Pseudonormo- and Pseudohypernatraemia in sick neonates

Introduction Pseudohyponatraemia is well recognised when using indirect ion selective electrodes (ISE) in samples with raised plasma protein or lipid. ‘Ion-exclusion’ may also cause pseudo-normo- or pseudohypernatraemia in low protein states which are common in sick neonates. We investigated the effect of hypoal-buminaemia on sodium measured by direct and indirect ISE in neonates. Method This retrospective study compared electrolyte results from paired samples, performed on the Abbott Architect (indirect ISE) and neonatal unit blood-gas analyser (direct ISE). Results Of 2420 paired results, hypoalbuminaemia ( r  = 0.309, p 3 mmol/L (the allowable limit of performance) in 31% and led to the misclassifi-cation of results as pseudonormonatraemia (9.4%) and pseudohypernatraemia (5.3%). Discussion Hypoalbuminaemia is common in sick neonates and concurrent measurement of electrolytes using direct and indirect ISEs frequently yields significantly different results leading to misclassification. In these patients, measurement of electrolytes by direct ISE may be more accurate.