Impact of Bicarbonate Interference on Routine Ion-Selective Electrode Chloride Measurements

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

INTRODUCTION

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.

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSION

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.

Stochastic Sensing of Chloride Anions Using an α-Hemolysin Pore with a semiaza-Bambusuril Adapter

Pores containing molecular adapters provide internal selective binding sites, thereby allowing the stochastic sensing of analytes. Herein, we demonstrate that semiaza-bambusuril (BU) acts as a non-covalent molecular adapter when lodged within the lumen of the wild-type α-hemolysin (WT-αHL) protein pore. Because the bambusurils are recognized as anion receptors, the anion binding site within the adapter-nanopore complex allows the detection of chloride anions, thus converting a non-selective pore into an anion sensor.

Case report: Salicylate intoxication can present with a normal anion gap metabolic acidosis depending on method used for measuring chloride

Severe salicylate intoxication usually presents with a high anion gap metabolic acidosis. We describe a patient with severe salicylate intoxication that presented with a surprisingly normal anion gap metabolic acidosis. Initial salicylate level was 594 mg/L (therapeutic range 50-300 mg/L). In this case, the anion gap was normal due to a falsely elevated chloride concentration measured using a direct ion-selective electrode (ISE) (ABL90-flex). Since earlier case reports have shown that salicylate ions can interfere with chloride measurement using different ISEs, available samples were re-analyzed using an indirect ISE (Roche Cobas 8000), in which salicylate levels up to 1000 mg/L were found to cause no significant interference. With this method, chloride concentration was found to be 115 instead of 122 mmol/L, leading to the expected elevated anion gap. We performed a spike experiment to investigate the impact of different salicylate levels and bicarbonate concentrations on the measured chloride concentration for both methods. This experiment showed that the difference between chloride concentrations was mainly explained by interference with bicarbonate. It is important for clinicians to be aware of this possible interference, since a high anion gap metabolic acidosis can be a clue to suspect salicylate poisoning and early recognition and appropriate treatment is important. The patient was successfully treated with hemodialysis and no rebound toxicity was observed.