Frozen Human Serum Reference Materialfor Standardizationof Sodiumand PotassiumMeasurementsin Serum or Plasmaby Ion-SelectiveElectrodeAnalyzers

Three interlaboratory round-robin studies (RR1, RR2, and RR3) were conducted to identifya serum-based reference material that would aid in the standardization of direct ion-selective electrode (ISE) measurements of sodium and potassium. Ultrafiltered frozen serum reference ma-terials requiring no reconstitution reduced between-labo-ratory variability (the largest source of imprecision) more than did other reference materials. ISE values for RR3 were normalized by the use of two points at the extremes of the clinical range for sodium (i.e., 120 and 160 mmol/L), with values assigned by the flame atomic emission spec-trometry (FAES) Reference Method. This FAES normal-ization of ISE raw values remarkably improved all sources of variability and unified the results from seven different direct ISE analyzers to the FAES Reference Metho