Errors due to heparin in the estimation of plasma sodium and potassium concentrations.

The effect of adding sodium heparin (5000 U/ml and 1000 U/ml) to whole blood on the concentrations of plasma sodium and potassium was examined. When the analysis was done by flame photometry both preparations of heparin caused an increase in plasma sodium concentration. If the analysis was done by a direct ion selective electrode method the 5000 U/ml heparin caused a decrease and the 1000 U/ml caused an increase in plasma sodium concentration. Plasma potassium concentration decreased when 1000 U/ml heparin was added irrespective of the method of analysis. It is recommended that sodium heparin should not be used in samples taken for estimation of sodium and potassium.