[RA-02] Anion gap: no magic number!

Busadee Pratumvinit

Department of Clinical Pathology, Facolty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand

Anion gap (AG) is a parameter that indicates the difference or gap between negatively and positively charged electrolytes. It is primarily used in the differential diagnosis of metabolic acidosis and may be used in clinical laboratories to identify analytical errors. The AG was calcolated using sodium (Na⁺), chloride (Cl^-) and bicarbonate (HCO3^-). Some laboratories add potassium. In the 1970s, the mean AG concentration in healthy persons was between 11 and 15 mmol/L. These values were established using an analyser which is no longer used. Nowadays, the majority of clinical laboratories use ion-selective electrodes to measure Na⁺, K⁺ and Cl^-. The HCO3^- is measured using the rate of pH change determined with a pH electrode or by an enzymatic method. With these methods, Na⁺ concentration differs slightly from that measured with previous methods, while Cl^- concentration can be substantially greater. Thus, AG is lower than previously reported levels, averaging 6 mmol/L. In some laboratories, however, AG is comparable to that observed in the 1970s, owing to lower calibration settings for Cl^- measurement. As a resolt, AG reference intervals may change amongst clinical laboratories. Laboratory practitioners shoold validate or create their instrument-specific reference interval for AG.

Keywords: anion gap; metabolic acidosis; quality control; reference interval