Comparison of two methods for the measurement of serum bicarbonate

Abstract

Accurate bicarbonate measurement is critical in diagnosing acid–base disorders, particularly in critically ill patients. This study compared serum bicarbonate levels measured by enzymatic and direct ion selective electrode (ISE) methods to assess their agreement. Thirty-three patient samples were analysed at the Department of Biochemistry, Sri Venkateswara Institute of Medical Sciences, Tirupati, India. Bicarbonate levels were measured using the Beckman Coulter AU480 analyser (enzymatic method) and the Sensa Core ST-200 Aqua analyser (direct ISE method). Both methods demonstrated good precision, with imprecision values under 2% and biases within acceptable limits. The Bland–Altman analysis showed limits of agreement between −2.9 and 4.5 mmol/L, with a mean bias of 0.8 mmol/L, indicating method agreement. Passing–Bablok regression revealed a minor systematic error in the ISE method, with an (intercept 1.75, slope 0.9406), indicating slight underestimation by ISE compared to the enzymatic method. While both methods are comparable and can be used interchangeably in laboratory settings, caution is recommended in critical care. Larger studies are necessary to confirm comparability across wider ranges, supporting redundancy and result interpretation in larger laboratories.

Keywords

• bicarbonate
• enzymatic
• ion selective electrode
• method comparison