A comparison of methods of blood withdrawal and sample preparation for potassium measurements.

The effect on measured potassium values of a number of factors involved in sample acquisition and preparation were investigated. Measured potassium values in either plasma or serum were not influenced by the site of sample withdrawal, the presence of a tourniquet, or the time elapsed between blood sampling and analysis. However, increasing heparin concentrations and/or volumes decreased potassium values. The highest values were obtained in samples obtained in a syringe that had been washed with minimum volumes of heparin. It is concluded that this common clinical practice is based on a sound scientific rationale.