Agreement between whole blood and plasma sodium measurements in profound hyponatremia

Objectives: We compared two different methods of whole blood sodium measurement to plasma sodium measurement using samples in the profoundly hyponatremic range (Na < 120 mmol/L).

Design and methods: Whole blood pools with a range of low sodium values were generated using combinations and dilutions of pooled electrolyte-balanced lithium heparin samples submitted for arterial blood gas analysis. Each pool was analyzed five times on a Radiometer 827 blood gas analyzer and iSTAT analyzer. Pools were centrifuged to produce plasma, which was analyzed five times on a Roche Cobas c501 chemistry analyzer. An additional 40 fresh (analyzed on day of collection) excess lithium heparin arterial blood gas samples from 36 patients were analyzed on the Radiometer 827, iSTAT, and Cobas c501 as described above. The setting was a tertiary referral center. Blood samples were collected from a combination of patients in the intensive care unit, operating theaters and emergency room.

Results: All methods demonstrated excellent precision, even in the profoundly hyponatremic measurement range (Na < 120 mmol/L using a plasma reference method). However, agreement between the methods varied with the degree of hyponatremia. In the profoundly hyponatremic range, Radiometer whole blood sodium values were nearly identical to plasma reference sodium, while iSTAT whole blood sodium showed a consistent positive bias relative to plasma sodium in this range.

Conclusion: If whole blood direct sodium measurements are compared with plasma sodium in profoundly hyponatremic patients consideration should be given to the use of Radiometer blood gas analyzers over iSTAT since the latter shows a positive bias relative to a plasma comparative method.