Indirect Ion Selective Electrode Methods Potentially Overestimate Peritoneal Dialysate Sodium Losses

Abstract

Measurements of dialysate sodium are used to estimate peritoneal dialysis sodium losses and sodium sieving, a measure of hydraulic permeability of the peritoneum. Peritoneal dialysates differ from serum samples in terms of pH, osmolality, protein and glucose concentration. We wished to determine whether these factors affected sodium measurement. Dialysate samples were taken from 52 consecutive peritoneal dialysis patients attending for a standard peritoneal dialysis equilibrium test (PET), 20 with standard lactate dialysate and 32 with neutral pH dialysate and sodium was measured by both flame photometry and indirect ion selective electrode (ISE). Sodium measured by ISE consistently overestimated that measured by flame photometer, mean bias 1.5 mmol/L (95% confidence limits 1.2 to 1.8), P < 0.001. Sodium was lower in fresh neutral pH dialysates by both methods - flame 125.3 ± 1.17 vs. 131.6 ± 0.39 mmol/L, than standard lactate dialysates ISE 127.4 ± 1.05 vs 132.7 ± 0.27 mmol/L, P < 0.001. Glucose was higher in fresh neutral pH dialysates 122.7 ± 1.1 vs. standard lactate dialysates 116.7 ± 0.4 mmol/L, P < 0.001. On multiple regression analysis, only glucose was found to be an independent factor for sodium measurement, F = 14.78, β = -0.0851, SEM 0.022, 95% confidence limits -1.28 to -0.042. In this study there was a small but consistent difference between sodium measurements by ISE and flame photometry during the PET. Sodium measurements by either method appeared to be affected by hypertonic dialysates, but there were differences with pH. This may potentially lead to errors in both overestimating peritoneal sodium losses and the proportion of patients with ultrafiltration failure due to loss of sodium sieving.

Peritoneal Protein Losses Depend on More Than Just Peritoneal Dialysis Modality and Peritoneal Membrane Transporter Status

“…* P < 0.05, ** P < 0.01, *** P < 0.001 vs. Fast transporter. Dialysate to serum creatinine ratio (4 h Dialysate/Serum creatinine) .). N terminal brain natriuretic peptide (NT‐proBNP), extracellular water/total body water ratio (ECW/TBW), skeletal muscle mass (SMM).…”

The Effect of Serum Sodium on Survival in Patients Treated by Peritoneal Dialysis in the United Kingdom

“…The higher effect estimate and the higher proportion of hyponatraemia in the younger age group might be partially explained by the higher proportion of patients with diabetic nephropathy in this group (50%) compared to middle (24%) and the older (25%) age groups. There has been debate as to the effect of increasing glucose concentrations on the measurement of serum sodium using indirect potentiometer and flame photometry [14], and potential correction factors for adjusting serum sodium for glucose, with some studies reporting no difference in serum sodium concentration between diabetic and non-diabetic dialysis patients [6,15]. Previous observational studies have suggested greater inter-dialytic weight gains in diabetic patients [16], and greater ratio of extracellular to total body water in diabetic peritoneal dialysis patients [17], suggesting that hyponatraemia in diabetic dialysis patients may have an over hydration component, perhaps due to increased thirst.…”

Does Peritoneal Protein Transport Increase with Peritoneal Dialysis Therapy Duration and Lead to Extracellular Water Overload in Peritoneal Dialysis Patients?

“…This method is linear up to 2.14 g/L (higher concentration samples were diluted to bring them into range). Serum albumin was measured by bromcresol green method (P module analyzer, Roche Integra, Roche Diagnostics ). C‐reactive protein (CRP) was measured using the same assay as the UK National Amlyoid Centre, with values reported down to < 1 mg/L, and hemoglobin by an automated counter (Sysmex XN900, Sysmex Corporation, Kobe, Japan) .…”