• T Lang, P Prinsloo, A F Broughton, N Lawson, and C B Marenah.
• Department of Clinical Chemistry, Nottingham City Hospital NHS Trust, UK.
• Ann. Clin. Biochem. 2002 Jan 1; 39 (Pt 1): 66-7.

BackgroundThe effect of high concentrations of protein or lipid on the measurement of plasma sodium by indirect ion selective electrode (ISE) causing pseudohyponatraemia and pseudonormonatraemia is well described. The effect of a low total protein concentration, however, has not been described.MethodsIn order to examine this, over a 2-week period the total protein concentration was measured on all samples received for urea and electrolyte measurement. All samples with a low (<50 g/L) or a high (> 80 g/L) total protein concentration had sodium measured by both direct and indirect ISE.ResultsThere were approximately equal numbers of samples with a protein concentration less than 50 g/L (1.3%) as samples with a protein concentration greater than 80 g/L (1.3%). The frequency of erroneous sodium results owing to the use of an indirect ISE was less in hypoproteinaemic (2%) than in hyperproteinaemic (20%) samples.

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