Determination of electrolytes--methodological problems.

Abstract

To avoid errors in plasma potassium determination clenching of the fist should be avoided and heparinized blood should be used. The patient should be fasting and samples taken at the same time of the day. When evaluating intracellular potassium from plasma potassium acid based status should be considered. The clinical status should also be considered, since severely ill patients may have a low transmembrane potential leading to loss of cell potassium and increase in plasma potassium. Measurements of exchangeable sodium and potassium with radioactive tracers of whole body potassium (40K) are inprecise for determination of absolute cellular concentrations but more suitable for longitudinal studies. Determination of red cell electrolytes is of limited value, since these cells are not typical for the body cells. Electrolyte determination in leukocytes may give more accurate information. Muscle water and electrolytes can be determined in needle biopsy material. The concentration of intracellular electrolytes are expressed in relation to different bases of reference (FFS, GFFS, alkali-soluble protein, phosphorus, and intracellular water); the advantages and fallacies in using these references are discussed.