Glucose interference in direct ion-sensitive electrode sodium measurements

Circulating sodium concentration is commonly measured by both direct and indirect ion-sensitive electrode (ISE). We describe an unusual case with a high elevation of serum glucose (162 mmol/L) where direct ISE sodium measurement was 9 mmol/L higher than the indirect measurement in the absence of any cause for pseudohyponatraemia. In vitro experiments showed that very high glucose concentrations increased the sodium in direct, but not in indirect ISE measurement. This effect was insufficient to account for the entire difference between the measurements seen in the patient, indicating that other factors, for example pH and bicarbonate concentration, must also be involved. This effect may influence interpretation of sodium status in patients with gross hyperglycaemia.