Determination of potassium in red blood cells using unmeasured volumes of whole blood and combined sodium/potassium-selective membrane electrode measurements.

Recent studies suggest that the measurement of intracellular potassium concentrations in red blood cells (RBC-K) can be a marker for assessing the risk, development, and treatment of hypertension. In this work, the combined use of miniature potassium- and sodium-selective membrane electrodes is evaluated as a simple means to determine RBC-K. The proposed method requires two separate sets of electrode measurements: (i) potassium and sodium concentrations in the plasma phase of an unmeasured volume of a whole blood sample, and (ii) determination of potassium and sodium concentrations in the same sample of blood after complete hemolysis by ultrasonic disruption of the RBC membranes. The dilution of sodium concentration after hemolysis can be used to determine hematocrit (Hct) (volume of red cells per unit volume of blood) of the blood. The concentration of potassium within the red blood cells (RBCs) is then calculated using the measured change in potassium levels before and after RBCs lysis and the hematocrit level determined from the sodium electrode measurements and/or a conventional centrifugation method. Good correlation for RBC-K between the proposed method and traditional flame photometry is observed for animal blood samples that possess the range of potassium levels found within human RBCs (80-120 mM). However, when potassium is much lower than that found in human RBCs (known to occur for certain animal species), the Hct measured by the sodium electrode method is falsely low, compared to traditional spun hematocrit values, because of an increased level of sodium within the RBCs, necessitating use of spun Hct levels to assess RBC-K accurately. It is envisioned that this new approach could be further miniaturized into a single-use disposable cartridge type electrode system that would enable rapid point-of-care screening of RBC-K levels in human subjects.