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Correct! Two methods of ion-selective electrodes (ISE) are currently in use within clinical laboratories. In direct ISE methods, the specimen is brought to the electrode surface without dilution. In indirect ISE methods, the specimen is diluted with high ionic strength buffer in order to control activity coefficient and keep it virtually constant. Although the electrode will respond to the activity, when signal is transformed, indirect ISE yields a very accurate estimate of serum sodium. Sodium values for plasma water are 6% higher for indirect ISE than direct ISE because not all sodium in plasma in present in ionized form. As explained by Mendoza et al, with dialysis, the difference is compensated by the Gibbs-Donnan effect that occurs in HD because of negatively charged non-diffusable proteins trapped over the dialysis membrane attract and decrease the sodium by ~5%. Therefore, using the indirect ISE to measure dialysate sodium could provide a way to accurately assess the patient’s plasma sodium.

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Correct! In Tassin, France, longer HD session (5-8 h) three times a week has been practiced since 1968. In long dialysis, no diet restrictions were requested of patients except for sodium restriction. On the contrary, they are asked to eat a large amount of calories and proteins. Mean NaCl intake was 5-6 g per day, allowing for a reasonable interdialytic weight gain of 1.7 kg, i.e., less than 3% of average body weight. The dialysate sodium used was between 135-138 mmol/L. With longer duration of HD, dietary sodium restriction, and dialysate sodium prescription between 135-138, blood pressure control in patients was achieved without the use of antihypertensive medications.

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Correct! The DOPPS (Dialysis Outcomes and Practice pattern) has shown that the prevalence of hypertension in HD patients is greatest in the US (83.2%), compared with Europe (72.7%) and Japan (55.9%) (Mendoza et al). A global study including almost 30,000 patients, DOPPS showed that dialysate sodium concentration 142 mEq/L was associated with the highest IDWG. IDWG increased by 0.17% of post-dialysis body weight per 2 mEq/L higher dialysate sodium concentration, independent of plasma sodium level. However, finding of this study showed high dialysate sodium was not associated with higher risk for hospitalization or death. The pathophysiology is nicely explained in Figure 2 of the article in AJKD. High dialysate sodium concentration leads to a high post-dialysis plasma sodium that stimulates thirst and water balance, leading to increased weight gain and hypertension.

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Correct! Arramreddy et al described a series of 13 patients undergoing conventional, thriceweekly in-center hemodialysis with an individualized dialysate Na+ prescription. In this study, individualized dialysate Na+ was achieved through a stepwise weekly reductionof the standard dialysate Na+ prescription (140 mEq/L) by 2-3 mEq/L until reaching aNa+ gradient of −2 mEq/L (dialysate Na+ minus average plasma Na+ over the preceding 3 months). When switched from the standard to the individualized dialysate Na+ concentration, IDWG decreased. There was no significant change in the proportion of treatments with cramps, intradialytic hypotension (drop in SBP>30), or intradialytic hypotension requiring an intervention. Once individualized reduction of dialysate Na+ was achieved, there was a reduction of IDWG without significantly increasing the frequency of cramps or hypotension. The study by Mendoza et al showed that when a similar strategy was used, there was a change in IDWG and pre-dialysis systolic blood pressure. No differences in pre-dialysis diastolic, mean arterial, or post-dialysis blood pressures were found in the intervention arm.

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