Effect of Protein Concentration on Serum Sodium Measurement Using Two Analytical Platforms

Saldaña-Orejón I.M.; Monsefú-Cerna L.D.; Benites-Ricra M.A. · Revista del Cuerpo Medico Hospital Nacional Almanzor Aguinaga Asenjo (2026) · DOI: 10.35434/rcmhnaaa.2026.191.3014

Resumen

Introduction: Reliable measurement of serum sodium is essential for the diagnosis and management of hydroelectrolytic disorders. Determination using ion-selective electrodes (ISE) provides reliability, although it is not free from interference. In particular, indirect ISE tends to underestimate sodium levels in the presence of hyperproteinemia. Objective: To evaluate the effect of protein concentration on sodium measurement by indirect ISE in two analytical platforms, using a direct ISE method as reference. Methods: Observational analytical comparative study, ex vivo in nature, based on the analysis of residual human serum samples. A total of 663 serum samples were processed and grouped into 10 protein ranges (<4 to ≥12 g/dL). Of these, 387 were measured with Atellica CH (Siemens) and 276 with DXC 700 AU (Beckman Coulter), both using indirect ISE, and compared with ABL 800 FLEX (Radiometer), based on direct ISE. Mean biases, medians, Spearman correlations, and the percentage of results within the CLIA allowable error (±4 mmol/L) were calculated. Results: Atellica CH showed a mean bias of -0.01 mmol/L with 63.3% of results within the acceptable limit, whereas DXC 700 AU showed -3.73 mmol/L with 59.4% acceptable results. In the normal protein range (6–8 g/dL), both exceeded 90% acceptability; however, at ≥8 g/dL biases exceeded ±4 mmol/L, reaching -9.17 mmol/L (Atellica CH) and -10.61 mmol/L (DXC 700 AU), with acceptability close to 0%. A strong negative correlation between bias and serum protein concentration was observed (ρ = -0.91 and -0.89; p < 0.001). Conclusions: Hyperproteinemia is a critical factor causing inaccuracy in sodium measurements obtained by indirect ISE. Confirmation using direct ISE or equivalent methods is recommended in patients with elevated serum protein levels to avoid erroneous diagnoses of hyponatremia.

Instituciones: Universidad Nacional Mayor de San Marcos; Universidad Privada Norbert Wiener