Analytical evaluation of a direct ion-selective-based analyser: Still gaps to close

Discrepancies between electrolyte concentrations determined by blood gas analysers (BGA) and core-lab chemistry analysers may create confusion in clinical practice. This problem is rooted in the different ion-selective electrode (ISE) methodologies that are used. Whilst most available chemistry analysers use indirect ISE, we evaluated the analytical performance of the new automated chemistry analyser Biossays™ E6 (Snibe), equipped with direct ISE, for the determination of sodium (Na⁺), potassium (K⁺), chloride (Cl^-), ionized calcium (iCa²⁺) and pH.

Total precision, estimated deviation and total error were evaluated for all analytes on the E6 analyser. Several patient cohorts were used to perform method comparisons between the E6 and the direct (RP500e BGA) and indirect (Architect c16000 analyser) ISE methods routinely used in the lab. Obtained data were compared against pre-set quality specifications and used for adjustment of the 2 direct ISE methods. For Na⁺ and iCa²⁺, agreement with the routinely used protein-corrected Na⁺ and total calcium (TCa²⁺) concentrations were assessed respectively.

The analytical performance for the 4 tested electrolytes (Na⁺, K⁺, Cl^-, iCa²⁺) and pH were acceptable and within the specified performance specifications. After adjustment of both direct methods, method comparison on an independent patient cohort showed good agreement. For Na⁺ and iCa²⁺, a good correlation with the protein corrected Na⁺ and TCa²⁺ results was observed.

The acceptable analytical performance and ease-of-use of the E6 direct ion selective instrument is making it feasible to optimize electrolyte determinations to direct methodology.