, using sera and heparinized plasma samples, regression analysis yielded correlations with slopes of 1.00 +/- 0.04 (median slope 1.005) and negligible intercepts. The reagent carrier system showed a linear response in the measuring range 2-12 mmol/l. Bilirubin (up to 513 mumol/l), triacylglycerols (up to 5,7 mmol/l), sodium (135-189 mmol/l) and ammonium ions (up to 590 mumol/l) did not interfere with the test. Comparison with the results from flame atomic emission spectrometry shows that the recoveries of Reflotron K+ and the direct potentiometric method are slightly and similarly influenced by total protein. With a panel of 28 drugs tested, no interference could be detected. Reflotron K+ provides a precise and reliable procedure for the measurement of potassium in serum and heparinized plasma.