The reliability of electrolyte measurements in plasma.

The difference between serum and plasma electrolyte measurements and the effects of delay, cooling and addition of sodium heparin on plasma electrolyte measurements were observed. Delay of more than 30 min from sampling to analysis renders plasma potassium measurements unreliable. All other factors assessed produced no changes in plasma electrolyte measurements. Arterial blood gas laboratories can, therefore, reliably provide rapid or serial measurements of plasma sodium and potassium provided the analysis is performed within 30 min of obtaining an appropriate arterial blood sample.